Messages in this thread | | | Date | Tue, 29 Aug 2023 08:21:47 +0200 | From | Willy Tarreau <> | Subject | Re: [PATCH v2 1/1] tools/nolibc: i386: Fix a stack misalign bug on _start |
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Hi Ammar,
On Mon, Aug 28, 2023 at 02:02:40PM +0700, Ammar Faizi wrote: > The ABI mandates that the %esp register must be a multiple of 16 when > executing a 'call' instruction. > > Commit 2ab446336b17 ("tools/nolibc: i386: shrink _start with _start_c") > simplified the _start function, but it didn't take care of the %esp > alignment, causing SIGSEGV on SSE and AVX programs that use aligned move > instruction (e.g., movdqa, movaps, and vmovdqa). > > The 'and $-16, %esp' aligns the %esp at a multiple of 16. Then 'push > %eax' will subtract the %esp by 4; thus, it breaks the 16-byte > alignment. Make sure the %esp is correctly aligned after the push by > subtracting 12 before the push.
Indeed, good catch! However if we want to do it cleany (i.e not punch a 16 to 28 byes hole in the stack), we should do this instead:
add $12, %esp /* the stack must be aligned to 16 ... */ and $-16, %esp /* ... bytes after eax is pushed and ... */ sub $12, %esp /* ... before the call. */
This will only add 0 to 12 bytes depending on the existing alignment.
What do you think ?
thanks, Willy
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