Signal-safe tracing without _dl_find_object

[collinjackson93]

We used to have some custom written stack trace-generating code in a project at work that we replaced with Cpptrace. We eventually got bit by ignoring the myriad warnings against generating stack traces in a signal handler without properly considering signal safety and crashed while calling malloc, leading to deadlocks. Unfortunately, the version of glibc that we have access to doesn't include _dl_find_object so we couldn't just migrate to the signal-safe API of Cpptrace.

One of my coworkers ended up making a modification to add a couple new functions to Cpptrace that avoid calling malloc from the main process and only do so from the child. We haven't seen any issues running this modified version of the code for a few months, so I want to offer up the changes in case they're useful for others.

As mentioned above, I wasn't the one that originally made these changes, so I can't answer any questions about why certain changes were made, but I'm happy to help get this change ready for a broader audience if it's deemed useful. Here are the changes and below is output showing that the signal_demo originally only included the addresses before this change and now includes a full stack trace.

$ ./signal_demo
SIGSEGV occurred:
Stack trace (most recent call first):
#0 0x000055555555b384
#1 0x00007ffff6e74d1f
#2 0x000055555555b529
#3 0x00007ffff6ac67e4
#4 0x000055555555b25d

$ ./signal_demo
SIGSEGV occurred:
Stack trace (most recent call first):
#0 0x000055555555b3c4 in handler(int, siginfo_t*, void*) at /home/cjackson/build/github/cpptrace/test/signal_demo.cpp:41:25
#1 0x00007ffff6e74d1f at /lib64/libpthread.so.0
#2 (inlined)          in trace() at /home/cjackson/build/github/cpptrace/test/signal_demo.cpp:15:24
#3 (inlined)          in bar() at /home/cjackson/build/github/cpptrace/test/signal_demo.cpp:19:5
#4 (inlined)          in foo() at /home/cjackson/build/github/cpptrace/test/signal_demo.cpp:23:5
#5 0x000055555555b569 in main at /home/cjackson/build/github/cpptrace/test/signal_demo.cpp:96:5
#6 0x00007ffff6ac67e4 in __libc_start_main at /lib64/libc.so.6
#7 0x000055555555b29d in _start at /build/cjackson/github/cpptrace/build/signal_demo