+/*
+ * 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 <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;
+ pid_t pid;
+ 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;
+ }
+ }
+
+ /*
+ * If the open failed because the file did not exist, try creating it
+ * ourself.
+ */
+ pid = fork();
+ if (pid > 0) {
+ int status;
+ /*
+ * Parent: wait for child to return, in which case the shared memory
+ * map will have been created.
+ */
+ pid = wait(&status);
+ if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {
+ goto error;
+ }
+
+ /*
+ * Try to open read-only again after creation.
+ */
+ wait_shm_fd = shm_open(shm_path, O_RDWR, 0);
+ if (wait_shm_fd < 0) {
+ /*
+ * Real-only open did not work. It's a failure that prohibits using
+ * shm.
+ */
+ ERR("Error opening shm %s", shm_path);
+ goto error;
+ }
+ goto end;
+ } else if (pid == 0) {
+ /*
+ * 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) {
+ 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);
+ }
+ exit(EXIT_SUCCESS);
+ }
+ ERR("Error opening shm %s", shm_path);
+ exit(EXIT_FAILURE);
+ } else {
+ return -1;
+ }
+
+end:
+ 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;
+}
projects / lttng-tools.git / commitdiff
