linux-kernel - [PATCH v2 1/1] tools/nolibc: i386: Fix a stack misalign bug on

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Date:   Mon, 28 Aug 2023 14:02:40 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Willy Tarreau <w@....eu>,
        Thomas Weißschuh <linux@...ssschuh.net>
Cc:     Ammar Faizi <ammarfaizi2@...weeb.org>,
        Zhangjin Wu <falcon@...ylab.org>,
        Nicholas Rosenberg <inori@...x.org>,
        Michael William Jonathan <moe@...weeb.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH v2 1/1] tools/nolibc: i386: Fix a stack misalign bug on _start

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.

Cc: Zhangjin Wu <falcon@...ylab.org>
Fixes: 2ab446336b17aad362c6decee29b4efd83a01979 ("tools/nolibc: i386: shrink _start with _start_c")
Reported-by: Nicholas Rosenberg <inori@...x.org>
Acked-by: Thomas Weißschuh <linux@...ssschuh.net>
Signed-off-by: Ammar Faizi <ammarfaizi2@...weeb.org>
---
 tools/include/nolibc/arch-i386.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/include/nolibc/arch-i386.h b/tools/include/nolibc/arch-i386.h
index 64415b9fac77f996..8079974424fa18b0 100644
--- a/tools/include/nolibc/arch-i386.h
+++ b/tools/include/nolibc/arch-i386.h
@@ -167,7 +167,8 @@ void __attribute__((weak, noreturn, optimize("Os", "omit-frame-pointer"))) __no_
 	__asm__ volatile (
 		"xor  %ebp, %ebp\n"       /* zero the stack frame                                */
 		"mov  %esp, %eax\n"       /* save stack pointer to %eax, as arg1 of _start_c     */
-		"and  $-16, %esp\n"       /* last pushed argument must be 16-byte aligned        */
+		"and  $-16, %esp\n"       /* align stack to 16 bytes                             */
+		"sub  $(16 - 4), %esp\n"  /* push %eax breaks 16-byte alignment, so sub 12 bytes */
 		"push %eax\n"             /* push arg1 on stack to support plain stack modes too */
 		"call _start_c\n"         /* transfer to c runtime                               */
 		"hlt\n"                   /* ensure it does not return                           */
-- 
Ammar Faizi