Fix UST SIGPIPE handling

When the consumerd dies (from a SIGKILL), it may close all of its file
descriptors rather abruptly.

We ensured that the UST command threads have all signals blocked, and
they use MSG_NOSIGNAL when sending messages to the sessiond over
sockets.

However, the consumer scheme uses a pipe(2) to transport the "wakeup"
info from the application tracing site to the consumer daemon. It may
send a SIGPIPE to the application in that case, which could kill the
application, an unwanted side-effect.

Block thread SIGPIPE around write() and wait for the signal to fix this.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Christian Babeux <christian.babeux@efficios.com>
CC: David Goulet <dgoulet@efficios.com>

diff --git a/libringbuffer/frontend_internal.h b/libringbuffer/frontend_internal.h
index 6d1a75bdceef83e06d982097b94e4cdc8d8bcf59..2d3b1071f9e4f10565ca52b0385c65359a8866f2 100644 (file)
--- a/libringbuffer/frontend_internal.h
+++ b/libringbuffer/frontend_internal.h
@@ -32,6 +32,8 @@
  */
 
 #include <urcu/compiler.h>
+#include <signal.h>
+#include <pthread.h>
 
 #include <lttng/ringbuffer-config.h>
 #include "backend_types.h"
@@ -397,7 +399,9 @@ void lib_ring_buffer_check_deliver(const struct lttng_ust_lib_ring_buffer_config
                                int wakeup_fd = shm_get_wakeup_fd(handle, &buf->self._ref);
 
                                if (wakeup_fd >= 0) {
-                                       int ret;
+                                       sigset_t sigpipe_set, pending_set, old_set;
+                                       int ret, sigpipe_was_pending = 0;
+
                                        /*
                                         * Wake-up the other end by
                                         * writing a null byte in the
@@ -416,13 +420,55 @@ void lib_ring_buffer_check_deliver(const struct lttng_ust_lib_ring_buffer_config
                                         * 2) check if there is data in
                                         *    the buffer.
                                         * 3) wait on the pipe (poll).
+                                        *
+                                        * Discard the SIGPIPE from write(), not
+                                        * disturbing any SIGPIPE that might be
+                                        * already pending. If a bogus SIGPIPE
+                                        * is sent to the entire process
+                                        * concurrently by a malicious user, it
+                                        * may be simply discarded.
+                                        */
+                                       ret = sigemptyset(&pending_set);
+                                       assert(!ret);
+                                       /*
+                                        * sigpending returns the mask
+                                        * of signals that are _both_
+                                        * blocked for the thread _and_
+                                        * pending for either the thread
+                                        * or the entire process.
                                         */
+                                       ret = sigpending(&pending_set);
+                                       assert(!ret);
+                                       sigpipe_was_pending = sigismember(&pending_set, SIGPIPE);
+                                       /*
+                                        * If sigpipe was pending, it
+                                        * means it was already blocked,
+                                        * so no need to block it.
+                                        */
+                                       if (!sigpipe_was_pending) {
+                                               ret = sigemptyset(&sigpipe_set);
+                                               assert(!ret);
+                                               ret = sigaddset(&sigpipe_set, SIGPIPE);
+                                               assert(!ret);
+                                               ret = pthread_sigmask(SIG_BLOCK, &sigpipe_set, &old_set);
+                                               assert(!ret);
+                                       }
                                        do {
                                                ret = write(wakeup_fd, "", 1);
                                        } while (ret == -1L && errno == EINTR);
+                                       if (ret == -1L && errno == EPIPE && !sigpipe_was_pending) {
+                                               struct timespec timeout = { 0, 0 };
+                                               do {
+                                                       ret = sigtimedwait(&sigpipe_set, NULL,
+                                                               &timeout);
+                                               } while (ret == -1L && errno == EINTR);
+                                       }
+                                       if (!sigpipe_was_pending) {
+                                               ret = pthread_sigmask(SIG_SETMASK, &old_set, NULL);
+                                               assert(!ret);
+                                       }
                                }
                        }
-
                }
        }
 }