Re: [PATCH v1 0/1] Fix a stack misalign bug on _start

[Zhangjin Wu <falcon@tinylab.org>]

Hi, Ammar

> On Sat, Aug 26, 2023 at 11:20:24PM +0800, Zhangjin Wu wrote:
> > >   $eax   : 0x56559000  →  0x00003f90
> > >   $ebx   : 0x56559000  →  0x00003f90
> > >   $ecx   : 0x1
> > >   $edx   : 0xf7fcaaa0  →   endbr32 
> > >   $esp   : 0xffffcdbc  →  0x00000001
> > >   $ebp   : 0x0
> > >   $esi   : 0xffffce7c  →  0xffffd096
> > >   $edi   : 0x56556060  →  <_start+0> xor %ebp, %ebp
> > >   $eip   : 0x56556489  →  <sse_pq_add+25> movaps %xmm0, 0x30(%esp)
> > > 
> > >     <sse_pq_add+11>  pop    %eax
> > >     <sse_pq_add+12>  add    $0x2b85, %eax
> > >     <sse_pq_add+18>  movups -0x1fd0(%eax), %xmm0
> > >   → <sse_pq_add+25>  movaps %xmm0, 0x30(%esp)     <== trapping instruction
> > >     <sse_pq_add+30>  movups -0x1fe0(%eax), %xmm1
> > >     <sse_pq_add+37>  movaps %xmm1, 0x20(%esp)
> > >     <sse_pq_add+42>  movups -0x1ff0(%eax), %xmm2
> > >     <sse_pq_add+49>  movaps %xmm2, 0x10(%esp)
> > >     <sse_pq_add+54>  movups -0x2000(%eax), %xmm3
> > > 
> > >   [#0] Id 1, Name: "test", stopped 0x56556489 in sse_pq_add (), reason: SIGSEGV
> > > 
> > >   (gdb)  bt
> > >   #0  0x56556489 in sse_pq_add ()
> > >   #1  0x5655608e in main ()
> > >
> > 
> > Since we have a new 'startup' test group, do you have a short function
> > to trigger this error?
> 
> Here is a simple program to test the stack alignment.
> 
> #include "tools/include/nolibc/nolibc.h"
> 
> __asm__ (
> "main:\n"
>     /*
>      * When the call main is executed, the
>      * %esp is 16 bytes aligned.
>      *
>      * Then, on function entry (%esp mod 16) == 12
>      * because the call instruction pushes 4 bytes
>      * onto the stack.
>      *
>      * subl $12, %esp will make (%esp mod 16) == 0
>      * again.
>      */
>     "subl  $12, %esp\n"
> 
>     /*
>      * These move instructions will crash if %esp is
>      * not a multiple of 16.
>      */
>     "movdqa (%esp), %xmm0\n"
>     "movdqa %xmm0, (%esp)\n"
>     "movaps (%esp), %xmm0\n"
>     "movaps %xmm0, (%esp)\n"
> 
>     "addl   $12, %esp\n"
>     "xorl   %eax, %eax\n"
>     "ret\n"
> );
>

Thanks very much for sharing this code.

> > Perhaps it is time for us to add a new 'stack alignment' test case for
> > all of the architectures.
> 
> I don't know the alignment rules for other architectures (I only work on
> x86 and x86-64). While waiting for the maintainers' comment, I'll leave
> the test case decision to you. Feel free to take the above code.
>

Yes, the stack alignment rule is architecture dependent, so, we need
more discussion and more work, not sure if there is a 'C' test function
for all, let's delay this after v6.6.

> Extra:
> It's also fine if you take my patch with the 'sub $(16 - 4), %esp'
> change and batch it together in your next series.
>

Ammar, your fixup patch is urgent since our _start_c() is for v6.6-rc1 (already
in linux-next), let's wait for comments from Thomas or Willy, they will
determine that merge it directly or require a v2. I'm ok with v1 code, but the
old comment looks not that clear.

Thanks,
Zhangjin

> -- 
> Ammar Faizi