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

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Oct 25, 2004 at 07:51:26PM -0700, Roland McGrath wrote:
> > 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?

Yeah, this is a known problem, but not yet dealt with.
See http://sources.redhat.com/bugzilla/show_bug.cgi?id=300
for details.
Option Three in Richard's #7 comment sounds the best thing to do, but
I didn't get to implement it yet.

	Jakub