compat send no SIGPIPE: multithread-safe

The current implementation of the no-SIGPIPE send in the compatibility
layer has side-effects on multithreaded processes due to use of
sigaction(). Although multithread-safety is not strictly needed since
Babeltrace is single-threaded for now, there is no reason to keep this
limitation deeply rooted in a compatibility layer.

Use the multithreaded-safe algorithm to catch SIGPIPE implemented in
LTTng-UST for the write() system call for platforms that do not have
MSG_NOSIGNAL. It was originally implented in LTTng-UST as part of the
ring buffer wakeup. This is a re-implementation of this same algorithm
under MIT license. It uses signal masks and sigtimedwait.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>

diff --git a/include/babeltrace/compat/send.h b/include/babeltrace/compat/send.h
index 98e1feb80e44f55550c7f4f352e99f960ab31aa9..3c6d01a14e7dfe53c6a4d49a081bb6e6c5a1cc4e 100644 (file)
--- a/include/babeltrace/compat/send.h
+++ b/include/babeltrace/compat/send.h
@@ -5,6 +5,7 @@
  * babeltrace/compat/send.h
  *
  * Copyright (C) 2015  Michael Jeanson <mjeanson@efficios.com>
+ *               2015  Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -27,9 +28,7 @@
 
 /*
  * This wrapper is used on platforms that have no way of ignoring SIGPIPE
- * during a send(). Instead, we set the signal action to ignore. This is OK
- * in a single-threaded app, but would be problematic in a multi-threaded app
- * since sigaction applies to all threads.
+ * during a send().
  */
 
 #ifndef MSG_NOSIGNAL
@@ -45,32 +44,69 @@ ssize_t bt_send_nosigpipe(int fd, const void *buffer, size_t size)
        return send(fd, buffer, size, MSG_NOSIGNAL);
 }
 #else
+
+#include <signal.h>
+
 static inline
 ssize_t bt_send_nosigpipe(int fd, const void *buffer, size_t size)
 {
        ssize_t sent;
        int saved_err;
-       struct sigaction act, oldact;
+       sigset_t sigpipe_set, pending_set, old_set;
+       int sigpipe_was_pending;
 
-       /* Set SIGPIPE action to ignore and save current signal action */
-       act.sa_handler = SIG_IGN;
-       if (sigaction(SIGPIPE, &act, &oldact)) {
-               perror("sigaction");
-               sent = -1;
-               goto end;
+       /*
+        * Discard the SIGPIPE from send(), 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.
+        */
+       if (sigemptyset(&pending_set)) {
+               return -1;
+       }
+       /*
+        * sigpending returns the mask of signals that are _both_
+        * blocked for the thread _and_ pending for either the thread or
+        * the entire process.
+        */
+       if (sigpending(&pending_set)) {
+               return -1;
+       }
+       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) {
+               if (sigemptyset(&sigpipe_set)) {
+                       return -1;
+               }
+               if (sigaddset(&sigpipe_set, SIGPIPE)) {
+                       return -1;
+               }
+               if (pthread_sigmask(SIG_BLOCK, &sigpipe_set, &old_set)) {
+                       return -1;
+               }
        }
 
-       /* Send and save errno */
+       /* Send and save errno. */
        sent = send(fd, buffer, size, 0);
        saved_err = errno;
 
-       /* Restore original signal action */
-       if (sigaction(SIGPIPE, &oldact, NULL)) {
-               perror("sigaction");
-               sent = -1;
-               goto end;
-       }
+       if (sent == -1 && errno == EPIPE && !sigpipe_was_pending) {
+               struct timespec timeout = { 0, 0 };
+               int ret;
 
+               do {
+                       ret = sigtimedwait(&sigpipe_set, NULL,
+                               &timeout);
+               } while (ret == -1 && errno == EINTR);
+       }
+       if (!sigpipe_was_pending) {
+               if (pthread_sigmask(SIG_SETMASK, &old_set, NULL)) {
+                       return -1;
+               }
+       }
        /* Restore send() errno */
        errno = saved_err;
 end: