From: Willy Tarreau <[email protected]>
To: Ammar Faizi <[email protected]>
Cc: "Thomas Weißschuh" <[email protected]>,
"Zhangjin Wu" <[email protected]>,
"Nicholas Rosenberg" <[email protected]>,
"Michael William Jonathan" <[email protected]>,
"GNU/Weeb Mailing List" <[email protected]>,
"Linux Kernel Mailing List" <[email protected]>
Subject: Re: [PATCH v2 1/1] tools/nolibc: i386: Fix a stack misalign bug on _start
Date: Tue, 29 Aug 2023 08:21:47 +0200 [thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
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
next prev parent reply other threads:[~2023-08-29 6:22 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-28 7:02 [PATCH v2 0/1] Fix a stack misalign bug on _start Ammar Faizi
2023-08-28 7:02 ` [PATCH v2 1/1] tools/nolibc: i386: " Ammar Faizi
2023-08-29 6:21 ` Willy Tarreau [this message]
2023-08-29 12:17 ` Ammar Faizi
2023-08-29 12:24 ` Willy Tarreau
2023-08-29 8:37 ` Zhangjin Wu
2023-08-29 12:25 ` Ammar Faizi
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