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outdated git snapshot (99e60be), but I'm currently trying to update to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
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