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

[Daniel Jacobowitz <drow@false.org>]

On Tue, Oct 26, 2004 at 04:17:44AM -0400, Jakub Jelinek wrote:
> 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.

Thanks for the pointer!  I've been thinking about #2; I'm not quite
sure about #3.  What would the expresson be used for - "evaluates
non-zero" for what condition?

[I've also needed something like DW_CFA_expression that didn't evaluate
to a location before; if that would be useful to solve this, then we
ought to add it.]

-- 
Daniel Jacobowitz