Re: [Libunwind-devel] unwinding from signal handler (ARM)

[Ken Werner]

On 08/19/2011 02:39 PM, Sven Neumann wrote:
> Hi,
>
> we are using libunwind for a while now to obtain backtraces for crash
> reports. This on an ARM with EABI. Right now we are using a somewhat
> outdated git snapshot (99e60be), but I'm currently trying to update to
> 1.0-rc1. I don't have much success though.
>
> With 99e60be we had the problem that we could not unwind from a signal
> handler. But that is exactly what we need to do since we want to get a
> backtrace from the signal handler that is called when the application
> crashes (usually signal 11). We found a workaround though, instead of
> calling unw_getcontext() to get an initial unw_context_t pointer to pass
> to unw_init_local(), we use the pointer that we got from the signal
> handler. Here's some pseudo code for this:
>
>      struct sigaction new_action;
>
>      new_action.sa_sigaction = logCrash;
>      sigemptyset (&new_action.sa_mask);
>      new_action.sa_flags = SA_SIGINFO;
>
>      sigaction(SIGSEGV,&new_action, NULL);
>      sigaction(SIGFPE,&new_action, NULL);
>
> and in logCrash we do something along the lines of:
>
> static void
> logCrash(int sig, siginfo_t *info, void *ctx)
> {
>    unw_init_local(&cursor, ctx);
>
>    do
>    {
>      char        name[128];
>      unw_word_t  valp;
>      unw_word_t  offset;
>   unw_getcontext()
>      if (unw_get_reg(&cursor, UNW_REG_IP,&valp) != 0)
>        break;
>
>      /* log function name etc. here */
>
>      ret = unw_step (&cursor);
>    }
>    while (ret>  0);
> }
>
> This worked reasonably well giving us a nice stack trace most of the
> time.

Hi Sven,

This code is looking strange to me because unw_getcontext() gets called 
in a loop. But we might focus on the new code instead.

> Now I've updated libunwind to 1.0-rc1 and this trick doesn't seem to
> work any longer. The code appears to crash in libunwind if used as
> above. I had a look at the ARM implementation and found that there is
> now code that's supposed to unw_step a signal handler. So I tried
> without our hack and just use unw_getcontext(). Now it doesn't crash any
> longer, but it also doesn't unwind over the signal handler. All I get is
> a stack trace like this:
>
> 0x4016cde0 logUnwind() from /usr/lib/libraumfeld-1.0.so.0
> 0x40205654 ?? from /usr/lib/libunwind.so.7
>
> Any ideas what we could do?

Hmm, this is interesting. How does your system look like (which libc, 
kernel version) and are you stepping over RT or non-RT signal frames? It 
would be helpful if you could share some code that shows the described 
behavior.
In the meanwhile I'll prepare a simple testcase where I'll verify that 
it's working on my system.

Regards
Ken