| From 937ed91c712273131de6d2a02caafd3ee84e0c72 Mon Sep 17 00:00:00 2001 |
| From: Ammar Faizi <ammar.faizi@students.amikom.ac.id> |
| Date: Sun, 24 Oct 2021 19:28:14 +0200 |
| Subject: tools/nolibc: x86-64: Fix startup code bug |
| |
| From: Ammar Faizi <ammar.faizi@students.amikom.ac.id> |
| |
| commit 937ed91c712273131de6d2a02caafd3ee84e0c72 upstream. |
| |
| Before this patch, the `_start` function looks like this: |
| ``` |
| 0000000000001170 <_start>: |
| 1170: pop %rdi |
| 1171: mov %rsp,%rsi |
| 1174: lea 0x8(%rsi,%rdi,8),%rdx |
| 1179: and $0xfffffffffffffff0,%rsp |
| 117d: sub $0x8,%rsp |
| 1181: call 1000 <main> |
| 1186: movzbq %al,%rdi |
| 118a: mov $0x3c,%rax |
| 1191: syscall |
| 1193: hlt |
| 1194: data16 cs nopw 0x0(%rax,%rax,1) |
| 119f: nop |
| ``` |
| Note the "and" to %rsp with $-16, it makes the %rsp be 16-byte aligned, |
| but then there is a "sub" with $0x8 which makes the %rsp no longer |
| 16-byte aligned, then it calls main. That's the bug! |
| |
| What actually the x86-64 System V ABI mandates is that right before the |
| "call", the %rsp must be 16-byte aligned, not after the "call". So the |
| "sub" with $0x8 here breaks the alignment. Remove it. |
| |
| An example where this rule matters is when the callee needs to align |
| its stack at 16-byte for aligned move instruction, like `movdqa` and |
| `movaps`. If the callee can't align its stack properly, it will result |
| in segmentation fault. |
| |
| x86-64 System V ABI also mandates the deepest stack frame should be |
| zero. Just to be safe, let's zero the %rbp on startup as the content |
| of %rbp may be unspecified when the program starts. Now it looks like |
| this: |
| ``` |
| 0000000000001170 <_start>: |
| 1170: pop %rdi |
| 1171: mov %rsp,%rsi |
| 1174: lea 0x8(%rsi,%rdi,8),%rdx |
| 1179: xor %ebp,%ebp # zero the %rbp |
| 117b: and $0xfffffffffffffff0,%rsp # align the %rsp |
| 117f: call 1000 <main> |
| 1184: movzbq %al,%rdi |
| 1188: mov $0x3c,%rax |
| 118f: syscall |
| 1191: hlt |
| 1192: data16 cs nopw 0x0(%rax,%rax,1) |
| 119d: nopl (%rax) |
| ``` |
| |
| Cc: Bedirhan KURT <windowz414@gnuweeb.org> |
| Cc: Louvian Lyndal <louvianlyndal@gmail.com> |
| Reported-by: Peter Cordes <peter@cordes.ca> |
| Signed-off-by: Ammar Faizi <ammar.faizi@students.amikom.ac.id> |
| [wt: I did this on purpose due to a misunderstanding of the spec, other |
| archs will thus have to be rechecked, particularly i386] |
| Cc: stable@vger.kernel.org |
| Signed-off-by: Willy Tarreau <w@1wt.eu> |
| Signed-off-by: Paul E. McKenney <paulmck@kernel.org> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| --- |
| tools/include/nolibc/nolibc.h | 10 ++++++++-- |
| 1 file changed, 8 insertions(+), 2 deletions(-) |
| |
| --- a/tools/include/nolibc/nolibc.h |
| +++ b/tools/include/nolibc/nolibc.h |
| @@ -399,14 +399,20 @@ struct stat { |
| }) |
| |
| /* startup code */ |
| +/* |
| + * x86-64 System V ABI mandates: |
| + * 1) %rsp must be 16-byte aligned right before the function call. |
| + * 2) The deepest stack frame should be zero (the %rbp). |
| + * |
| + */ |
| asm(".section .text\n" |
| ".global _start\n" |
| "_start:\n" |
| "pop %rdi\n" // argc (first arg, %rdi) |
| "mov %rsp, %rsi\n" // argv[] (second arg, %rsi) |
| "lea 8(%rsi,%rdi,8),%rdx\n" // then a NULL then envp (third arg, %rdx) |
| - "and $-16, %rsp\n" // x86 ABI : esp must be 16-byte aligned when |
| - "sub $8, %rsp\n" // entering the callee |
| + "xor %ebp, %ebp\n" // zero the stack frame |
| + "and $-16, %rsp\n" // x86 ABI : esp must be 16-byte aligned before call |
| "call main\n" // main() returns the status code, we'll exit with it. |
| "movzb %al, %rdi\n" // retrieve exit code from 8 lower bits |
| "mov $60, %rax\n" // NR_exit == 60 |
