Re: [PATCH v4.1 06/10] x86/entry/vdso32: remove open-coded DWARF in sigreturn.S

["H. Peter Anvin" <hpa@zytor.com>]

On 2026-02-09 19:11, Xi Ruoyao wrote:
> On Mon, 2026-02-02 at 19:57 -0800, H. Peter Anvin wrote:
>> That hack dates back from before the signal frame extension. It is no
>> longer necessary.
> 
> Unfortunately at least it seems libgcc unwinder does not handle the
> signal frame extension properly.  The code reads:
> 
>   fde = _Unwind_Find_FDE (context->ra + _Unwind_IsSignalFrame (context) - 1, 
>                           &context->bases);
>   if (fde == NULL)
>     {    
> #ifdef MD_FALLBACK_FRAME_STATE_FOR
>       /* Couldn't find frame unwind info for this function.  Try a
>          target-specific fallback mechanism.  This will necessarily
>          not provide a personality routine or LSDA.  */
>       return MD_FALLBACK_FRAME_STATE_FOR (context, fs); 
> #else
>       return _URC_END_OF_STACK;
> #endif
>     }    
> 
>   fs->pc = context->bases.func;
> 
>   cie = get_cie (fde);
>   insn = extract_cie_info (cie, context, fs); 
> 
> Thus, it indeed attempts to avoid subtracting 1 for a signal frame, but
> ... _Unwind_IsSignalFrame (context) actually extracts a flag in context
> which will only be raised up by extract_cie_info.
> 
> Or am I missing something here?
> 

Well, it looks like this might be a non-manifest bug on x86 *IN THIS CASE*,
because the fallback handler looks explicitly for the byte sequences:

	popl %eax
	movl $__NR_ia32_sigreturn, %eax
	int $0x80

	movl $__NR_ia32_rt_sigreturn, %eax
	int $0x80

	movq $__NR_x64_rt_sigreturn, %rax	/* !!!! */
	syscall

... so the vdso patchset is still OK.

Hard-coding a reliance on movq in the 64-bit sequence some serious brain damage...

Could you file a gcc bug on this?

	-hpa