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Re: [Libunwind-devel] use_prev_instr breaking unwind - how to stop it?

From: Doug Moore
Subject: Re: [Libunwind-devel] use_prev_instr breaking unwind - how to stop it?
Date: Fri, 24 Mar 2017 12:05:00 -0500
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Linux 3.10.0-123.el7.x86_64 #1 SMP Mon Jun 30 12:09:22 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

fetch_proc_info checks c->use_prev_instr and, if it is set, decrements the ip value as its first action. Later, it might update c->use_prev_instr, but for the first call, I'm stuck with the initial value, as set by unw_init_local.


Quoting Lassi Tuura <address@hidden>:

What platform are you on? fetch_proc_info() should auto-detect signal frame
and set use_prev_instr accordingly using the (kernel-provided) frame
annotations detected in parse_cie(). I don't know if that works for
anything but linux (and possibly x86_64 only at that). Looks like there may
be some support for freebsd.

On Thu, Mar 23, 2017 at 8:45 PM, Doug Moore <address@hidden> wrote:

When unw_init_local uses common_init to set use_prev_instr, and then
fetch_proc_info decrements ip, based on use_prev_instr, unw_step fails for

I’m putting a temp break point at the start of _dl_runtime_resolve, and
when gdb gets there, I’m sending a signal to trigger a signal handler that
invokes unw_step.  What happens, sadly, is that the instruction pointer is
decremented, so the eh_frames search doesn’t find the right frame, so the
step fails.

If only I could clear that use_prev_instr field.  So far, I don’t have any
better choices than to hack the libunwind source and maintain my own local
branch.  Are there any better ideas?

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