lib: make sure SEGV handler cannot lock up

Just adding -pthread to gcc options changes libc's behaviour, e.g.
making malloc() use proper locking.  This means a SEGV inside malloc()
(e.g. because malloc bookkeeping structures have been damaged by writing
to a broken pointer) can lead to a lockup by the following chain:

- random_function()
- malloc()
--- SEGV
- core_handler()
- zlog_backtrace_sigsafe()
- backtrace()
- malloc()

This will hang forever waiting for the malloc() lock to be released.

Another failure mode is dynamic linking with lazy binding (-z lazy,
default).  Since backtrace() is seldomly used, this means the call to
backtrace() in the core handler can in fact result in the dynamic linker
trying to resolve the "backtrace" symbol, which can also deadlock.

Add several safeguards to prevent any of this from happening.

(Unfortunately, these are not theoretical issues - I found them by
running into them headfirst.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/lib/sigevent.c b/lib/sigevent.c
index 09f07180ce..b2059a17bf 100644
--- a/lib/sigevent.c
+++ b/lib/sigevent.c
@@ -233,6 +233,18 @@ core_handler(int signo
 #endif
 	    )
 {
+  /* make sure we don't hang in here.  default for SIGALRM is terminate.
+   * - if we're in backtrace for more than a second, abort. */
+  struct sigaction sa_default = { .sa_handler = SIG_DFL };
+  sigaction (SIGALRM, &sa_default, NULL);
+
+  sigset_t sigset;
+  sigemptyset (&sigset);
+  sigaddset (&sigset, SIGALRM);
+  sigprocmask (SIG_UNBLOCK, &sigset, NULL);
+
+  alarm (1);
+
   zlog_signal(signo, "aborting..."
 #ifdef SA_SIGINFO
 	      , siginfo, program_counter(context)
@@ -327,6 +339,11 @@ trap_default_signals(void)
 		  act.sa_handler = sigmap[i].handler;
 		  act.sa_flags = 0;
 #endif
+#ifdef SA_RESETHAND
+                  /* don't try to print backtraces recursively */
+                  if (sigmap[i].handler == core_handler)
+                    act.sa_flags |= SA_RESETHAND;
+#endif
 	        }
 	      if (sigaction(sigmap[i].sigs[j],&act,NULL) < 0)
 	        zlog_warn("Unable to set signal handler for signal %d: %s",