| From b329452ad87286adfb90522b453807c9c1a218a7 Mon Sep 17 00:00:00 2001 |
| From: Sasha Levin <sashal@kernel.org> |
| Date: Tue, 21 Dec 2021 16:58:59 +1100 |
| Subject: powerpc/toc: Future proof kernel toc |
| |
| From: Alan Modra <amodra@au1.ibm.com> |
| |
| [ Upstream commit a3ad84da076009c94969fa97f604257667e2980f ] |
| |
| This patch future-proofs the kernel against linker changes that might |
| put the toc pointer at some location other than .got+0x8000, by |
| replacing __toc_start+0x8000 with .TOC. throughout. If the kernel's |
| idea of the toc pointer doesn't agree with the linker, bad things |
| happen. |
| |
| prom_init.c code relocating its toc is also changed so that a symbolic |
| __prom_init_toc_start toc-pointer relative address is calculated |
| rather than assuming that it is always at toc-pointer - 0x8000. The |
| length calculations loading values from the toc are also avoided. |
| It's a little incestuous to do that with unreloc_toc picking up |
| adjusted values (which is fine in practice, they both adjust by the |
| same amount if all goes well). |
| |
| I've also changed the way .got is aligned in vmlinux.lds and |
| zImage.lds, mostly so that dumping out section info by objdump or |
| readelf plainly shows the alignment is 256. This linker script |
| feature was added 2005-09-27, available in FSF binutils releases from |
| 2.17 onwards. Should be safe to use in the kernel, I think. |
| |
| Finally, put *(.got) before the prom_init.o entry which only needs |
| *(.toc), so that the GOT header goes in the correct place. I don't |
| believe this makes any difference for the kernel as it would for |
| dynamic objects being loaded by ld.so. That change is just to stop |
| lusers who blindly copy kernel scripts being led astray. Of course, |
| this change needs the prom_init.c changes. |
| |
| Some notes on .toc and .got. |
| |
| .toc is a compiler generated section of addresses. .got is a linker |
| generated section of addresses, generally built when the linker sees |
| R_*_*GOT* relocations. In the case of powerpc64 ld.bfd, there are |
| multiple generated .got sections, one per input object file. So you |
| can somewhat reasonably write in a linker script an input section |
| statement like *prom_init.o(.got .toc) to mean "the .got and .toc |
| section for files matching *prom_init.o". On other architectures that |
| doesn't make sense, because the linker generally has just one .got |
| section. Even on powerpc64, note well that the GOT entries for |
| prom_init.o may be merged with GOT entries from other objects. That |
| means that if prom_init.o references, say, _end via some GOT |
| relocation, and some other object also references _end via a GOT |
| relocation, the GOT entry for _end may be in the range |
| __prom_init_toc_start to __prom_init_toc_end and if the kernel does |
| something special to GOT/TOC entries in that range then the value of |
| _end as seen by objects other than prom_init.o will be affected. On |
| the other hand the GOT entry for _end may not be in the range |
| __prom_init_toc_start to __prom_init_toc_end. Which way it turns out |
| is deterministic but a detail of linker operation that should not be |
| relied on. |
| |
| A feature of ld.bfd is that input .toc (and .got) sections matching |
| one linker input section statement may be sorted, to put entries used |
| by small-model code first, near the toc base. This is why scripts for |
| powerpc64 normally use *(.got .toc) rather than *(.got) *(.toc), since |
| the first form allows more freedom to sort. |
| |
| Another feature of ld.bfd is that indirect addressing sequences using |
| the GOT/TOC may be edited by the linker to relative addressing. In |
| many cases relative addressing would be emitted by gcc for |
| -mcmodel=medium if you appropriately decorate variable declarations |
| with non-default visibility. |
| |
| The original patch is here: |
| https://lore.kernel.org/linuxppc-dev/20210310034813.GM6042@bubble.grove.modra.org/ |
| |
| Signed-off-by: Alan Modra <amodra@au1.ibm.com> |
| [aik: removed non-relocatable which is gone in 24d33ac5b8ffb] |
| [aik: added <=2.24 check] |
| [aik: because of llvm-as, kernel_toc_addr() uses "mr" instead of global register variable] |
| Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru> |
| Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> |
| Link: https://lore.kernel.org/r/20211221055904.555763-2-aik@ozlabs.ru |
| Stable-dep-of: 1b1e38002648 ("powerpc: add crtsavres.o to always-y instead of extra-y") |
| Signed-off-by: Sasha Levin <sashal@kernel.org> |
| --- |
| arch/powerpc/Makefile | 5 +++-- |
| arch/powerpc/boot/crt0.S | 2 +- |
| arch/powerpc/boot/zImage.lds.S | 7 ++----- |
| arch/powerpc/include/asm/sections.h | 16 ++++++++-------- |
| arch/powerpc/kernel/head_64.S | 2 +- |
| arch/powerpc/kernel/vmlinux.lds.S | 8 +++----- |
| 6 files changed, 18 insertions(+), 22 deletions(-) |
| |
| diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile |
| index 3dc75040a756..41604a37b385 100644 |
| --- a/arch/powerpc/Makefile |
| +++ b/arch/powerpc/Makefile |
| @@ -437,10 +437,11 @@ PHONY += checkbin |
| # Check toolchain versions: |
| # - gcc-4.6 is the minimum kernel-wide version so nothing required. |
| checkbin: |
| - @if test "x${CONFIG_CPU_LITTLE_ENDIAN}" = "xy" \ |
| - && $(LD) --version | head -1 | grep ' 2\.24$$' >/dev/null ; then \ |
| + @if test "x${CONFIG_LD_IS_LLD}" != "xy" -a \ |
| + "x$(call ld-ifversion, -le, 22400, y)" = "xy" ; then \ |
| echo -n '*** binutils 2.24 miscompiles weak symbols ' ; \ |
| echo 'in some circumstances.' ; \ |
| + echo '*** binutils 2.23 do not define the TOC symbol ' ; \ |
| echo -n '*** Please use a different binutils version.' ; \ |
| false ; \ |
| fi |
| diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S |
| index e8f10a599659..feadee18e271 100644 |
| --- a/arch/powerpc/boot/crt0.S |
| +++ b/arch/powerpc/boot/crt0.S |
| @@ -28,7 +28,7 @@ p_etext: .8byte _etext |
| p_bss_start: .8byte __bss_start |
| p_end: .8byte _end |
| |
| -p_toc: .8byte __toc_start + 0x8000 - p_base |
| +p_toc: .8byte .TOC. - p_base |
| p_dyn: .8byte __dynamic_start - p_base |
| p_rela: .8byte __rela_dyn_start - p_base |
| p_prom: .8byte 0 |
| diff --git a/arch/powerpc/boot/zImage.lds.S b/arch/powerpc/boot/zImage.lds.S |
| index d6f072865627..d65cd55a6f38 100644 |
| --- a/arch/powerpc/boot/zImage.lds.S |
| +++ b/arch/powerpc/boot/zImage.lds.S |
| @@ -36,12 +36,9 @@ SECTIONS |
| } |
| |
| #ifdef CONFIG_PPC64_BOOT_WRAPPER |
| - . = ALIGN(256); |
| - .got : |
| + .got : ALIGN(256) |
| { |
| - __toc_start = .; |
| - *(.got) |
| - *(.toc) |
| + *(.got .toc) |
| } |
| #endif |
| |
| diff --git a/arch/powerpc/include/asm/sections.h b/arch/powerpc/include/asm/sections.h |
| index e92d39c0cd1d..34d82ae1774c 100644 |
| --- a/arch/powerpc/include/asm/sections.h |
| +++ b/arch/powerpc/include/asm/sections.h |
| @@ -26,16 +26,16 @@ extern char start_virt_trampolines[]; |
| extern char end_virt_trampolines[]; |
| #endif |
| |
| +/* |
| + * This assumes the kernel is never compiled -mcmodel=small or |
| + * the total .toc is always less than 64k. |
| + */ |
| static inline unsigned long kernel_toc_addr(void) |
| { |
| - /* Defined by the linker, see vmlinux.lds.S */ |
| - extern unsigned long __toc_start; |
| - |
| - /* |
| - * The TOC register (r2) points 32kB into the TOC, so that 64kB of |
| - * the TOC can be addressed using a single machine instruction. |
| - */ |
| - return (unsigned long)(&__toc_start) + 0x8000UL; |
| + unsigned long toc_ptr; |
| + |
| + asm volatile("mr %0, 2" : "=r" (toc_ptr)); |
| + return toc_ptr; |
| } |
| |
| static inline int overlaps_interrupt_vector_text(unsigned long start, |
| diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S |
| index f17ae2083733..a08c050ff645 100644 |
| --- a/arch/powerpc/kernel/head_64.S |
| +++ b/arch/powerpc/kernel/head_64.S |
| @@ -904,7 +904,7 @@ _GLOBAL(relative_toc) |
| blr |
| |
| .balign 8 |
| -p_toc: .8byte __toc_start + 0x8000 - 0b |
| +p_toc: .8byte .TOC. - 0b |
| |
| /* |
| * This is where the main kernel code starts. |
| diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S |
| index d8301ce7c675..70bf67ed87b5 100644 |
| --- a/arch/powerpc/kernel/vmlinux.lds.S |
| +++ b/arch/powerpc/kernel/vmlinux.lds.S |
| @@ -352,15 +352,13 @@ SECTIONS |
| *(.branch_lt) |
| } |
| |
| - . = ALIGN(256); |
| - .got : AT(ADDR(.got) - LOAD_OFFSET) { |
| - __toc_start = .; |
| + .got : AT(ADDR(.got) - LOAD_OFFSET) ALIGN(256) { |
| + *(.got) |
| #ifndef CONFIG_RELOCATABLE |
| __prom_init_toc_start = .; |
| - arch/powerpc/kernel/prom_init.o*(.toc .got) |
| + arch/powerpc/kernel/prom_init.o*(.toc) |
| __prom_init_toc_end = .; |
| #endif |
| - *(.got) |
| *(.toc) |
| } |
| #endif |
| -- |
| 2.43.0 |
| |
