[ovs-dev,PATCHv3] fatal-signal: Catch SIGSEGV and print backtrace.

diff --git a/.travis.yml b/.travis.yml
index 68026312ba84..b547eb041791 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -25,6 +25,7 @@  addons:
       - selinux-policy-dev
       - libunbound-dev
       - libunbound-dev:i386
+      - libunwind-dev
 
 before_install: ./.travis/${TRAVIS_OS_NAME}-prepare.sh
 
diff --git a/configure.ac b/configure.ac
index 1d45c4fdd153..15922418062b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -139,6 +139,7 @@  OVS_LIBTOOL_VERSIONS
 OVS_CHECK_CXX
 AX_FUNC_POSIX_MEMALIGN
 OVS_CHECK_UNBOUND
+OVS_CHECK_UNWIND
 
 OVS_CHECK_INCLUDE_NEXT([stdio.h string.h])
 AC_CONFIG_FILES([
diff --git a/lib/backtrace.c b/lib/backtrace.c
index 5cb2954f816b..265af342ad50 100644
--- a/lib/backtrace.c
+++ b/lib/backtrace.c
@@ -15,7 +15,11 @@ 
  */
 
 #include <config.h>
+#include <errno.h>
+#include <fcntl.h>
 #include <inttypes.h>
+#include <string.h>
+#include <unistd.h>
 
 #include "backtrace.h"
 #include "openvswitch/vlog.h"
@@ -76,3 +80,37 @@  log_backtrace_at(const char *msg, const char *where)
 
     ds_destroy(&ds);
 }
+
+#ifdef HAVE_UNWIND
+void
+log_received_backtrace(int fd) {
+    int byte_read;
+    struct unw_backtrace backtrace[UNW_MAX_DEPTH];
+
+    VLOG_WARN("%s fd %d", __func__, fd);
+    fcntl(fd, F_SETFL, O_NONBLOCK);
+    memset(backtrace, 0, UNW_MAX_BUF);
+
+    byte_read = read(fd, backtrace, UNW_MAX_BUF);
+    if (byte_read < 0) {
+        VLOG_ERR("Read fd %d failed: %s", fd,
+                 ovs_strerror(errno));
+    } else if (byte_read > 0) {
+        VLOG_WARN("SIGSEGV detected, backtrace:");
+        for (int i = 0; i < UNW_MAX_DEPTH; i++) {
+            if (backtrace[i].func[0] == 0) {
+                break;
+            }
+            VLOG_WARN("0x%016lx <%s+0x%lx>\n",
+                      backtrace[i].ip,
+                      backtrace[i].func,
+                      backtrace[i].offset);
+        }
+    }
+}
+#else /* !HAVE_UNWIND */
+void
+log_received_backtrace(int daemonize_fd OVS_UNUSED) {
+    VLOG_WARN("Backtrace using libunwind not supported.");
+}
+#endif /* HAVE_UNWIND */
diff --git a/lib/backtrace.h b/lib/backtrace.h
index 384f2700d94c..5708bf9c6831 100644
--- a/lib/backtrace.h
+++ b/lib/backtrace.h
@@ -20,6 +20,11 @@ 
 #include <stdint.h>
 #include "openvswitch/dynamic-string.h"
 
+#ifdef HAVE_UNWIND
+#define UNW_LOCAL_ONLY
+#include <libunwind.h>
+#endif
+
 /* log_backtrace() will save the backtrace of a running program
  * into the log at the DEBUG level.
  *
@@ -68,7 +73,21 @@  struct backtrace {
     uintptr_t frames[BACKTRACE_MAX_FRAMES];
 };
 
+#ifdef HAVE_UNWIND
+#define UNW_MAX_DEPTH 32
+#define UNW_MAX_FUNCN 32
+#define UNW_MAX_BUF \
+    (UNW_MAX_DEPTH * sizeof(struct unw_backtrace))
+
+struct unw_backtrace {
+    char func[UNW_MAX_FUNCN];
+    unw_word_t ip;
+    unw_word_t offset;
+};
+#endif
+
 void backtrace_capture(struct backtrace *);
 void log_backtrace_at(const char *msg, const char *where);
+void log_received_backtrace(int fd);
 
 #endif /* backtrace.h */
diff --git a/lib/daemon-private.h b/lib/daemon-private.h
index 8bc71e2a638d..4e0667601001 100644
--- a/lib/daemon-private.h
+++ b/lib/daemon-private.h
@@ -19,6 +19,7 @@ 
 
 extern bool detach;
 extern char *pidfile;
+extern int daemonize_fd;
 
 char *make_pidfile_name(const char *name);
 
diff --git a/lib/daemon-unix.c b/lib/daemon-unix.c
index 6169763c294c..7e48630f0e93 100644
--- a/lib/daemon-unix.c
+++ b/lib/daemon-unix.c
@@ -15,6 +15,7 @@ 
  */
 
 #include <config.h>
+#include "backtrace.h"
 #include "daemon.h"
 #include "daemon-private.h"
 #include <errno.h>
@@ -75,7 +76,7 @@  static bool overwrite_pidfile;
 static bool chdir_ = true;
 
 /* File descriptor used by daemonize_start() and daemonize_complete(). */
-static int daemonize_fd = -1;
+int daemonize_fd = -1;
 
 /* --monitor: Should a supervisory process monitor the daemon and restart it if
  * it dies due to an error signal? */
@@ -291,8 +292,7 @@  fork_and_wait_for_startup(int *fdp, pid_t *child_pid)
                 OVS_NOT_REACHED();
             }
         }
-        close(fds[0]);
-        *fdp = -1;
+        *fdp = fds[0];
     } else if (!pid) {
         /* Running in child process. */
         close(fds[0]);
@@ -313,8 +313,6 @@  fork_notify_startup(int fd)
         if (error) {
             VLOG_FATAL("pipe write failed (%s)", ovs_strerror(error));
         }
-
-        close(fd);
     }
 }
 
@@ -393,6 +391,8 @@  monitor_daemon(pid_t daemon_pid)
                     }
                 }
 
+                log_received_backtrace(daemonize_fd);
+
                 /* Throttle restarts to no more than once every 10 seconds. */
                 if (time(NULL) < last_restart + 10) {
                     VLOG_WARN("%s, waiting until 10 seconds since last "
@@ -508,7 +508,6 @@  daemonize_complete(void)
         detached = true;
 
         fork_notify_startup(daemonize_fd);
-        daemonize_fd = -1;
         daemonize_post_detach();
     }
 }
diff --git a/lib/fatal-signal.c b/lib/fatal-signal.c
index 3b905b6de766..7733850d5c4b 100644
--- a/lib/fatal-signal.c
+++ b/lib/fatal-signal.c
@@ -14,6 +14,7 @@ 
  * limitations under the License.
  */
 #include <config.h>
+#include "backtrace.h"
 #include "fatal-signal.h"
 #include <errno.h>
 #include <signal.h>
@@ -34,6 +35,10 @@ 
 
 #include "openvswitch/type-props.h"
 
+#ifdef HAVE_UNWIND
+#include "daemon-private.h"
+#endif
+
 #ifndef SIG_ATOMIC_MAX
 #define SIG_ATOMIC_MAX TYPE_MAXIMUM(sig_atomic_t)
 #endif
@@ -42,7 +47,8 @@  VLOG_DEFINE_THIS_MODULE(fatal_signal);
 
 /* Signals to catch. */
 #ifndef _WIN32
-static const int fatal_signals[] = { SIGTERM, SIGINT, SIGHUP, SIGALRM };
+static const int fatal_signals[] = { SIGTERM, SIGINT, SIGHUP, SIGALRM,
+                                     SIGSEGV };
 #else
 static const int fatal_signals[] = { SIGTERM };
 #endif
@@ -151,6 +157,44 @@  fatal_signal_add_hook(void (*hook_cb)(void *aux), void (*cancel_cb)(void *aux),
     ovs_mutex_unlock(&mutex);
 }
 
+#ifdef HAVE_UNWIND
+/* Send the backtrace buffer to monitor thread.
+ *
+ * Note that this runs in the signal handling context, any system
+ * library functions used here must be async-signal-safe.
+ */
+static inline void
+send_backtrace_to_monitor(void) {
+    int dep;
+    struct unw_backtrace unw_bt[UNW_MAX_DEPTH];
+    unw_cursor_t cursor;
+    unw_context_t uc;
+
+    if (daemonize_fd == -1) {
+        return;
+    }
+
+    dep = 0;
+    unw_getcontext(&uc);
+    unw_init_local(&cursor, &uc);
+
+    while (dep < UNW_MAX_DEPTH && unw_step(&cursor)) {
+        memset(unw_bt[dep].func, 0, UNW_MAX_FUNCN);
+        unw_get_reg(&cursor, UNW_REG_IP, &unw_bt[dep].ip);
+        unw_get_proc_name(&cursor, unw_bt[dep].func, UNW_MAX_FUNCN,
+                          &unw_bt[dep].offset);
+       dep++;
+    }
+
+    ignore(write(daemonize_fd, unw_bt, dep * sizeof(struct unw_backtrace)));
+}
+#else
+static inline void
+send_backtrace_to_monitor(void) {
+    /* Nothing. */
+}
+#endif
+
 /* Handles fatal signal number 'sig_nr'.
  *
  * Ordinarily this is the actual signal handler.  When other code needs to
@@ -164,6 +208,11 @@  void
 fatal_signal_handler(int sig_nr)
 {
 #ifndef _WIN32
+    if (sig_nr == SIGSEGV) {
+        signal(sig_nr, SIG_DFL); /* Set it back immediately. */
+        send_backtrace_to_monitor();
+        raise(sig_nr);
+    }
     ignore(write(signal_fds[1], "", 1));
 #else
     SetEvent(wevent);
diff --git a/m4/openvswitch.m4 b/m4/openvswitch.m4
index ba912e58780a..465d9c5ba368 100644
--- a/m4/openvswitch.m4
+++ b/m4/openvswitch.m4
@@ -700,3 +700,13 @@  AC_DEFUN([OVS_CHECK_UNBOUND],
    fi
    AM_CONDITIONAL([HAVE_UNBOUND], [test "$HAVE_UNBOUND" = yes])
    AC_SUBST([HAVE_UNBOUND])])
+
+dnl Checks for libunwind.
+AC_DEFUN([OVS_CHECK_UNWIND],
+  [AC_CHECK_LIB(unwind, unw_backtrace, [HAVE_UNWIND=yes], [HAVE_UNWIND=no])
+   if test "$HAVE_UNWIND" = yes; then
+     AC_DEFINE([HAVE_UNWIND], [1], [Define to 1 if unwind is detected.])
+     LIBS="$LIBS -lunwind"
+   fi
+   AM_CONDITIONAL([HAVE_UNWIND], [test "$HAVE_UNWIND" = yes])
+   AC_SUBST([HAVE_UNWIND])])