Thanks for those links.
You ask
Would unw_init_remote() work for your use?
I tried, but the resulting binary crashed. That may be because
unw_init_remote does not include "c->validate = 0;" as does
unw_init_local. Even if it did work, I don't think I could stand the
performance penalty of using remote instead of local.
In any case, the implementations of local and remote clearly are not
compliant with the online documentation for unw_init_local, which
states:
From a behavioral point of view, the call:
ret = unw_init_local(&cursor, &ucontext);
is equivalent to:
ret = unw_init_remote(&cursor, unw_local_addr_space,
&ucontext);
which is untrue because of the different values of use_prev_instr and
validate assigned by the two.
The rationale from 2010 includes
This seems almost impossible to run into with local unwinding - it's
not possible for unw_getcontext() to have signal trampoline as the
caller.
but I am trying local unwinding with a context delivered by a signal
handler, not by unw_getcontext, and so stepping, for me, fails as you
described in 2010:
2) Failure to trace at function entry; .eh_frame information exists
and is correct. I suspect fetch_proc_info() should use "ip", not
"--ip" to locate the FDE. In all cases I examined there is no
adjacent preceding FDE, so lookup by ip-1 would come up empty.
At the moment, I'm defining a third flavor of unw_init to get me a
cursor initialized to let me avoid this problem. I'd rather be
staying within the documented libunwind interface. Can libunwind
change to help me?
Doug
Quoting Lassi Tuura <l...@iki.fi>:
The rationale from back in 2010 is in
http://lists.gnu.org/archive/html/libunwind-devel/2010-04/msg00060.html
plus surrounding thread, with earlier context on unwind failures in
http://lists.gnu.org/archive/html/libunwind-devel/2010-03/msg00015.html
There's also related earlier context in
http://lists.gnu.org/archive/html/libunwind-devel/2008-01/msg00017.html
As I mentioned in the first linked message, it seems improbable that
unw_init_local() could ever be invoked from a signal frame itself. At that
time, your use case of single stepping was mentioned as a counter-example.
Would unw_init_remote() work for your use?
On Fri, Mar 24, 2017 at 11:23 PM, Doug Moore <do...@rice.edu> wrote:
Your change of Apr 24 2010 introduced the second argument to common_init,
and used it to pass 1 (use_prev_instr set) from unw_init_local and 0
(use_prev_instr clear) from unw_init_remote. I don’t know how those
choices were made, but for my purposes, the choice for unw_init_local was
wrong, and I have to change that decision in the copy of libunwind that
we’ll use here. I’d welcome an explanation of that decision.
Doug
On Mar 24, 2017, at 11:37 AM, Lassi Tuura <l...@iki.fi> wrote:
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 <do...@rice.edu> 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
me.
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?
Doug
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