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Re: [PATCH 1/2] avoid infinite loop with bad debuginfo
- From: Pedro Alves <palves at redhat dot com>
- To: Tom Tromey <tromey at redhat dot com>
- Cc: gdb-patches at sourceware dot org
- Date: Tue, 19 Nov 2013 19:07:44 +0000
- Subject: Re: [PATCH 1/2] avoid infinite loop with bad debuginfo
- Authentication-results: sourceware.org; auth=none
- References: <1384375873-32160-1-git-send-email-tromey at redhat dot com> <1384375873-32160-2-git-send-email-tromey at redhat dot com> <52850730 dot 1060109 at redhat dot com> <87d2lxpo1l dot fsf at fleche dot redhat dot com> <528B7F15 dot 7040605 at redhat dot com> <87vbzomm78 dot fsf at fleche dot redhat dot com> <528B8FF6 dot 7000406 at redhat dot com> <87siusl10r dot fsf at fleche dot redhat dot com>
On 11/19/2013 06:06 PM, Tom Tromey wrote:
> It seems to me that the loop in question could perhaps be reached from
> some path outside the unwinder.
Yes, that's actually what my example was about. I was assuming the
recursion fixed already.
> If so, bad DWARF would be able to cause
> an internal error -- clearly incorrect.
I don't think so, because get_prev_frame_1 would not link in
the dup frame. The loop in question would never see it.
Hmm, I think one of us is missing something.
while (VALUE_LVAL (new_val) == lval_register && value_lazy (new_val))
frame = frame_find_by_id (VALUE_FRAME_ID (new_val));
new_val = get_frame_register_value (frame, regnum);
get_frame_register_value unwinds the value in question from the
struct value *
get_frame_register_value (struct frame_info *frame, int regnum)
return frame_unwind_register_value (frame->next, regnum);
IOW, if we get a lazy lval_register, it should have
the frame ID of the _next_ frame, never of FRAME.
At this point, the whole relevant chunk of the stack has
already been unwound -- note the loop always "unlazies"
lval_registers in the "next/innermost" direction, not in
the "prev/unwind further/outermost" direction.
So the bad loop can only ever happen (outside the unwinder code)
if we ever let outselves get in the dup frame_id situation:
> #4 0x0000007fb7f0956c in clone () from /lib64/libc.so.6
> #5 0x0000007fb7f0956c in clone () from /lib64/libc.so.6
> Backtrace stopped: previous frame identical to this frame (corrupt stack?)
At least, I'm not seeing any other way.