Re: [Libunwind-devel] unwind within a signal handler (in linux / arm)

[Harald Servat]

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Al 26/04/12 11:40, En/na Ken Werner ha escrit:
> On 04/19/2012 05:44 PM, Harald Servat wrote:
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>> Dear all,
>>
>>    I've just downloaded a GIT clone of the libunwind and installed it in
>> a linux/ARM machine.
>>
>>    We use libunwind from an library that monitors an application. This
>> library uses periodical sampling to collect some information (including
>> the callstack). I've found that libunwind segfaults when is issued from
>> the signal handler. I'm attaching the gdb output below.
>>
>>    Has anyone seen this issue before?
>>
>> Thank you in advance.
>>
>> warning: Breakpoint address adjusted from 0x4002eb25 to 0x4002eb24.
>> Core was generated by `./mpi_ping'.
>> Program terminated with signal 11, Segmentation fault.
>> #0  dwarf_get (cursor=0xbe9edbf8) at
>> ../include/tdep-arm/libunwind_i.h:114
>> 114      *val = *(unw_word_t *) DWARF_GET_LOC (loc);
>> (gdb) where
>> #0  dwarf_get (cursor=0xbe9edbf8) at
>> ../include/tdep-arm/libunwind_i.h:114
>> #1  _ULarm_step (cursor=0xbe9edbf8) at arm/Gstep.c:228
>> #2  0x4017e8c2 in trace_callers (time=<value optimized out>, offset=6,
>> type=1) at calltrace.c:78
>> #3  0x4017ebe6 in Extrae_SamplingHandler (address=0xbe9f1d50) at
>> sampling.c:83
>> #4  0x4017ec68 in TimeSamplingHandler (sig=<value optimized out>,
>> siginfo=0x0, context=<value optimized out>) at sampling.c:151
>> #5<signal handler called>
>> #6  0x42112e9a in MPIDI_CH3I_Progress () from /usr/lib/libmpich.so.3
>> #7  0x42160038 in MPID_nem_network_poll () from /usr/lib/libmpich.so.3
>> #8  0x42160038 in MPID_nem_network_poll () from /usr/lib/libmpich.so.3
>> Backtrace stopped: previous frame identical to this frame (corrupt
>> stack?)
> 
> Hi Harald,
> 
> It looks like libunwind wasn't able to walk a particular frame of the
> stack and segfaults when attempting to parse APCS frame (which won't
> work in case the ARM EABI is used). By default libunwind uses DWARF
> information. If that fails it tries to use the ARM specific unwind
> information (requires your code to be compiled with -funwind-tables) and
> as a last resort it falls back on APCS frame parsing. The unwind
> mechanism used by libunwind can be controlled at runtime using the
> following environment variable:
>   UNW_ARM_UNWIND_METHOD=255
> In case you want restrict libunwind to DWARF only:
>   UNW_ARM_UNWIND_METHOD=1
> To restrict it to ARM specific unwind information only:
>   UNW_ARM_UNWIND_METHOD=4
> 
> Of course this doesn't explain why unwinding via DWARF and ARM-extbl
> failed in the first place. Maybe there is no correct DWARF information
> and this piece of code wasn't compiled with -funwind-tables but I'm just
> guessing. In case of signals there are a couple of things involved - the
> kernel version (pre-/post 2.6.18), the type of the signal (rt vs-
> non-rt) and so on. I've attached a simple testcase that would be easy to
> debug. Could you check whether it fails on your system?
> 
> Regards,
> Ken
> 

Arun, Ken,

  thank you very much for your comments (and test-cases!). I'll take a
look into this when the machine becomes available again. What I can say
for sure is that the application was not compiled with -funwind-tables.

Regards.
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