Re: [PATCH v1 1/1] tools/nolibc: i386: Fix a stack misalign bug on _start

[Zhangjin Wu]

Hi, Ammar, Hi, Nicholas

Thanks very much for your report and fixup, sorry for the missing alignment for
the new single one push instruction.

> The ABI mandates that the %esp register must be a multiple of 16 when
> executing a call instruction.
> 
> Commit 2ab446336b17 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).
> 
> Ensure the %esp is a multiple of 16 when executing the call instruction.
> 
> Cc: Zhangjin Wu <falcon@xxxxxxxxxxx>
> Fixes: 2ab446336b17aad362c6decee29b4efd83a01979 ("tools/nolibc: i386: shrink _start with _start_c")
> Reported-by: Nicholas Rosenberg <inori@xxxxxxxx>
> Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
> ---
>  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..4edf238eeac67aa2 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  $12, %esp\n"        /* last pushed argument must be 16-byte aligned        */

Ammar, the new call of _start_c() only requires a single push, it pushes
the argument and minus %esp by 4, so, the alignment of %esp requires to
minus 12 to reserve 16-byte alignment, is this description right?

If so, What about further?

    "and  $-16, %esp\n"       /* align stack to 16 bytes                             */
    "sub ($16 - $4), %esp"    /* the 'push %eax' breaks stack alignment, fix up it   */

Thanks,
Zhangjin Wu

>  		"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