Re: [rfa] Attach vsyscall support for GNU/Linux

[Roland McGrath <roland@redhat.com>]

I'm punting this to Jakub as the person more knowledgeable about how the
unwind info is used for exception handling in glibc.

> I just don't think this problem is solvable within the existing CFI.
> I don't know whether it is solvable by extending DWARF.  Just to make
> sure I'm on the right page, I'll recap one instance of why GDB needs to
> know it's found a signal handler.
> 
> Here's __kernel_rt_sigreturn (starts at 0xffffe440).
> ffffe43f:       90                      nop
> ffffe440:       b8 ad 00 00 00          mov    $0xad,%eax
> ffffe445:       cd 80                   int    $0x80
> 
> Here's the unwind information:
> 000000c4 00000044 00000084 FDE cie=00000044 pc=ffffe43f..ffffe447
>   DW_CFA_def_cfa_expression (DW_OP_breg4: 188; DW_OP_deref)
>   DW_CFA_expression: r0 (DW_OP_breg4: 204)
>   DW_CFA_expression: r1 (DW_OP_breg4: 200)
>   DW_CFA_expression: r2 (DW_OP_breg4: 196)
>   DW_CFA_expression: r3 (DW_OP_breg4: 192)
>   DW_CFA_expression: r5 (DW_OP_breg4: 184)
>   DW_CFA_expression: r6 (DW_OP_breg4: 180)
>   DW_CFA_expression: r7 (DW_OP_breg4: 176)
>   DW_CFA_expression: r8 (DW_OP_breg4: 216)
>   DW_CFA_nop
> 
> This is accurate.  It correctly locates the saved values of all
> registers.  However, this is the frame_address_in_block problem; if the
> first instruction of a function generates a synchronous signal, then
> the restored value of r8 (the PC) will point to the first byte of the
> function.  GDB will use the unwind information for the previous
> function.
> 
> I bet you could reproduce the corresponding problem by an extremely
> signal-heavy stress test using NPTL and asynchronous cancellation.
> Roland, am I missing something?  Won't we go off into never-never land
> if we're at the first instruction of a function call when a signal is
> received and we try to do a forced unwind?