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 2ab4463 ("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.

Extra:
Add 'add $12, %esp' before the 'and $-16, %esp' to avoid over-estimating
for particular cases as suggested by Willy.

A test program to validate the %esp alignment on _start can be found at:

   https://lore.kernel.org/lkml/[email protected]

[ Thomas: trim Fixes tag commit id ]

Cc: Zhangjin Wu <[email protected]>
Fixes: 2ab4463 ("tools/nolibc: i386: shrink _start with _start_c")
Reported-by: Nicholas Rosenberg <[email protected]>
Acked-by: Thomas Weißschuh <[email protected]>
Signed-off-by: Ammar Faizi <[email protected]>
Reviewed-by: Alviro Iskandar Setiawan <[email protected]>
Signed-off-by: Willy Tarreau <[email protected]>
Signed-off-by: Thomas Weißschuh <[email protected]>

Author: ammarfaizi2

tools/include/nolibc/arch-i386.h

@@ -167,7 +167,9 @@ 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        */
+		"add  $12, %esp\n"        /* avoid over-estimating after the 'and' & 'sub' below */
+		"and  $-16, %esp\n"       /* the %esp must be 16-byte aligned on 'call'          */
+		"sub  $12, %esp\n"        /* sub 12 to keep it aligned after the push %eax       */
 		"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                           */