Re: [PATCH v2 1/1] tools/nolibc: i386: Fix a stack misalign bug on _start

[Willy Tarreau]

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