Make backtrace handler handle signals correctly.

This meant moving a fair bit of code around, and writing a signal
cleanup function.  Still pretty nice from what I can tell, though.

src/common/backtrace.c

@@ -7,6 +7,8 @@
 #include "util.h"
 #include "torlog.h"
 
+#define __USE_GNU
+
 #ifdef HAVE_EXECINFO_H
 #include <execinfo.h>
 #endif
@@ -16,6 +18,15 @@
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>
 #endif
+#ifdef HAVE_SIGNAL_H
+#include <signal.h>
+#endif
+#ifdef HAVE_UCONTEXT_H
+#include <ucontext.h>
+#endif
+#ifdef HAVE_SYS_UCONTEXT_H
+#include <sys/ucontext.h>
+#endif
 
 #if defined(HAVE_EXECINFO_H) && defined(HAVE_BACKTRACE) && \
   defined(HAVE_BACKTRACE_SYMBOLS_FD) && defined(HAVE_SIGACTION)
@@ -26,53 +37,135 @@
 #define NO_BACKTRACE_IMPL
 #endif
 
+/** Version of Tor to report in backtrace messages. */
 static char *bt_version = NULL;
 
 #ifdef USE_BACKTRACE
+/** Largest stack depth to try to dump. */
 #define MAX_DEPTH 256
+/** Static allocation of stack to dump. This is static so we avoid stack
+ * pressure. */
 static void *cb_buf[MAX_DEPTH];
 
-/**DOCDOC*/
-void
-dump_backtrace(const char *msg)
+/** Change a stacktrace in <b>stack</b> of depth <b>depth</b> so that it will
+ * log the correct function from which a signal was received with context
+ * <b>ctx</b>.  (When we get a signal, the current function will not have
+ * called any other function, and will therefore have not pushed its address
+ * onto the stack.  Fortunately, we usually have the program counter in the
+ * ucontext_t structure.
+ */
+static void
+clean_backtrace(void **stack, int depth, const ucontext_t *ctx)
 {
-  int depth;
-  const int *fds;
-  int n_fds;
-  int i;
+#ifdef PC_FROM_UCONTEXT
+#if defined(__linux__)
+  const int n = 1;
+#elif defined(__darwin__) || defined(__APPLE__) || defined(__OpenBSD__) \
+  || defined(__FreeBSD__)
+  const int n = 2;
+#else
+  const int n = 1;
+#endif
+  if (depth <= n)
+    return;
 
-  if (!msg) msg = "unspecified crash";
+  stack[n] = (void*) ctx->PC_FROM_UCONTEXT;
+#else
+  (void) depth;
+  (void) ctx;
+#endif
+}
+
+/** Log a message <b>msg</b> at <b>severity</b> in <b>domain</b>, and follow
+ * that with a backtrace log. */
+void
+log_backtrace(int severity, int domain, const char *msg)
+{
+  int depth = backtrace(cb_buf, MAX_DEPTH);
+  char **symbols = backtrace_symbols(cb_buf, depth);
+  int i;
+  tor_log(severity, domain, "%s. Stack trace:", msg);
+  if (!symbols) {
+    tor_log(severity, domain, "    Unable to generate backtrace.");
+    return;
+  }
+  for (i=0; i < depth; ++i) {
+    tor_log(severity, domain, "    %s", symbols[i]);
+  }
+  free(symbols);
+}
+
+static void crash_handler(int sig, siginfo_t *si, void *ctx_)
+  __attribute__((noreturn));
+
+/** Signal handler: write a crash message with a stack trace, and die. */
+static void
+crash_handler(int sig, siginfo_t *si, void *ctx_)
+{
+  char buf[40];
+  int depth;
+  ucontext_t *ctx = (ucontext_t *) ctx_;
+  int n_fds, i;
+  const int *fds = NULL;
+
+  (void) si;
 
   depth = backtrace(cb_buf, MAX_DEPTH);
+  /* Clean up the top stack frame so we get the real function
+   * name for the most recently failing function. */
+  clean_backtrace(cb_buf, depth, ctx);
 
-  tor_log_err_sigsafe(bt_version, " died: ", msg, "\n",
+  format_dec_number_sigsafe((unsigned)sig, buf, sizeof(buf));
+
+  tor_log_err_sigsafe(bt_version, " died: Caught signal ", buf, "\n",
                       NULL);
+
   n_fds = tor_log_get_sigsafe_err_fds(&fds);
   for (i=0; i < n_fds; ++i)
     backtrace_symbols_fd(cb_buf, depth, fds[i]);
+
+  abort();
 }
 
-/**DOCDOC*/
+/** Install signal handlers as needed so that when we crash, we produce a
+ * useful stack trace. Return 0 on success, -1 on failure. */
 static int
 install_bt_handler(void)
 {
-  /*XXXX add signal handlers */
   /*XXXX make this idempotent */
-  return 0;
+  int trap_signals[] = { SIGSEGV, SIGILL, SIGFPE, SIGBUS, SIGSYS,
+                         SIGIO, -1 };
+  int i, rv=0;
+
+  struct sigaction sa;
+  memset(&sa, 0, sizeof(sa));
+  sa.sa_sigaction = crash_handler;
+  sa.sa_flags = SA_SIGINFO;
+  sigfillset(&sa.sa_mask);
+
+  for (i = 0; trap_signals[i] >= 0; ++i) {
+    if (sigaction(trap_signals[i], &sa, NULL) == -1) {
+      log_warn(LD_BUG, "Sigaction failed: %s", strerror(errno));
+      rv = -1;
+    }
+  }
+  return rv;
 }
-/**DOCDOC*/
+
+/** Uninstall crash handlers. */
 static void
 remove_bt_handler(void)
 {
+  /*XXXX writeme*/
 }
 #endif
 
 #ifdef NO_BACKTRACE_IMPL
-/**DOCDOC*/
+/**DOCDOC */
 void
-dump_backtrace(const char *msg)
+log_backtrace(int severity, int domain, const char *msg)
 {
-  tor_log_err_sigsafe(bt_version, " died: ", msg, "\n", NULL);
+  tor_log(severity, domain, "%s. (Stack trace not available)", msg);
 }
 
 /**DOCDOC*/
@@ -95,8 +188,8 @@ configure_backtrace_handler(const char *tor_version)
 {
   tor_free(bt_version);
   if (!tor_version)
-    tor_version = "Tor";
-  bt_version = tor_strdup(tor_version);
+    tor_version = "";
+  tor_asprintf(&bt_version, "Tor %s", tor_version);
 
   return install_bt_handler();
 }

src/common/backtrace.h

@@ -4,7 +4,7 @@
 #ifndef TOR_BACKTRACE_H
 #define TOR_BACKTRACE_H
 
-void dump_backtrace(const char *msg);
+void log_backtrace(int severity, int domain, const char *msg);
 int configure_backtrace_handler(const char *tor_version);
 void clean_up_backtrace_handler(void);
 

src/common/util.c

@@ -108,9 +108,7 @@ tor_assertion_failed_(const char *fname, unsigned int line,
   tor_snprintf(buf, sizeof(buf),
                "Assertion %s failed in %s at %s:%u",
                expr, func, fname, line);
-  dump_backtrace(buf);
-  fprintf(stderr,"%s:%u: %s: Assertion %s failed; aborting.\n",
-          fname, line, func, expr);
+  log_backtrace(LOG_ERR, LD_BUG, buf);
 }
 
 /* =====