Complete change of the source directory tree

[lttng-tools.git] / src / bin / lttng-sessiond / shm.c

diff --git a/src/bin/lttng-sessiond/shm.c b/src/bin/lttng-sessiond/shm.c
new file mode 100644 (file)
index 0000000..64c712e
--- /dev/null
+++ b/src/bin/lttng-sessiond/shm.c
@@ -0,0 +1,160 @@
+/*
+ * Copyright (C)  2011 - David Goulet <david.goulet@polymtl.ca>
+ *                       Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; only version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program; if not, write to the Free Software Foundation, Inc., 59 Temple
+ * Place - Suite 330, Boston, MA  02111-1307, USA.
+ */
+
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <limits.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <urcu.h>
+
+#include <common/lttngerr.h>
+
+#include "shm.h"
+
+/*
+ * Using fork to set umask in the child process (not multi-thread safe). We
+ * deal with the shm_open vs ftruncate race (happening when the sessiond owns
+ * the shm and does not let everybody modify it, to ensure safety against
+ * shm_unlink) by simply letting the mmap fail and retrying after a few
+ * seconds. For global shm, everybody has rw access to it until the sessiond
+ * starts.
+ */
+static int get_wait_shm(char *shm_path, size_t mmap_size, int global)
+{
+       int wait_shm_fd, ret;
+       mode_t mode;
+
+       /* Default permissions */
+       mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP;
+
+       /* Change owner of the shm path */
+       if (global) {
+               ret = chown(shm_path, 0, 0);
+               if (ret < 0) {
+                       if (errno != ENOENT) {
+                               perror("chown wait shm");
+                               goto error;
+                       }
+               }
+
+               /*
+                * If global session daemon, any application can register so the shm
+                * needs to be set in read-only mode for others.
+                */
+               mode |= S_IROTH;
+       } else {
+               ret = chown(shm_path, getuid(), getgid());
+               if (ret < 0) {
+                       if (errno != ENOENT) {
+                               perror("chown wait shm");
+                               goto error;
+                       }
+               }
+       }
+
+       /*
+        * Set permissions to the shm even if we did not create the shm.
+        */
+       ret = chmod(shm_path, mode);
+       if (ret < 0) {
+               if (errno != ENOENT) {
+                       perror("chmod wait shm");
+                       goto error;
+               }
+       }
+
+       /*
+        * We're alone in a child process, so we can modify the process-wide
+        * umask.
+        */
+       umask(~mode);
+
+       /*
+        * Try creating shm (or get rw access). We don't do an exclusive open,
+        * because we allow other processes to create+ftruncate it concurrently.
+        */
+       wait_shm_fd = shm_open(shm_path, O_RDWR | O_CREAT, mode);
+       if (wait_shm_fd < 0) {
+               perror("shm_open wait shm");
+               goto error;
+       }
+
+       ret = ftruncate(wait_shm_fd, mmap_size);
+       if (ret < 0) {
+               perror("ftruncate wait shm");
+               exit(EXIT_FAILURE);
+       }
+
+       ret = fchmod(wait_shm_fd, mode);
+       if (ret < 0) {
+               perror("fchmod");
+               exit(EXIT_FAILURE);
+       }
+
+       DBG("Got the wait shm fd %d", wait_shm_fd);
+
+       return wait_shm_fd;
+
+error:
+       DBG("Failing to get the wait shm fd");
+
+       return -1;
+}
+
+/*
+ * Return the wait shm mmap for UST application notification. The global
+ * variable is used to indicate if the the session daemon is global
+ * (root:tracing) or running with an unprivileged user.
+ *
+ * This returned value is used by futex_wait_update() in futex.c to WAKE all
+ * waiters which are UST application waiting for a session daemon.
+ */
+char *shm_ust_get_mmap(char *shm_path, int global)
+{
+       size_t mmap_size = sysconf(_SC_PAGE_SIZE);
+       int wait_shm_fd, ret;
+       char *wait_shm_mmap;
+
+       wait_shm_fd = get_wait_shm(shm_path, mmap_size, global);
+       if (wait_shm_fd < 0) {
+               goto error;
+       }
+
+       wait_shm_mmap = mmap(NULL, mmap_size, PROT_WRITE | PROT_READ,
+                       MAP_SHARED, wait_shm_fd, 0);
+
+       /* close shm fd immediately after taking the mmap reference */
+       ret = close(wait_shm_fd);
+       if (ret) {
+               perror("Error closing fd");
+       }
+
+       if (wait_shm_mmap == MAP_FAILED) {
+               DBG("mmap error (can be caused by race with ust).");
+               goto error;
+       }
+
+       return wait_shm_mmap;
+
+error:
+       return NULL;
+}