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kernel/pub/scm/linux/kernel/git/rafael/linux-pm/bleeding-edge/./rust/zerocopy/src/layout.rs
blob: 6015d0f2de52adc3c0d3c726af3e4b9b6fde1cd4 [file]
// SPDX-License-Identifier: (BSD-2-Clause OR Apache-2.0) OR MIT
// Copyright 2024 The Fuchsia Authors
//
// Licensed under the 2-Clause BSD License <LICENSE-BSD or
// https://opensource.org/license/bsd-2-clause>, Apache License, Version 2.0
// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
// This file may not be copied, modified, or distributed except according to
// those terms.
use core::{mem, num::NonZeroUsize};
use crate::util;
/// The target pointer width, counted in bits.
const POINTER_WIDTH_BITS: usize = mem::size_of::<usize>() * 8;
/// The layout of a type which might be dynamically-sized.
///
/// `DstLayout` describes the layout of sized types, slice types, and "slice
/// DSTs" - ie, those that are known by the type system to have a trailing slice
/// (as distinguished from `dyn Trait` types - such types *might* have a
/// trailing slice type, but the type system isn't aware of it).
///
/// Note that `DstLayout` does not have any internal invariants, so no guarantee
/// is made that a `DstLayout` conforms to any of Rust's requirements regarding
/// the layout of real Rust types or instances of types.
#[doc(hidden)]
#[allow(missing_debug_implementations, missing_copy_implementations)]
#[cfg_attr(any(kani, test), derive(Debug, PartialEq, Eq))]
#[derive(Copy, Clone)]
pub struct DstLayout {
pub(crate) align: NonZeroUsize,
pub(crate) size_info: SizeInfo,
// Is it guaranteed statically (without knowing a value's runtime metadata)
// that the top-level type contains no padding? This does *not* apply
// recursively - for example, `[(u8, u16)]` has `statically_shallow_unpadded
// = true` even though this type likely has padding inside each `(u8, u16)`.
pub(crate) statically_shallow_unpadded: bool,
}
#[cfg_attr(any(kani, test), derive(Debug, PartialEq, Eq))]
#[derive(Copy, Clone)]
pub(crate) enum SizeInfo<E = usize> {
Sized { size: usize },
SliceDst(TrailingSliceLayout<E>),
}
#[cfg_attr(any(kani, test), derive(Debug, PartialEq, Eq))]
#[derive(Copy, Clone)]
pub(crate) struct TrailingSliceLayout<E = usize> {
// The offset of the first byte of the trailing slice field. Note that this
// is NOT the same as the minimum size of the type. For example, consider
// the following type:
//
// struct Foo {
// a: u16,
// b: u8,
// c: [u8],
// }
//
// In `Foo`, `c` is at byte offset 3. When `c.len() == 0`, `c` is followed
// by a padding byte.
pub(crate) offset: usize,
// The size of the element type of the trailing slice field.
pub(crate) elem_size: E,
}
impl SizeInfo {
/// Attempts to create a `SizeInfo` from `Self` in which `elem_size` is a
/// `NonZeroUsize`. If `elem_size` is 0, returns `None`.
#[allow(unused)]
const fn try_to_nonzero_elem_size(&self) -> Option<SizeInfo<NonZeroUsize>> {
Some(match *self {
SizeInfo::Sized { size } => SizeInfo::Sized { size },
SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size }) => {
if let Some(elem_size) = NonZeroUsize::new(elem_size) {
SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size })
} else {
return None;
}
}
})
}
}
#[doc(hidden)]
#[derive(Copy, Clone)]
#[cfg_attr(test, derive(Debug))]
#[allow(missing_debug_implementations)]
pub enum CastType {
Prefix,
Suffix,
}
#[cfg_attr(test, derive(Debug))]
pub(crate) enum MetadataCastError {
Alignment,
Size,
}
impl DstLayout {
/// The minimum possible alignment of a type.
const MIN_ALIGN: NonZeroUsize = match NonZeroUsize::new(1) {
Some(min_align) => min_align,
None => const_unreachable!(),
};
/// The maximum theoretic possible alignment of a type.
///
/// For compatibility with future Rust versions, this is defined as the
/// maximum power-of-two that fits into a `usize`. See also
/// [`DstLayout::CURRENT_MAX_ALIGN`].
pub(crate) const THEORETICAL_MAX_ALIGN: NonZeroUsize =
match NonZeroUsize::new(1 << (POINTER_WIDTH_BITS - 1)) {
Some(max_align) => max_align,
None => const_unreachable!(),
};
/// The current, documented max alignment of a type \[1\].
///
/// \[1\] Per <https://doc.rust-lang.org/reference/type-layout.html#the-alignment-modifiers>:
///
/// The alignment value must be a power of two from 1 up to
/// 2<sup>29</sup>.
#[cfg(not(kani))]
#[cfg(not(target_pointer_width = "16"))]
pub(crate) const CURRENT_MAX_ALIGN: NonZeroUsize = match NonZeroUsize::new(1 << 28) {
Some(max_align) => max_align,
None => const_unreachable!(),
};
#[cfg(not(kani))]
#[cfg(target_pointer_width = "16")]
pub(crate) const CURRENT_MAX_ALIGN: NonZeroUsize = match NonZeroUsize::new(1 << 15) {
Some(max_align) => max_align,
None => const_unreachable!(),
};
/// The maximum size of an allocation \[1\].
///
/// \[1\] Per <https://doc.rust-lang.org/1.91.1/std/ptr/index.html#allocation>:
///
/// For any allocation with base `address`, `size`, and a set of `addresses`,
/// the following are guaranteed: [..]
///
/// - `size <= isize::MAX`
///
#[allow(clippy::as_conversions)]
pub(crate) const MAX_SIZE: usize = isize::MAX as usize;
/// Assumes that this layout lacks static shallow padding.
///
/// # Panics
///
/// This method does not panic.
///
/// # Safety
///
/// If `self` describes the size and alignment of type that lacks static
/// shallow padding, unsafe code may assume that the result of this method
/// accurately reflects the size, alignment, and lack of static shallow
/// padding of that type.
const fn assume_shallow_unpadded(self) -> Self {
Self { statically_shallow_unpadded: true, ..self }
}
/// Constructs a `DstLayout` for a zero-sized type with `repr_align`
/// alignment (or 1). If `repr_align` is provided, then it must be a power
/// of two.
///
/// # Panics
///
/// This function panics if the supplied `repr_align` is not a power of two.
///
/// # Safety
///
/// Unsafe code may assume that the contract of this function is satisfied.
#[doc(hidden)]
#[must_use]
#[inline]
pub const fn new_zst(repr_align: Option<NonZeroUsize>) -> DstLayout {
let align = match repr_align {
Some(align) => align,
None => Self::MIN_ALIGN,
};
const_assert!(align.get().is_power_of_two());
DstLayout {
align,
size_info: SizeInfo::Sized { size: 0 },
statically_shallow_unpadded: true,
}
}
/// Constructs a `DstLayout` which describes `T` and assumes `T` may contain
/// padding.
///
/// # Safety
///
/// Unsafe code may assume that `DstLayout` is the correct layout for `T`.
#[doc(hidden)]
#[must_use]
#[inline]
pub const fn for_type<T>() -> DstLayout {
// SAFETY: `align` is correct by construction. `T: Sized`, and so it is
// sound to initialize `size_info` to `SizeInfo::Sized { size }`; the
// `size` field is also correct by construction. `unpadded` can safely
// default to `false`.
DstLayout {
align: match NonZeroUsize::new(mem::align_of::<T>()) {
Some(align) => align,
None => const_unreachable!(),
},
size_info: SizeInfo::Sized { size: mem::size_of::<T>() },
statically_shallow_unpadded: false,
}
}
/// Constructs a `DstLayout` which describes a `T` that does not contain
/// padding.
///
/// # Safety
///
/// Unsafe code may assume that `DstLayout` is the correct layout for `T`.
#[doc(hidden)]
#[must_use]
#[inline]
pub const fn for_unpadded_type<T>() -> DstLayout {
Self::for_type::<T>().assume_shallow_unpadded()
}
/// Constructs a `DstLayout` which describes `[T]`.
///
/// # Safety
///
/// Unsafe code may assume that `DstLayout` is the correct layout for `[T]`.
pub(crate) const fn for_slice<T>() -> DstLayout {
// SAFETY: The alignment of a slice is equal to the alignment of its
// element type, and so `align` is initialized correctly.
//
// Since this is just a slice type, there is no offset between the
// beginning of the type and the beginning of the slice, so it is
// correct to set `offset: 0`. The `elem_size` is correct by
// construction. Since `[T]` is a (degenerate case of a) slice DST, it
// is correct to initialize `size_info` to `SizeInfo::SliceDst`.
DstLayout {
align: match NonZeroUsize::new(mem::align_of::<T>()) {
Some(align) => align,
None => const_unreachable!(),
},
size_info: SizeInfo::SliceDst(TrailingSliceLayout {
offset: 0,
elem_size: mem::size_of::<T>(),
}),
statically_shallow_unpadded: true,
}
}
/// Constructs a complete `DstLayout` reflecting a `repr(C)` struct with the
/// given alignment modifiers and fields.
///
/// This method cannot be used to match the layout of a record with the
/// default representation, as that representation is mostly unspecified.
///
/// # Safety
///
/// For any definition of a `repr(C)` struct, if this method is invoked with
/// alignment modifiers and fields corresponding to that definition, the
/// resulting `DstLayout` will correctly encode the layout of that struct.
///
/// We make no guarantees to the behavior of this method when it is invoked
/// with arguments that cannot correspond to a valid `repr(C)` struct.
#[must_use]
#[inline]
pub const fn for_repr_c_struct(
repr_align: Option<NonZeroUsize>,
repr_packed: Option<NonZeroUsize>,
fields: &[DstLayout],
) -> DstLayout {
let mut layout = DstLayout::new_zst(repr_align);
let mut i = 0;
#[allow(clippy::arithmetic_side_effects)]
while i < fields.len() {
#[allow(clippy::indexing_slicing)]
let field = fields[i];
layout = layout.extend(field, repr_packed);
i += 1;
}
layout = layout.pad_to_align();
// SAFETY: `layout` accurately describes the layout of a `repr(C)`
// struct with `repr_align` or `repr_packed` alignment modifications and
// the given `fields`. The `layout` is constructed using a sequence of
// invocations of `DstLayout::{new_zst,extend,pad_to_align}`. The
// documentation of these items vows that invocations in this manner
// will accurately describe a type, so long as:
//
// - that type is `repr(C)`,
// - its fields are enumerated in the order they appear,
// - the presence of `repr_align` and `repr_packed` are correctly accounted for.
//
// We respect all three of these preconditions above.
layout
}
/// Like `Layout::extend`, this creates a layout that describes a record
/// whose layout consists of `self` followed by `next` that includes the
/// necessary inter-field padding, but not any trailing padding.
///
/// In order to match the layout of a `#[repr(C)]` struct, this method
/// should be invoked for each field in declaration order. To add trailing
/// padding, call `DstLayout::pad_to_align` after extending the layout for
/// all fields. If `self` corresponds to a type marked with
/// `repr(packed(N))`, then `repr_packed` should be set to `Some(N)`,
/// otherwise `None`.
///
/// This method cannot be used to match the layout of a record with the
/// default representation, as that representation is mostly unspecified.
///
/// # Safety
///
/// If a (potentially hypothetical) valid `repr(C)` Rust type begins with
/// fields whose layout are `self`, and those fields are immediately
/// followed by a field whose layout is `field`, then unsafe code may rely
/// on `self.extend(field, repr_packed)` producing a layout that correctly
/// encompasses those two components.
///
/// We make no guarantees to the behavior of this method if these fragments
/// cannot appear in a valid Rust type (e.g., the concatenation of the
/// layouts would lead to a size larger than `isize::MAX`).
#[doc(hidden)]
#[must_use]
#[inline]
pub const fn extend(self, field: DstLayout, repr_packed: Option<NonZeroUsize>) -> Self {
use util::{max, min, padding_needed_for};
// If `repr_packed` is `None`, there are no alignment constraints, and
// the value can be defaulted to `THEORETICAL_MAX_ALIGN`.
let max_align = match repr_packed {
Some(max_align) => max_align,
None => Self::THEORETICAL_MAX_ALIGN,
};
const_assert!(max_align.get().is_power_of_two());
// We use Kani to prove that this method is robust to future increases
// in Rust's maximum allowed alignment. However, if such a change ever
// actually occurs, we'd like to be notified via assertion failures.
#[cfg(not(kani))]
{
const_debug_assert!(self.align.get() <= DstLayout::CURRENT_MAX_ALIGN.get());
const_debug_assert!(field.align.get() <= DstLayout::CURRENT_MAX_ALIGN.get());
if let Some(repr_packed) = repr_packed {
const_debug_assert!(repr_packed.get() <= DstLayout::CURRENT_MAX_ALIGN.get());
}
}
// The field's alignment is clamped by `repr_packed` (i.e., the
// `repr(packed(N))` attribute, if any) [1].
//
// [1] Per https://doc.rust-lang.org/reference/type-layout.html#the-alignment-modifiers:
//
// The alignments of each field, for the purpose of positioning
// fields, is the smaller of the specified alignment and the alignment
// of the field's type.
let field_align = min(field.align, max_align);
// The struct's alignment is the maximum of its previous alignment and
// `field_align`.
let align = max(self.align, field_align);
let (interfield_padding, size_info) = match self.size_info {
// If the layout is already a DST, we panic; DSTs cannot be extended
// with additional fields.
SizeInfo::SliceDst(..) => const_panic!("Cannot extend a DST with additional fields."),
SizeInfo::Sized { size: preceding_size } => {
// Compute the minimum amount of inter-field padding needed to
// satisfy the field's alignment, and offset of the trailing
// field. [1]
//
// [1] Per https://doc.rust-lang.org/reference/type-layout.html#the-alignment-modifiers:
//
// Inter-field padding is guaranteed to be the minimum
// required in order to satisfy each field's (possibly
// altered) alignment.
let padding = padding_needed_for(preceding_size, field_align);
// This will not panic (and is proven to not panic, with Kani)
// if the layout components can correspond to a leading layout
// fragment of a valid Rust type, but may panic otherwise (e.g.,
// combining or aligning the components would create a size
// exceeding `isize::MAX`).
let offset = match preceding_size.checked_add(padding) {
Some(offset) => offset,
None => const_panic!("Adding padding to `self`'s size overflows `usize`."),
};
(
padding,
match field.size_info {
SizeInfo::Sized { size: field_size } => {
// If the trailing field is sized, the resulting layout
// will be sized. Its size will be the sum of the
// preceding layout, the size of the new field, and the
// size of inter-field padding between the two.
//
// This will not panic (and is proven with Kani to not
// panic) if the layout components can correspond to a
// leading layout fragment of a valid Rust type, but may
// panic otherwise (e.g., combining or aligning the
// components would create a size exceeding
// `usize::MAX`).
let size = match offset.checked_add(field_size) {
Some(size) => size,
None => const_panic!("`field` cannot be appended without the total size overflowing `usize`"),
};
SizeInfo::Sized { size }
}
SizeInfo::SliceDst(TrailingSliceLayout {
offset: trailing_offset,
elem_size,
}) => {
// If the trailing field is dynamically sized, so too
// will the resulting layout. The offset of the trailing
// slice component is the sum of the offset of the
// trailing field and the trailing slice offset within
// that field.
//
// This will not panic (and is proven with Kani to not
// panic) if the layout components can correspond to a
// leading layout fragment of a valid Rust type, but may
// panic otherwise (e.g., combining or aligning the
// components would create a size exceeding
// `usize::MAX`).
let offset = match offset.checked_add(trailing_offset) {
Some(offset) => offset,
None => const_panic!("`field` cannot be appended without the total size overflowing `usize`"),
};
SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size })
}
},
)
}
};
let statically_shallow_unpadded = self.statically_shallow_unpadded
&& field.statically_shallow_unpadded
&& interfield_padding == 0;
DstLayout { align, size_info, statically_shallow_unpadded }
}
/// Like `Layout::pad_to_align`, this routine rounds the size of this layout
/// up to the nearest multiple of this type's alignment or `repr_packed`
/// (whichever is less). This method leaves DST layouts unchanged, since the
/// trailing padding of DSTs is computed at runtime.
///
/// The accompanying boolean is `true` if the resulting composition of
/// fields necessitated static (as opposed to dynamic) padding; otherwise
/// `false`.
///
/// In order to match the layout of a `#[repr(C)]` struct, this method
/// should be invoked after the invocations of [`DstLayout::extend`]. If
/// `self` corresponds to a type marked with `repr(packed(N))`, then
/// `repr_packed` should be set to `Some(N)`, otherwise `None`.
///
/// This method cannot be used to match the layout of a record with the
/// default representation, as that representation is mostly unspecified.
///
/// # Safety
///
/// If a (potentially hypothetical) valid `repr(C)` type begins with fields
/// whose layout are `self` followed only by zero or more bytes of trailing
/// padding (not included in `self`), then unsafe code may rely on
/// `self.pad_to_align(repr_packed)` producing a layout that correctly
/// encapsulates the layout of that type.
///
/// We make no guarantees to the behavior of this method if `self` cannot
/// appear in a valid Rust type (e.g., because the addition of trailing
/// padding would lead to a size larger than `isize::MAX`).
#[doc(hidden)]
#[must_use]
#[inline]
pub const fn pad_to_align(self) -> Self {
use util::padding_needed_for;
let (static_padding, size_info) = match self.size_info {
// For sized layouts, we add the minimum amount of trailing padding
// needed to satisfy alignment.
SizeInfo::Sized { size: unpadded_size } => {
let padding = padding_needed_for(unpadded_size, self.align);
let size = match unpadded_size.checked_add(padding) {
Some(size) => size,
None => const_panic!("Adding padding caused size to overflow `usize`."),
};
(padding, SizeInfo::Sized { size })
}
// For DST layouts, trailing padding depends on the length of the
// trailing DST and is computed at runtime. This does not alter the
// offset or element size of the layout, so we leave `size_info`
// unchanged.
size_info @ SizeInfo::SliceDst(_) => (0, size_info),
};
let statically_shallow_unpadded = self.statically_shallow_unpadded && static_padding == 0;
DstLayout { align: self.align, size_info, statically_shallow_unpadded }
}
/// Produces `true` if `self` requires static padding; otherwise `false`.
#[must_use]
#[inline(always)]
pub const fn requires_static_padding(self) -> bool {
!self.statically_shallow_unpadded
}
/// Produces `true` if there exists any metadata for which a type of layout
/// `self` would require dynamic trailing padding; otherwise `false`.
#[must_use]
#[inline(always)]
pub const fn requires_dynamic_padding(self) -> bool {
// A `% self.align.get()` cannot panic, since `align` is non-zero.
#[allow(clippy::arithmetic_side_effects)]
match self.size_info {
SizeInfo::Sized { .. } => false,
SizeInfo::SliceDst(trailing_slice_layout) => {
// SAFETY: This predicate is formally proved sound by
// `proofs::prove_requires_dynamic_padding`.
trailing_slice_layout.offset % self.align.get() != 0
|| trailing_slice_layout.elem_size % self.align.get() != 0
}
}
}
/// Validates that a cast is sound from a layout perspective.
///
/// Validates that the size and alignment requirements of a type with the
/// layout described in `self` would not be violated by performing a
/// `cast_type` cast from a pointer with address `addr` which refers to a
/// memory region of size `bytes_len`.
///
/// If the cast is valid, `validate_cast_and_convert_metadata` returns
/// `(elems, split_at)`. If `self` describes a dynamically-sized type, then
/// `elems` is the maximum number of trailing slice elements for which a
/// cast would be valid (for sized types, `elem` is meaningless and should
/// be ignored). `split_at` is the index at which to split the memory region
/// in order for the prefix (suffix) to contain the result of the cast, and
/// in order for the remaining suffix (prefix) to contain the leftover
/// bytes.
///
/// There are three conditions under which a cast can fail:
/// - The smallest possible value for the type is larger than the provided
/// memory region
/// - A prefix cast is requested, and `addr` does not satisfy `self`'s
/// alignment requirement
/// - A suffix cast is requested, and `addr + bytes_len` does not satisfy
/// `self`'s alignment requirement (as a consequence, since all instances
/// of the type are a multiple of its alignment, no size for the type will
/// result in a starting address which is properly aligned)
///
/// # Safety
///
/// The caller may assume that this implementation is correct, and may rely
/// on that assumption for the soundness of their code. In particular, the
/// caller may assume that, if `validate_cast_and_convert_metadata` returns
/// `Some((elems, split_at))`, then:
/// - A pointer to the type (for dynamically sized types, this includes
/// `elems` as its pointer metadata) describes an object of size `size <=
/// bytes_len`
/// - If this is a prefix cast:
/// - `addr` satisfies `self`'s alignment
/// - `size == split_at`
/// - If this is a suffix cast:
/// - `split_at == bytes_len - size`
/// - `addr + split_at` satisfies `self`'s alignment
///
/// Note that this method does *not* ensure that a pointer constructed from
/// its return values will be a valid pointer. In particular, this method
/// does not reason about `isize` overflow, which is a requirement of many
/// Rust pointer APIs, and may at some point be determined to be a validity
/// invariant of pointer types themselves. This should never be a problem so
/// long as the arguments to this method are derived from a known-valid
/// pointer (e.g., one derived from a safe Rust reference), but it is
/// nonetheless the caller's responsibility to justify that pointer
/// arithmetic will not overflow based on a safety argument *other than* the
/// mere fact that this method returned successfully.
///
/// # Panics
///
/// `validate_cast_and_convert_metadata` will panic if `self` describes a
/// DST whose trailing slice element is zero-sized.
///
/// If `addr + bytes_len` overflows `usize`,
/// `validate_cast_and_convert_metadata` may panic, or it may return
/// incorrect results. No guarantees are made about when
/// `validate_cast_and_convert_metadata` will panic. The caller should not
/// rely on `validate_cast_and_convert_metadata` panicking in any particular
/// condition, even if `debug_assertions` are enabled.
#[allow(unused)]
#[inline(always)]
pub(crate) const fn validate_cast_and_convert_metadata(
&self,
addr: usize,
bytes_len: usize,
cast_type: CastType,
) -> Result<(usize, usize), MetadataCastError> {
// `debug_assert!`, but with `#[allow(clippy::arithmetic_side_effects)]`.
macro_rules! __const_debug_assert {
($e:expr $(, $msg:expr)?) => {
const_debug_assert!({
#[allow(clippy::arithmetic_side_effects)]
let e = $e;
e
} $(, $msg)?);
};
}
// Note that, in practice, `self` is always a compile-time constant. We
// do this check earlier than needed to ensure that we always panic as a
// result of bugs in the program (such as calling this function on an
// invalid type) instead of allowing this panic to be hidden if the cast
// would have failed anyway for runtime reasons (such as a too-small
// memory region).
//
// FIXME(#67): Once our MSRV is 1.65, use let-else:
// https://blog.rust-lang.org/2022/11/03/Rust-1.65.0.html#let-else-statements
let size_info = match self.size_info.try_to_nonzero_elem_size() {
Some(size_info) => size_info,
None => const_panic!("attempted to cast to slice type with zero-sized element"),
};
// Precondition
__const_debug_assert!(
addr.checked_add(bytes_len).is_some(),
"`addr` + `bytes_len` > usize::MAX"
);
// Alignment checks go in their own block to avoid introducing variables
// into the top-level scope.
{
// We check alignment for `addr` (for prefix casts) or `addr +
// bytes_len` (for suffix casts). For a prefix cast, the correctness
// of this check is trivial - `addr` is the address the object will
// live at.
//
// For a suffix cast, we know that all valid sizes for the type are
// a multiple of the alignment (and by safety precondition, we know
// `DstLayout` may only describe valid Rust types). Thus, a
// validly-sized instance which lives at a validly-aligned address
// must also end at a validly-aligned address. Thus, if the end
// address for a suffix cast (`addr + bytes_len`) is not aligned,
// then no valid start address will be aligned either.
let offset = match cast_type {
CastType::Prefix => 0,
CastType::Suffix => bytes_len,
};
// Addition is guaranteed not to overflow because `offset <=
// bytes_len`, and `addr + bytes_len <= usize::MAX` is a
// precondition of this method. Modulus is guaranteed not to divide
// by 0 because `align` is non-zero.
#[allow(clippy::arithmetic_side_effects)]
if (addr + offset) % self.align.get() != 0 {
return Err(MetadataCastError::Alignment);
}
}
let (elems, self_bytes) = match size_info {
SizeInfo::Sized { size } => {
if size > bytes_len {
return Err(MetadataCastError::Size);
}
(0, size)
}
SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size }) => {
// Calculate the maximum number of bytes that could be consumed
// - any number of bytes larger than this will either not be a
// multiple of the alignment, or will be larger than
// `bytes_len`.
let max_total_bytes =
util::round_down_to_next_multiple_of_alignment(bytes_len, self.align);
// Calculate the maximum number of bytes that could be consumed
// by the trailing slice.
//
// FIXME(#67): Once our MSRV is 1.65, use let-else:
// https://blog.rust-lang.org/2022/11/03/Rust-1.65.0.html#let-else-statements
let max_slice_and_padding_bytes = match max_total_bytes.checked_sub(offset) {
Some(max) => max,
// `bytes_len` too small even for 0 trailing slice elements.
None => return Err(MetadataCastError::Size),
};
// Calculate the number of elements that fit in
// `max_slice_and_padding_bytes`; any remaining bytes will be
// considered padding.
//
// Guaranteed not to divide by zero: `elem_size` is non-zero.
#[allow(clippy::arithmetic_side_effects)]
let elems = max_slice_and_padding_bytes / elem_size.get();
// Guaranteed not to overflow on multiplication: `usize::MAX >=
// max_slice_and_padding_bytes >= (max_slice_and_padding_bytes /
// elem_size) * elem_size`.
//
// Guaranteed not to overflow on addition:
// - max_slice_and_padding_bytes == max_total_bytes - offset
// - elems * elem_size <= max_slice_and_padding_bytes == max_total_bytes - offset
// - elems * elem_size + offset <= max_total_bytes <= usize::MAX
#[allow(clippy::arithmetic_side_effects)]
let without_padding = offset + elems * elem_size.get();
// `self_bytes` is equal to the offset bytes plus the bytes
// consumed by the trailing slice plus any padding bytes
// required to satisfy the alignment. Note that we have computed
// the maximum number of trailing slice elements that could fit
// in `self_bytes`, so any padding is guaranteed to be less than
// the size of an extra element.
//
// Guaranteed not to overflow:
// - By previous comment: without_padding == elems * elem_size +
// offset <= max_total_bytes
// - By construction, `max_total_bytes` is a multiple of
// `self.align`.
// - At most, adding padding needed to round `without_padding`
// up to the next multiple of the alignment will bring
// `self_bytes` up to `max_total_bytes`.
#[allow(clippy::arithmetic_side_effects)]
let self_bytes =
without_padding + util::padding_needed_for(without_padding, self.align);
(elems, self_bytes)
}
};
__const_debug_assert!(self_bytes <= bytes_len);
let split_at = match cast_type {
CastType::Prefix => self_bytes,
// Guaranteed not to underflow:
// - In the `Sized` branch, only returns `size` if `size <=
// bytes_len`.
// - In the `SliceDst` branch, calculates `self_bytes <=
// max_toatl_bytes`, which is upper-bounded by `bytes_len`.
#[allow(clippy::arithmetic_side_effects)]
CastType::Suffix => bytes_len - self_bytes,
};
Ok((elems, split_at))
}
}
pub(crate) use cast_from::CastFrom;
mod cast_from {
use crate::*;
pub(crate) struct CastFrom<Dst: ?Sized> {
_never: core::convert::Infallible,
_marker: PhantomData<Dst>,
}
// SAFETY: The implementation of `Project::project` preserves the address
// of the referent – it only modifies pointer metadata.
unsafe impl<Src, Dst> crate::pointer::cast::Cast<Src, Dst> for CastFrom<Dst>
where
Src: KnownLayout + ?Sized,
Dst: KnownLayout + ?Sized,
{
}
// SAFETY: The implementation of `Project::project` preserves the size of
// the referent (see inline comments for a more detailed proof of this).
unsafe impl<Src, Dst> crate::pointer::cast::CastExact<Src, Dst> for CastFrom<Dst>
where
Src: KnownLayout + ?Sized,
Dst: KnownLayout + ?Sized,
{
}
// SAFETY: `project` produces a pointer which refers to the same referent
// bytes as its input, or to a subset of them (see inline comments for a
// more detailed proof of this). It does this using provenance-preserving
// operations.
unsafe impl<Src, Dst> crate::pointer::cast::Project<Src, Dst> for CastFrom<Dst>
where
Src: KnownLayout + ?Sized,
Dst: KnownLayout + ?Sized,
{
/// # PME
///
/// Generates a post-monomorphization error if it is not possible to
/// implement soundly.
//
// FIXME(#1817): Support Sized->Unsized and Unsized->Sized casts
fn project(src: PtrInner<'_, Src>) -> *mut Dst {
/// The parameters required in order to perform a pointer cast from
/// `Src` to `Dst`.
///
/// These are a compile-time function of the layouts of `Src`
/// and `Dst`.
///
/// # Safety
///
/// `Src`'s alignment must not be smaller than `Dst`'s alignment.
struct CastParams<Src: ?Sized, Dst: ?Sized> {
inner: CastParamsInner,
_src: PhantomData<Src>,
_dst: PhantomData<Dst>,
}
#[derive(Copy, Clone)]
enum CastParamsInner {
// At compile time (specifically, post-monomorphization time),
// we need to compute two things:
// - Whether, given *any* `*Src`, it is possible to construct a
// `*Dst` which addresses the same number of bytes (ie,
// whether, for any `Src` pointer metadata, there exists `Dst`
// pointer metadata that addresses the same number of bytes)
// - If this is possible, any information necessary to perform
// the `Src`->`Dst` metadata conversion at runtime.
//
// Assume that `Src` and `Dst` are slice DSTs, and define:
// - `S_OFF = Src::LAYOUT.size_info.offset`
// - `S_ELEM = Src::LAYOUT.size_info.elem_size`
// - `D_OFF = Dst::LAYOUT.size_info.offset`
// - `D_ELEM = Dst::LAYOUT.size_info.elem_size`
//
// We are trying to solve the following equation:
//
// D_OFF + d_meta * D_ELEM = S_OFF + s_meta * S_ELEM
//
// At runtime, we will be attempting to compute `d_meta`, given
// `s_meta` (a runtime value) and all other parameters (which
// are compile-time values). We can solve like so:
//
// D_OFF + d_meta * D_ELEM = S_OFF + s_meta * S_ELEM
//
// d_meta * D_ELEM = S_OFF - D_OFF + s_meta * S_ELEM
//
// d_meta = (S_OFF - D_OFF + s_meta * S_ELEM)/D_ELEM
//
// Since `d_meta` will be a `usize`, we need the right-hand side
// to be an integer, and this needs to hold for *any* value of
// `s_meta` (in order for our conversion to be infallible - ie,
// to not have to reject certain values of `s_meta` at runtime).
// This means that:
//
// - `s_meta * S_ELEM` must be a multiple of `D_ELEM`
// - Since this must hold for any value of `s_meta`, `S_ELEM`
// must be a multiple of `D_ELEM`
// - `S_OFF - D_OFF` must be a multiple of `D_ELEM`
//
// Thus, let `OFFSET_DELTA_ELEMS = (S_OFF - D_OFF)/D_ELEM` and
// `ELEM_MULTIPLE = S_ELEM/D_ELEM`. We can rewrite the above
// expression as:
//
// d_meta = (S_OFF - D_OFF + s_meta * S_ELEM)/D_ELEM
//
// d_meta = OFFSET_DELTA_ELEMS + s_meta * ELEM_MULTIPLE
//
// Thus, we just need to compute the following and confirm that
// they have integer solutions in order to both a) determine
// whether infallible `Src` -> `Dst` casts are possible and, b)
// pre-compute the parameters necessary to perform those casts
// at runtime. These parameters are encapsulated in
// `CastParams`, which acts as a witness that such infallible
// casts are possible.
/// The parameters required in order to perform an
/// unsized-to-unsized pointer cast from `Src` to `Dst` as
/// described above.
///
/// # Safety
///
/// `Src` and `Dst` must both be slice DSTs.
///
/// `offset_delta_elems` and `elem_multiple` must be valid as
/// described above.
UnsizedToUnsized { offset_delta_elems: usize, elem_multiple: usize },
/// The metadata of a `Dst` which has the same size as `Src:
/// Sized`.
///
/// # Safety
///
/// `Src: Sized` and `Dst` must be a slice DST.
///
/// A raw `Dst` pointer with metadata `dst_meta` must address
/// `size_of::<Src>()` bytes.
SizedToUnsized { dst_meta: usize },
/// The metadata of a `Dst` which has the same size as `Src:
/// Sized`.
///
/// # Safety
///
/// `Src` and `Dst` must both be `Sized` and `size_of::<Src>()
/// == size_of::<Dst>()`.
SizedToSized,
}
impl<Src: ?Sized, Dst: ?Sized> Copy for CastParams<Src, Dst> {}
impl<Src: ?Sized, Dst: ?Sized> Clone for CastParams<Src, Dst> {
fn clone(&self) -> Self {
*self
}
}
impl<Src: ?Sized, Dst: ?Sized> CastParams<Src, Dst> {
const fn try_compute(
src: &DstLayout,
dst: &DstLayout,
) -> Option<CastParams<Src, Dst>> {
if src.align.get() < dst.align.get() {
return None;
}
let inner = match (src.size_info, dst.size_info) {
(
SizeInfo::Sized { size: src_size },
SizeInfo::Sized { size: dst_size },
) => {
if src_size != dst_size {
return None;
}
// SAFETY: We checked above that `src_size ==
// dst_size`.
CastParamsInner::SizedToSized
}
(SizeInfo::Sized { size: src_size }, SizeInfo::SliceDst(dst)) => {
let offset_delta = if let Some(od) = src_size.checked_sub(dst.offset) {
od
} else {
return None;
};
let dst_elem_size = if let Some(e) = NonZeroUsize::new(dst.elem_size) {
e
} else {
return None;
};
// PANICS: `dst_elem_size: NonZeroUsize`, so this won't
// divide by zero.
#[allow(clippy::arithmetic_side_effects)]
let delta_mod_other_elem = offset_delta % dst_elem_size.get();
if delta_mod_other_elem != 0 {
return None;
}
// PANICS: `dst_elem_size: NonZeroUsize`, so this won't
// divide by zero.
#[allow(clippy::arithmetic_side_effects)]
let dst_meta = offset_delta / dst_elem_size.get();
// SAFETY: The preceding math ensures that a `Dst`
// with `dst_meta` addresses `src_size` bytes.
CastParamsInner::SizedToUnsized { dst_meta }
}
(SizeInfo::SliceDst(src), SizeInfo::SliceDst(dst)) => {
let offset_delta = if let Some(od) = src.offset.checked_sub(dst.offset)
{
od
} else {
return None;
};
let dst_elem_size = if let Some(e) = NonZeroUsize::new(dst.elem_size) {
e
} else {
return None;
};
// PANICS: `dst_elem_size: NonZeroUsize`, so this won't
// divide by zero.
#[allow(clippy::arithmetic_side_effects)]
let delta_mod_other_elem = offset_delta % dst_elem_size.get();
// PANICS: `dst_elem_size: NonZeroUsize`, so this won't
// divide by zero.
#[allow(clippy::arithmetic_side_effects)]
let elem_remainder = src.elem_size % dst_elem_size.get();
if delta_mod_other_elem != 0
|| src.elem_size < dst.elem_size
|| elem_remainder != 0
{
return None;
}
// PANICS: `dst_elem_size: NonZeroUsize`, so this won't
// divide by zero.
#[allow(clippy::arithmetic_side_effects)]
let offset_delta_elems = offset_delta / dst_elem_size.get();
// PANICS: `dst_elem_size: NonZeroUsize`, so this won't
// divide by zero.
#[allow(clippy::arithmetic_side_effects)]
let elem_multiple = src.elem_size / dst_elem_size.get();
CastParamsInner::UnsizedToUnsized {
// SAFETY: We checked above that this is an exact ratio.
offset_delta_elems,
// SAFETY: We checked above that this is an exact ratio.
elem_multiple,
}
}
_ => return None,
};
// SAFETY: We checked above that `src.align >= dst.align`.
Some(CastParams { inner, _src: PhantomData, _dst: PhantomData })
}
}
impl<Src: KnownLayout + ?Sized, Dst: KnownLayout + ?Sized> CastParams<Src, Dst> {
/// # Safety
///
/// `src_meta` describes a `Src` whose size is no larger than
/// `isize::MAX`.
///
/// The returned metadata describes a `Dst` of the same size as
/// the original `Src`.
#[inline(always)]
unsafe fn cast_metadata(
self,
src_meta: Src::PointerMetadata,
) -> Dst::PointerMetadata {
#[allow(unused)]
use crate::util::polyfills::*;
let dst_meta = match self.inner {
CastParamsInner::UnsizedToUnsized { offset_delta_elems, elem_multiple } => {
let src_meta = src_meta.to_elem_count();
#[allow(
unstable_name_collisions,
clippy::multiple_unsafe_ops_per_block
)]
// SAFETY: `self` is a witness that the following
// equation holds:
//
// D_OFF + d_meta * D_ELEM = S_OFF + s_meta * S_ELEM
//
// Since the caller promises that `src_meta` is
// valid `Src` metadata, this math will not
// overflow, and the returned value will describe a
// `Dst` of the same size.
unsafe {
offset_delta_elems
.unchecked_add(src_meta.unchecked_mul(elem_multiple))
}
}
CastParamsInner::SizedToUnsized { dst_meta } => dst_meta,
CastParamsInner::SizedToSized => 0,
};
Dst::PointerMetadata::from_elem_count(dst_meta)
}
}
trait Params<Src: ?Sized> {
const CAST_PARAMS: CastParams<Src, Self>;
}
impl<Src, Dst> Params<Src> for Dst
where
Src: KnownLayout + ?Sized,
Dst: KnownLayout + ?Sized,
{
const CAST_PARAMS: CastParams<Src, Dst> =
match CastParams::try_compute(&Src::LAYOUT, &Dst::LAYOUT) {
Some(params) => params,
None => const_panic!(
"cannot `transmute_ref!` or `transmute_mut!` between incompatible types"
),
};
}
let src_meta = <Src as KnownLayout>::pointer_to_metadata(src.as_ptr());
let params = <Dst as Params<Src>>::CAST_PARAMS;
// SAFETY: `src: PtrInner` guarantees that `src`'s referent is zero
// bytes or lives in a single allocation, which means that it is no
// larger than `isize::MAX` bytes [1].
//
// [1] https://doc.rust-lang.org/1.92.0/std/ptr/index.html#allocation
let dst_meta = unsafe { params.cast_metadata(src_meta) };
<Dst as KnownLayout>::raw_from_ptr_len(src.as_non_null().cast(), dst_meta).as_ptr()
}
}
}
// FIXME(#67): For some reason, on our MSRV toolchain, this `allow` isn't
// enforced despite having `#![allow(unknown_lints)]` at the crate root, but
// putting it here works. Once our MSRV is high enough that this bug has been
// fixed, remove this `allow`.
#[allow(unknown_lints)]
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_dst_layout_for_slice() {
let layout = DstLayout::for_slice::<u32>();
match layout.size_info {
SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size }) => {
assert_eq!(offset, 0);
assert_eq!(elem_size, 4);
}
_ => panic!("Expected SliceDst"),
}
assert_eq!(layout.align.get(), 4);
}
/// Tests of when a sized `DstLayout` is extended with a sized field.
#[allow(clippy::decimal_literal_representation)]
#[test]
fn test_dst_layout_extend_sized_with_sized() {
// This macro constructs a layout corresponding to a `u8` and extends it
// with a zero-sized trailing field of given alignment `n`. The macro
// tests that the resulting layout has both size and alignment `min(n,
// P)` for all valid values of `repr(packed(P))`.
macro_rules! test_align_is_size {
($n:expr) => {
let base = DstLayout::for_type::<u8>();
let trailing_field = DstLayout::for_type::<elain::Align<$n>>();
let packs =
core::iter::once(None).chain((0..29).map(|p| NonZeroUsize::new(2usize.pow(p))));
for pack in packs {
let composite = base.extend(trailing_field, pack);
let max_align = pack.unwrap_or(DstLayout::CURRENT_MAX_ALIGN);
let align = $n.min(max_align.get());
assert_eq!(
composite,
DstLayout {
align: NonZeroUsize::new(align).unwrap(),
size_info: SizeInfo::Sized { size: align },
statically_shallow_unpadded: false,
}
)
}
};
}
test_align_is_size!(1);
test_align_is_size!(2);
test_align_is_size!(4);
test_align_is_size!(8);
test_align_is_size!(16);
test_align_is_size!(32);
test_align_is_size!(64);
test_align_is_size!(128);
test_align_is_size!(256);
test_align_is_size!(512);
test_align_is_size!(1024);
test_align_is_size!(2048);
test_align_is_size!(4096);
test_align_is_size!(8192);
test_align_is_size!(16384);
test_align_is_size!(32768);
test_align_is_size!(65536);
test_align_is_size!(131072);
test_align_is_size!(262144);
test_align_is_size!(524288);
test_align_is_size!(1048576);
test_align_is_size!(2097152);
test_align_is_size!(4194304);
test_align_is_size!(8388608);
test_align_is_size!(16777216);
test_align_is_size!(33554432);
test_align_is_size!(67108864);
test_align_is_size!(33554432);
test_align_is_size!(134217728);
test_align_is_size!(268435456);
}
/// Tests of when a sized `DstLayout` is extended with a DST field.
#[test]
fn test_dst_layout_extend_sized_with_dst() {
// Test that for all combinations of real-world alignments and
// `repr_packed` values, that the extension of a sized `DstLayout`` with
// a DST field correctly computes the trailing offset in the composite
// layout.
let aligns = (0..29).map(|p| NonZeroUsize::new(2usize.pow(p)).unwrap());
let packs = core::iter::once(None).chain(aligns.clone().map(Some));
for align in aligns {
for pack in packs.clone() {
let base = DstLayout::for_type::<u8>();
let elem_size = 42;
let trailing_field_offset = 11;
let trailing_field = DstLayout {
align,
size_info: SizeInfo::SliceDst(TrailingSliceLayout { elem_size, offset: 11 }),
statically_shallow_unpadded: false,
};
let composite = base.extend(trailing_field, pack);
let max_align = pack.unwrap_or(DstLayout::CURRENT_MAX_ALIGN).get();
let align = align.get().min(max_align);
assert_eq!(
composite,
DstLayout {
align: NonZeroUsize::new(align).unwrap(),
size_info: SizeInfo::SliceDst(TrailingSliceLayout {
elem_size,
offset: align + trailing_field_offset,
}),
statically_shallow_unpadded: false,
}
)
}
}
}
/// Tests that calling `pad_to_align` on a sized `DstLayout` adds the
/// expected amount of trailing padding.
#[test]
fn test_dst_layout_pad_to_align_with_sized() {
// For all valid alignments `align`, construct a one-byte layout aligned
// to `align`, call `pad_to_align`, and assert that the size of the
// resulting layout is equal to `align`.
for align in (0..29).map(|p| NonZeroUsize::new(2usize.pow(p)).unwrap()) {
let layout = DstLayout {
align,
size_info: SizeInfo::Sized { size: 1 },
statically_shallow_unpadded: true,
};
assert_eq!(
layout.pad_to_align(),
DstLayout {
align,
size_info: SizeInfo::Sized { size: align.get() },
statically_shallow_unpadded: align.get() == 1
}
);
}
// Test explicitly-provided combinations of unpadded and padded
// counterparts.
macro_rules! test {
(unpadded { size: $unpadded_size:expr, align: $unpadded_align:expr }
=> padded { size: $padded_size:expr, align: $padded_align:expr }) => {
let unpadded = DstLayout {
align: NonZeroUsize::new($unpadded_align).unwrap(),
size_info: SizeInfo::Sized { size: $unpadded_size },
statically_shallow_unpadded: false,
};
let padded = unpadded.pad_to_align();
assert_eq!(
padded,
DstLayout {
align: NonZeroUsize::new($padded_align).unwrap(),
size_info: SizeInfo::Sized { size: $padded_size },
statically_shallow_unpadded: false,
}
);
};
}
test!(unpadded { size: 0, align: 4 } => padded { size: 0, align: 4 });
test!(unpadded { size: 1, align: 4 } => padded { size: 4, align: 4 });
test!(unpadded { size: 2, align: 4 } => padded { size: 4, align: 4 });
test!(unpadded { size: 3, align: 4 } => padded { size: 4, align: 4 });
test!(unpadded { size: 4, align: 4 } => padded { size: 4, align: 4 });
test!(unpadded { size: 5, align: 4 } => padded { size: 8, align: 4 });
test!(unpadded { size: 6, align: 4 } => padded { size: 8, align: 4 });
test!(unpadded { size: 7, align: 4 } => padded { size: 8, align: 4 });
test!(unpadded { size: 8, align: 4 } => padded { size: 8, align: 4 });
let current_max_align = DstLayout::CURRENT_MAX_ALIGN.get();
test!(unpadded { size: 1, align: current_max_align }
=> padded { size: current_max_align, align: current_max_align });
test!(unpadded { size: current_max_align + 1, align: current_max_align }
=> padded { size: current_max_align * 2, align: current_max_align });
}
/// Tests that calling `pad_to_align` on a DST `DstLayout` is a no-op.
#[test]
fn test_dst_layout_pad_to_align_with_dst() {
for align in (0..29).map(|p| NonZeroUsize::new(2usize.pow(p)).unwrap()) {
for offset in 0..10 {
for elem_size in 0..10 {
let layout = DstLayout {
align,
size_info: SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size }),
statically_shallow_unpadded: false,
};
assert_eq!(layout.pad_to_align(), layout);
}
}
}
}
// This test takes a long time when running under Miri, so we skip it in
// that case. This is acceptable because this is a logic test that doesn't
// attempt to expose UB.
#[test]
#[cfg_attr(miri, ignore)]
fn test_validate_cast_and_convert_metadata() {
#[allow(non_local_definitions)]
impl From<usize> for SizeInfo {
fn from(size: usize) -> SizeInfo {
SizeInfo::Sized { size }
}
}
#[allow(non_local_definitions)]
impl From<(usize, usize)> for SizeInfo {
fn from((offset, elem_size): (usize, usize)) -> SizeInfo {
SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size })
}
}
fn layout<S: Into<SizeInfo>>(s: S, align: usize) -> DstLayout {
DstLayout {
size_info: s.into(),
align: NonZeroUsize::new(align).unwrap(),
statically_shallow_unpadded: false,
}
}
/// This macro accepts arguments in the form of:
///
/// layout(_, _).validate(_, _, _), Ok(Some((_, _)))
/// | | | | | | |
/// size ---------+ | | | | | |
/// align -----------+ | | | | |
/// addr ------------------------+ | | | |
/// bytes_len ----------------------+ | | |
/// cast_type -------------------------+ | |
/// elems ------------------------------------------+ |
/// split_at ------------------------------------------+
///
/// `.validate` is shorthand for `.validate_cast_and_convert_metadata`
/// for brevity.
///
/// Each argument can either be an iterator or a wildcard. Each
/// wildcarded variable is implicitly replaced by an iterator over a
/// representative sample of values for that variable. Each `test!`
/// invocation iterates over every combination of values provided by
/// each variable's iterator (ie, the cartesian product) and validates
/// that the results are expected.
///
/// The final argument uses the same syntax, but it has a different
/// meaning:
/// - If it is `Ok(pat)`, then the pattern `pat` is supplied to
/// a matching assert to validate the computed result for each
/// combination of input values.
/// - If it is `Err(Some(msg) | None)`, then `test!` validates that the
/// call to `validate_cast_and_convert_metadata` panics with the given
/// panic message or, if the current Rust toolchain version is too
/// early to support panicking in `const fn`s, panics with *some*
/// message. In the latter case, the `const_panic!` macro is used,
/// which emits code which causes a non-panicking error at const eval
/// time, but which does panic when invoked at runtime. Thus, it is
/// merely difficult to predict the *value* of this panic. We deem
/// that testing against the real panic strings on stable and nightly
/// toolchains is enough to ensure correctness.
///
/// Note that the meta-variables that match these variables have the
/// `tt` type, and some valid expressions are not valid `tt`s (such as
/// `a..b`). In this case, wrap the expression in parentheses, and it
/// will become valid `tt`.
macro_rules! test {
(
layout($size:tt, $align:tt)
.validate($addr:tt, $bytes_len:tt, $cast_type:tt), $expect:pat $(,)?
) => {
itertools::iproduct!(
test!(@generate_size $size),
test!(@generate_align $align),
test!(@generate_usize $addr),
test!(@generate_usize $bytes_len),
test!(@generate_cast_type $cast_type)
).for_each(|(size_info, align, addr, bytes_len, cast_type)| {
// Temporarily disable the panic hook installed by the test
// harness. If we don't do this, all panic messages will be
// kept in an internal log. On its own, this isn't a
// problem, but if a non-caught panic ever happens (ie, in
// code later in this test not in this macro), all of the
// previously-buffered messages will be dumped, hiding the
// real culprit.
let previous_hook = std::panic::take_hook();
// I don't understand why, but this seems to be required in
// addition to the previous line.
std::panic::set_hook(Box::new(|_| {}));
let actual = std::panic::catch_unwind(|| {
layout(size_info, align).validate_cast_and_convert_metadata(addr, bytes_len, cast_type)
}).map_err(|d| {
let msg = d.downcast::<&'static str>().ok().map(|s| *s.as_ref());
assert!(msg.is_some() || cfg!(no_zerocopy_panic_in_const_and_vec_try_reserve_1_57_0), "non-string panic messages are not permitted when usage of panic in const fn is enabled");
msg
});
std::panic::set_hook(previous_hook);
assert!(
matches!(actual, $expect),
"layout({:?}, {}).validate_cast_and_convert_metadata({}, {}, {:?})" ,size_info, align, addr, bytes_len, cast_type
);
});
};
(@generate_usize _) => { 0..8 };
// Generate sizes for both Sized and !Sized types.
(@generate_size _) => {
test!(@generate_size (_)).chain(test!(@generate_size (_, _)))
};
// Generate sizes for both Sized and !Sized types by chaining
// specified iterators for each.
(@generate_size ($sized_sizes:tt | $unsized_sizes:tt)) => {
test!(@generate_size ($sized_sizes)).chain(test!(@generate_size $unsized_sizes))
};
// Generate sizes for Sized types.
(@generate_size (_)) => { test!(@generate_size (0..8)) };
(@generate_size ($sizes:expr)) => { $sizes.into_iter().map(Into::<SizeInfo>::into) };
// Generate sizes for !Sized types.
(@generate_size ($min_sizes:tt, $elem_sizes:tt)) => {
itertools::iproduct!(
test!(@generate_min_size $min_sizes),
test!(@generate_elem_size $elem_sizes)
).map(Into::<SizeInfo>::into)
};
(@generate_fixed_size _) => { (0..8).into_iter().map(Into::<SizeInfo>::into) };
(@generate_min_size _) => { 0..8 };
(@generate_elem_size _) => { 1..8 };
(@generate_align _) => { [1, 2, 4, 8, 16] };
(@generate_opt_usize _) => { [None].into_iter().chain((0..8).map(Some).into_iter()) };
(@generate_cast_type _) => { [CastType::Prefix, CastType::Suffix] };
(@generate_cast_type $variant:ident) => { [CastType::$variant] };
// Some expressions need to be wrapped in parentheses in order to be
// valid `tt`s (required by the top match pattern). See the comment
// below for more details. This arm removes these parentheses to
// avoid generating an `unused_parens` warning.
(@$_:ident ($vals:expr)) => { $vals };
(@$_:ident $vals:expr) => { $vals };
}
const EVENS: [usize; 8] = [0, 2, 4, 6, 8, 10, 12, 14];
const ODDS: [usize; 8] = [1, 3, 5, 7, 9, 11, 13, 15];
// base_size is too big for the memory region.
test!(
layout(((1..8) | ((1..8), (1..8))), _).validate([0], [0], _),
Ok(Err(MetadataCastError::Size))
);
test!(
layout(((2..8) | ((2..8), (2..8))), _).validate([0], [1], Prefix),
Ok(Err(MetadataCastError::Size))
);
test!(
layout(((2..8) | ((2..8), (2..8))), _).validate([0x1000_0000 - 1], [1], Suffix),
Ok(Err(MetadataCastError::Size))
);
// addr is unaligned for prefix cast
test!(layout(_, [2]).validate(ODDS, _, Prefix), Ok(Err(MetadataCastError::Alignment)));
test!(layout(_, [2]).validate(ODDS, _, Prefix), Ok(Err(MetadataCastError::Alignment)));
// addr is aligned, but end of buffer is unaligned for suffix cast
test!(layout(_, [2]).validate(EVENS, ODDS, Suffix), Ok(Err(MetadataCastError::Alignment)));
test!(layout(_, [2]).validate(EVENS, ODDS, Suffix), Ok(Err(MetadataCastError::Alignment)));
// Unfortunately, these constants cannot easily be used in the
// implementation of `validate_cast_and_convert_metadata`, since
// `panic!` consumes a string literal, not an expression.
//
// It's important that these messages be in a separate module. If they
// were at the function's top level, we'd pass them to `test!` as, e.g.,
// `Err(TRAILING)`, which would run into a subtle Rust footgun - the
// `TRAILING` identifier would be treated as a pattern to match rather
// than a value to check for equality.
mod msgs {
pub(super) const TRAILING: &str =
"attempted to cast to slice type with zero-sized element";
pub(super) const OVERFLOW: &str = "`addr` + `bytes_len` > usize::MAX";
}
// casts with ZST trailing element types are unsupported
test!(layout((_, [0]), _).validate(_, _, _), Err(Some(msgs::TRAILING) | None),);
// addr + bytes_len must not overflow usize
test!(layout(_, _).validate([usize::MAX], (1..100), _), Err(Some(msgs::OVERFLOW) | None));
test!(layout(_, _).validate((1..100), [usize::MAX], _), Err(Some(msgs::OVERFLOW) | None));
test!(
layout(_, _).validate(
[usize::MAX / 2 + 1, usize::MAX],
[usize::MAX / 2 + 1, usize::MAX],
_
),
Err(Some(msgs::OVERFLOW) | None)
);
// Validates that `validate_cast_and_convert_metadata` satisfies its own
// documented safety postconditions, and also a few other properties
// that aren't documented but we want to guarantee anyway.
fn validate_behavior(
(layout, addr, bytes_len, cast_type): (DstLayout, usize, usize, CastType),
) {
if let Ok((elems, split_at)) =
layout.validate_cast_and_convert_metadata(addr, bytes_len, cast_type)
{
let (size_info, align) = (layout.size_info, layout.align);
let debug_str = format!(
"layout({:?}, {}).validate_cast_and_convert_metadata({}, {}, {:?}) => ({}, {})",
size_info, align, addr, bytes_len, cast_type, elems, split_at
);
// If this is a sized type (no trailing slice), then `elems` is
// meaningless, but in practice we set it to 0. Callers are not
// allowed to rely on this, but a lot of math is nicer if
// they're able to, and some callers might accidentally do that.
let sized = matches!(layout.size_info, SizeInfo::Sized { .. });
assert!(!(sized && elems != 0), "{}", debug_str);
let resulting_size = match layout.size_info {
SizeInfo::Sized { size } => size,
SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size }) => {
let padded_size = |elems| {
let without_padding = offset + elems * elem_size;
without_padding + util::padding_needed_for(without_padding, align)
};
let resulting_size = padded_size(elems);
// Test that `validate_cast_and_convert_metadata`
// computed the largest possible value that fits in the
// given range.
assert!(padded_size(elems + 1) > bytes_len, "{}", debug_str);
resulting_size
}
};
// Test safety postconditions guaranteed by
// `validate_cast_and_convert_metadata`.
assert!(resulting_size <= bytes_len, "{}", debug_str);
match cast_type {
CastType::Prefix => {
assert_eq!(addr % align, 0, "{}", debug_str);
assert_eq!(resulting_size, split_at, "{}", debug_str);
}
CastType::Suffix => {
assert_eq!(split_at, bytes_len - resulting_size, "{}", debug_str);
assert_eq!((addr + split_at) % align, 0, "{}", debug_str);
}
}
} else {
let min_size = match layout.size_info {
SizeInfo::Sized { size } => size,
SizeInfo::SliceDst(TrailingSliceLayout { offset, .. }) => {
offset + util::padding_needed_for(offset, layout.align)
}
};
// If a cast is invalid, it is either because...
// 1. there are insufficient bytes at the given region for type:
let insufficient_bytes = bytes_len < min_size;
// 2. performing the cast would misalign type:
let base = match cast_type {
CastType::Prefix => 0,
CastType::Suffix => bytes_len,
};
let misaligned = (base + addr) % layout.align != 0;
assert!(insufficient_bytes || misaligned);
}
}
let sizes = 0..8;
let elem_sizes = 1..8;
let size_infos = sizes
.clone()
.map(Into::<SizeInfo>::into)
.chain(itertools::iproduct!(sizes, elem_sizes).map(Into::<SizeInfo>::into));
let layouts = itertools::iproduct!(size_infos, [1, 2, 4, 8, 16, 32])
.filter(|(size_info, align)| !matches!(size_info, SizeInfo::Sized { size } if size % align != 0))
.map(|(size_info, align)| layout(size_info, align));
itertools::iproduct!(layouts, 0..8, 0..8, [CastType::Prefix, CastType::Suffix])
.for_each(validate_behavior);
}
#[test]
#[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS)]
fn test_validate_rust_layout() {
use core::{
convert::TryInto as _,
ptr::{self, NonNull},
};
use crate::util::testutil::*;
// This test synthesizes pointers with various metadata and uses Rust's
// built-in APIs to confirm that Rust makes decisions about type layout
// which are consistent with what we believe is guaranteed by the
// language. If this test fails, it doesn't just mean our code is wrong
// - it means we're misunderstanding the language's guarantees.
#[derive(Debug)]
struct MacroArgs {
offset: usize,
align: NonZeroUsize,
elem_size: Option<usize>,
}
/// # Safety
///
/// `test` promises to only call `addr_of_slice_field` on a `NonNull<T>`
/// which points to a valid `T`.
///
/// `with_elems` must produce a pointer which points to a valid `T`.
fn test<T: ?Sized, W: Fn(usize) -> NonNull<T>>(
args: MacroArgs,
with_elems: W,
addr_of_slice_field: Option<fn(NonNull<T>) -> NonNull<u8>>,
) {
let dst = args.elem_size.is_some();
let layout = {
let size_info = match args.elem_size {
Some(elem_size) => {
SizeInfo::SliceDst(TrailingSliceLayout { offset: args.offset, elem_size })
}
None => SizeInfo::Sized {
// Rust only supports types whose sizes are a multiple
// of their alignment. If the macro created a type like
// this:
//
// #[repr(C, align(2))]
// struct Foo([u8; 1]);
//
// ...then Rust will automatically round the type's size
// up to 2.
size: args.offset + util::padding_needed_for(args.offset, args.align),
},
};
DstLayout { size_info, align: args.align, statically_shallow_unpadded: false }
};
for elems in 0..128 {
let ptr = with_elems(elems);
if let Some(addr_of_slice_field) = addr_of_slice_field {
let slc_field_ptr = addr_of_slice_field(ptr).as_ptr();
// SAFETY: Both `slc_field_ptr` and `ptr` are pointers to
// the same valid Rust object.
// Work around https://github.com/rust-lang/rust-clippy/issues/12280
let offset: usize =
unsafe { slc_field_ptr.byte_offset_from(ptr.as_ptr()).try_into().unwrap() };
assert_eq!(offset, args.offset);
}
// SAFETY: `ptr` points to a valid `T`.
#[allow(clippy::multiple_unsafe_ops_per_block)]
let (size, align) = unsafe {
(mem::size_of_val_raw(ptr.as_ptr()), mem::align_of_val_raw(ptr.as_ptr()))
};
// Avoid expensive allocation when running under Miri.
let assert_msg = if !cfg!(miri) {
format!("\n{:?}\nsize:{}, align:{}", args, size, align)
} else {
String::new()
};
let without_padding =
args.offset + args.elem_size.map(|elem_size| elems * elem_size).unwrap_or(0);
assert!(size >= without_padding, "{}", assert_msg);
assert_eq!(align, args.align.get(), "{}", assert_msg);
// This encodes the most important part of the test: our
// understanding of how Rust determines the layout of repr(C)
// types. Sized repr(C) types are trivial, but DST types have
// some subtlety. Note that:
// - For sized types, `without_padding` is just the size of the
// type that we constructed for `Foo`. Since we may have
// requested a larger alignment, `Foo` may actually be larger
// than this, hence `padding_needed_for`.
// - For unsized types, `without_padding` is dynamically
// computed from the offset, the element size, and element
// count. We expect that the size of the object should be
// `offset + elem_size * elems` rounded up to the next
// alignment.
let expected_size =
without_padding + util::padding_needed_for(without_padding, args.align);
assert_eq!(expected_size, size, "{}", assert_msg);
// For zero-sized element types,
// `validate_cast_and_convert_metadata` just panics, so we skip
// testing those types.
if args.elem_size.map(|elem_size| elem_size > 0).unwrap_or(true) {
let addr = ptr.addr().get();
let (got_elems, got_split_at) = layout
.validate_cast_and_convert_metadata(addr, size, CastType::Prefix)
.unwrap();
// Avoid expensive allocation when running under Miri.
let assert_msg = if !cfg!(miri) {
format!(
"{}\nvalidate_cast_and_convert_metadata({}, {})",
assert_msg, addr, size,
)
} else {
String::new()
};
assert_eq!(got_split_at, size, "{}", assert_msg);
if dst {
assert!(got_elems >= elems, "{}", assert_msg);
if got_elems != elems {
// If `validate_cast_and_convert_metadata`
// returned more elements than `elems`, that
// means that `elems` is not the maximum number
// of elements that can fit in `size` - in other
// words, there is enough padding at the end of
// the value to fit at least one more element.
// If we use this metadata to synthesize a
// pointer, despite having a different element
// count, we still expect it to have the same
// size.
let got_ptr = with_elems(got_elems);
// SAFETY: `got_ptr` is a pointer to a valid `T`.
let size_of_got_ptr = unsafe { mem::size_of_val_raw(got_ptr.as_ptr()) };
assert_eq!(size_of_got_ptr, size, "{}", assert_msg);
}
} else {
// For sized casts, the returned element value is
// technically meaningless, and we don't guarantee any
// particular value. In practice, it's always zero.
assert_eq!(got_elems, 0, "{}", assert_msg)
}
}
}
}
macro_rules! validate_against_rust {
($offset:literal, $align:literal $(, $elem_size:literal)?) => {{
#[repr(C, align($align))]
struct Foo([u8; $offset]$(, [[u8; $elem_size]])?);
let args = MacroArgs {
offset: $offset,
align: $align.try_into().unwrap(),
elem_size: {
#[allow(unused)]
let ret = None::<usize>;
$(let ret = Some($elem_size);)?
ret
}
};
#[repr(C, align($align))]
struct FooAlign;
// Create an aligned buffer to use in order to synthesize
// pointers to `Foo`. We don't ever load values from these
// pointers - we just do arithmetic on them - so having a "real"
// block of memory as opposed to a validly-aligned-but-dangling
// pointer is only necessary to make Miri happy since we run it
// with "strict provenance" checking enabled.
let aligned_buf = Align::<_, FooAlign>::new([0u8; 1024]);
let with_elems = |elems| {
let slc = NonNull::slice_from_raw_parts(NonNull::from(&aligned_buf.t), elems);
#[allow(clippy::as_conversions)]
NonNull::new(slc.as_ptr() as *mut Foo).unwrap()
};
let addr_of_slice_field = {
#[allow(unused)]
let f = None::<fn(NonNull<Foo>) -> NonNull<u8>>;
$(
// SAFETY: `test` promises to only call `f` with a `ptr`
// to a valid `Foo`.
let f: Option<fn(NonNull<Foo>) -> NonNull<u8>> = Some(|ptr: NonNull<Foo>| unsafe {
NonNull::new(ptr::addr_of_mut!((*ptr.as_ptr()).1)).unwrap().cast::<u8>()
});
let _ = $elem_size;
)?
f
};
test::<Foo, _>(args, with_elems, addr_of_slice_field);
}};
}
// Every permutation of:
// - offset in [0, 4]
// - align in [1, 16]
// - elem_size in [0, 4] (plus no elem_size)
validate_against_rust!(0, 1);
validate_against_rust!(0, 1, 0);
validate_against_rust!(0, 1, 1);
validate_against_rust!(0, 1, 2);
validate_against_rust!(0, 1, 3);
validate_against_rust!(0, 1, 4);
validate_against_rust!(0, 2);
validate_against_rust!(0, 2, 0);
validate_against_rust!(0, 2, 1);
validate_against_rust!(0, 2, 2);
validate_against_rust!(0, 2, 3);
validate_against_rust!(0, 2, 4);
validate_against_rust!(0, 4);
validate_against_rust!(0, 4, 0);
validate_against_rust!(0, 4, 1);
validate_against_rust!(0, 4, 2);
validate_against_rust!(0, 4, 3);
validate_against_rust!(0, 4, 4);
validate_against_rust!(0, 8);
validate_against_rust!(0, 8, 0);
validate_against_rust!(0, 8, 1);
validate_against_rust!(0, 8, 2);
validate_against_rust!(0, 8, 3);
validate_against_rust!(0, 8, 4);
validate_against_rust!(0, 16);
validate_against_rust!(0, 16, 0);
validate_against_rust!(0, 16, 1);
validate_against_rust!(0, 16, 2);
validate_against_rust!(0, 16, 3);
validate_against_rust!(0, 16, 4);
validate_against_rust!(1, 1);
validate_against_rust!(1, 1, 0);
validate_against_rust!(1, 1, 1);
validate_against_rust!(1, 1, 2);
validate_against_rust!(1, 1, 3);
validate_against_rust!(1, 1, 4);
validate_against_rust!(1, 2);
validate_against_rust!(1, 2, 0);
validate_against_rust!(1, 2, 1);
validate_against_rust!(1, 2, 2);
validate_against_rust!(1, 2, 3);
validate_against_rust!(1, 2, 4);
validate_against_rust!(1, 4);
validate_against_rust!(1, 4, 0);
validate_against_rust!(1, 4, 1);
validate_against_rust!(1, 4, 2);
validate_against_rust!(1, 4, 3);
validate_against_rust!(1, 4, 4);
validate_against_rust!(1, 8);
validate_against_rust!(1, 8, 0);
validate_against_rust!(1, 8, 1);
validate_against_rust!(1, 8, 2);
validate_against_rust!(1, 8, 3);
validate_against_rust!(1, 8, 4);
validate_against_rust!(1, 16);
validate_against_rust!(1, 16, 0);
validate_against_rust!(1, 16, 1);
validate_against_rust!(1, 16, 2);
validate_against_rust!(1, 16, 3);
validate_against_rust!(1, 16, 4);
validate_against_rust!(2, 1);
validate_against_rust!(2, 1, 0);
validate_against_rust!(2, 1, 1);
validate_against_rust!(2, 1, 2);
validate_against_rust!(2, 1, 3);
validate_against_rust!(2, 1, 4);
validate_against_rust!(2, 2);
validate_against_rust!(2, 2, 0);
validate_against_rust!(2, 2, 1);
validate_against_rust!(2, 2, 2);
validate_against_rust!(2, 2, 3);
validate_against_rust!(2, 2, 4);
validate_against_rust!(2, 4);
validate_against_rust!(2, 4, 0);
validate_against_rust!(2, 4, 1);
validate_against_rust!(2, 4, 2);
validate_against_rust!(2, 4, 3);
validate_against_rust!(2, 4, 4);
validate_against_rust!(2, 8);
validate_against_rust!(2, 8, 0);
validate_against_rust!(2, 8, 1);
validate_against_rust!(2, 8, 2);
validate_against_rust!(2, 8, 3);
validate_against_rust!(2, 8, 4);
validate_against_rust!(2, 16);
validate_against_rust!(2, 16, 0);
validate_against_rust!(2, 16, 1);
validate_against_rust!(2, 16, 2);
validate_against_rust!(2, 16, 3);
validate_against_rust!(2, 16, 4);
validate_against_rust!(3, 1);
validate_against_rust!(3, 1, 0);
validate_against_rust!(3, 1, 1);
validate_against_rust!(3, 1, 2);
validate_against_rust!(3, 1, 3);
validate_against_rust!(3, 1, 4);
validate_against_rust!(3, 2);
validate_against_rust!(3, 2, 0);
validate_against_rust!(3, 2, 1);
validate_against_rust!(3, 2, 2);
validate_against_rust!(3, 2, 3);
validate_against_rust!(3, 2, 4);
validate_against_rust!(3, 4);
validate_against_rust!(3, 4, 0);
validate_against_rust!(3, 4, 1);
validate_against_rust!(3, 4, 2);
validate_against_rust!(3, 4, 3);
validate_against_rust!(3, 4, 4);
validate_against_rust!(3, 8);
validate_against_rust!(3, 8, 0);
validate_against_rust!(3, 8, 1);
validate_against_rust!(3, 8, 2);
validate_against_rust!(3, 8, 3);
validate_against_rust!(3, 8, 4);
validate_against_rust!(3, 16);
validate_against_rust!(3, 16, 0);
validate_against_rust!(3, 16, 1);
validate_against_rust!(3, 16, 2);
validate_against_rust!(3, 16, 3);
validate_against_rust!(3, 16, 4);
validate_against_rust!(4, 1);
validate_against_rust!(4, 1, 0);
validate_against_rust!(4, 1, 1);
validate_against_rust!(4, 1, 2);
validate_against_rust!(4, 1, 3);
validate_against_rust!(4, 1, 4);
validate_against_rust!(4, 2);
validate_against_rust!(4, 2, 0);
validate_against_rust!(4, 2, 1);
validate_against_rust!(4, 2, 2);
validate_against_rust!(4, 2, 3);
validate_against_rust!(4, 2, 4);
validate_against_rust!(4, 4);
validate_against_rust!(4, 4, 0);
validate_against_rust!(4, 4, 1);
validate_against_rust!(4, 4, 2);
validate_against_rust!(4, 4, 3);
validate_against_rust!(4, 4, 4);
validate_against_rust!(4, 8);
validate_against_rust!(4, 8, 0);
validate_against_rust!(4, 8, 1);
validate_against_rust!(4, 8, 2);
validate_against_rust!(4, 8, 3);
validate_against_rust!(4, 8, 4);
validate_against_rust!(4, 16);
validate_against_rust!(4, 16, 0);
validate_against_rust!(4, 16, 1);
validate_against_rust!(4, 16, 2);
validate_against_rust!(4, 16, 3);
validate_against_rust!(4, 16, 4);
}
}
#[cfg(kani)]
mod proofs {
use core::alloc::Layout;
use super::*;
impl kani::Arbitrary for DstLayout {
fn any() -> Self {
let align: NonZeroUsize = kani::any();
let size_info: SizeInfo = kani::any();
kani::assume(align.is_power_of_two());
kani::assume(align < DstLayout::THEORETICAL_MAX_ALIGN);
// For testing purposes, we most care about instantiations of
// `DstLayout` that can correspond to actual Rust types. We use
// `Layout` to verify that our `DstLayout` satisfies the validity
// conditions of Rust layouts.
kani::assume(
match size_info {
SizeInfo::Sized { size } => Layout::from_size_align(size, align.get()),
SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size: _ }) => {
// `SliceDst` cannot encode an exact size, but we know
// it is at least `offset` bytes.
Layout::from_size_align(offset, align.get())
}
}
.is_ok(),
);
Self { align: align, size_info: size_info, statically_shallow_unpadded: kani::any() }
}
}
impl kani::Arbitrary for SizeInfo {
fn any() -> Self {
let is_sized: bool = kani::any();
match is_sized {
true => {
let size: usize = kani::any();
kani::assume(size <= DstLayout::MAX_SIZE);
SizeInfo::Sized { size }
}
false => SizeInfo::SliceDst(kani::any()),
}
}
}
impl kani::Arbitrary for TrailingSliceLayout {
fn any() -> Self {
let elem_size: usize = kani::any();
let offset: usize = kani::any();
kani::assume(elem_size < DstLayout::MAX_SIZE);
kani::assume(offset < DstLayout::MAX_SIZE);
TrailingSliceLayout { elem_size, offset }
}
}
#[kani::proof]
fn prove_requires_dynamic_padding() {
let layout: DstLayout = kani::any();
let SizeInfo::SliceDst(size_info) = layout.size_info else {
kani::assume(false);
loop {}
};
let meta: usize = kani::any();
let Some(trailing_slice_size) = size_info.elem_size.checked_mul(meta) else {
// The `trailing_slice_size` exceeds `usize::MAX`; `meta` is invalid.
kani::assume(false);
loop {}
};
let Some(unpadded_size) = size_info.offset.checked_add(trailing_slice_size) else {
// The `unpadded_size` exceeds `usize::MAX`; `meta`` is invalid.
kani::assume(false);
loop {}
};
if unpadded_size >= DstLayout::MAX_SIZE {
// The `unpadded_size` exceeds `isize::MAX`; `meta` is invalid.
kani::assume(false);
loop {}
}
let trailing_padding = util::padding_needed_for(unpadded_size, layout.align);
if !layout.requires_dynamic_padding() {
assert!(trailing_padding == 0);
}
}
#[kani::proof]
fn prove_dst_layout_extend() {
use crate::util::{max, min, padding_needed_for};
let base: DstLayout = kani::any();
let field: DstLayout = kani::any();
let packed: Option<NonZeroUsize> = kani::any();
if let Some(max_align) = packed {
kani::assume(max_align.is_power_of_two());
kani::assume(base.align <= max_align);
}
// The base can only be extended if it's sized.
kani::assume(matches!(base.size_info, SizeInfo::Sized { .. }));
let base_size = if let SizeInfo::Sized { size } = base.size_info {
size
} else {
unreachable!();
};
// Under the above conditions, `DstLayout::extend` will not panic.
let composite = base.extend(field, packed);
// The field's alignment is clamped by `max_align` (i.e., the
// `packed` attribute, if any) [1].
//
// [1] Per https://doc.rust-lang.org/reference/type-layout.html#the-alignment-modifiers:
//
// The alignments of each field, for the purpose of positioning
// fields, is the smaller of the specified alignment and the
// alignment of the field's type.
let field_align = min(field.align, packed.unwrap_or(DstLayout::THEORETICAL_MAX_ALIGN));
// The struct's alignment is the maximum of its previous alignment and
// `field_align`.
assert_eq!(composite.align, max(base.align, field_align));
// Compute the minimum amount of inter-field padding needed to
// satisfy the field's alignment, and offset of the trailing field.
// [1]
//
// [1] Per https://doc.rust-lang.org/reference/type-layout.html#the-alignment-modifiers:
//
// Inter-field padding is guaranteed to be the minimum required in
// order to satisfy each field's (possibly altered) alignment.
let padding = padding_needed_for(base_size, field_align);
let offset = base_size + padding;
// For testing purposes, we'll also construct `alloc::Layout`
// stand-ins for `DstLayout`, and show that `extend` behaves
// comparably on both types.
let base_analog = Layout::from_size_align(base_size, base.align.get()).unwrap();
match field.size_info {
SizeInfo::Sized { size: field_size } => {
if let SizeInfo::Sized { size: composite_size } = composite.size_info {
// If the trailing field is sized, the resulting layout will
// be sized. Its size will be the sum of the preceding
// layout, the size of the new field, and the size of
// inter-field padding between the two.
assert_eq!(composite_size, offset + field_size);
let field_analog =
Layout::from_size_align(field_size, field_align.get()).unwrap();
if let Ok((actual_composite, actual_offset)) = base_analog.extend(field_analog)
{
assert_eq!(actual_offset, offset);
assert_eq!(actual_composite.size(), composite_size);
assert_eq!(actual_composite.align(), composite.align.get());
} else {
// An error here reflects that composite of `base`
// and `field` cannot correspond to a real Rust type
// fragment, because such a fragment would violate
// the basic invariants of a valid Rust layout. At
// the time of writing, `DstLayout` is a little more
// permissive than `Layout`, so we don't assert
// anything in this branch (e.g., unreachability).
}
} else {
panic!("The composite of two sized layouts must be sized.")
}
}
SizeInfo::SliceDst(TrailingSliceLayout {
offset: field_offset,
elem_size: field_elem_size,
}) => {
if let SizeInfo::SliceDst(TrailingSliceLayout {
offset: composite_offset,
elem_size: composite_elem_size,
}) = composite.size_info
{
// The offset of the trailing slice component is the sum
// of the offset of the trailing field and the trailing
// slice offset within that field.
assert_eq!(composite_offset, offset + field_offset);
// The elem size is unchanged.
assert_eq!(composite_elem_size, field_elem_size);
let field_analog =
Layout::from_size_align(field_offset, field_align.get()).unwrap();
if let Ok((actual_composite, actual_offset)) = base_analog.extend(field_analog)
{
assert_eq!(actual_offset, offset);
assert_eq!(actual_composite.size(), composite_offset);
assert_eq!(actual_composite.align(), composite.align.get());
} else {
// An error here reflects that composite of `base`
// and `field` cannot correspond to a real Rust type
// fragment, because such a fragment would violate
// the basic invariants of a valid Rust layout. At
// the time of writing, `DstLayout` is a little more
// permissive than `Layout`, so we don't assert
// anything in this branch (e.g., unreachability).
}
} else {
panic!("The extension of a layout with a DST must result in a DST.")
}
}
}
}
#[kani::proof]
#[kani::should_panic]
fn prove_dst_layout_extend_dst_panics() {
let base: DstLayout = kani::any();
let field: DstLayout = kani::any();
let packed: Option<NonZeroUsize> = kani::any();
if let Some(max_align) = packed {
kani::assume(max_align.is_power_of_two());
kani::assume(base.align <= max_align);
}
kani::assume(matches!(base.size_info, SizeInfo::SliceDst(..)));
let _ = base.extend(field, packed);
}
#[kani::proof]
fn prove_dst_layout_pad_to_align() {
use crate::util::padding_needed_for;
let layout: DstLayout = kani::any();
let padded = layout.pad_to_align();
// Calling `pad_to_align` does not alter the `DstLayout`'s alignment.
assert_eq!(padded.align, layout.align);
if let SizeInfo::Sized { size: unpadded_size } = layout.size_info {
if let SizeInfo::Sized { size: padded_size } = padded.size_info {
// If the layout is sized, it will remain sized after padding is
// added. Its sum will be its unpadded size and the size of the
// trailing padding needed to satisfy its alignment
// requirements.
let padding = padding_needed_for(unpadded_size, layout.align);
assert_eq!(padded_size, unpadded_size + padding);
// Prove that calling `DstLayout::pad_to_align` behaves
// identically to `Layout::pad_to_align`.
let layout_analog =
Layout::from_size_align(unpadded_size, layout.align.get()).unwrap();
let padded_analog = layout_analog.pad_to_align();
assert_eq!(padded_analog.align(), layout.align.get());
assert_eq!(padded_analog.size(), padded_size);
} else {
panic!("The padding of a sized layout must result in a sized layout.")
}
} else {
// If the layout is a DST, padding cannot be statically added.
assert_eq!(padded.size_info, layout.size_info);
}
}
}