/*
- * Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca>
- * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
+ * Copyright (C) 2011 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, version 2 only,
- * as published by the Free Software Foundation.
+ * SPDX-License-Identifier: GPL-2.0-only
*
- * 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.,
- * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/
-#define _GNU_SOURCE
+#define _LGPL_SOURCE
#include <fcntl.h>
#include <limits.h>
#include <sys/mman.h>
/*
* Try creating shm (or get rw access). We don't do an exclusive open,
* because we allow other processes to create+ftruncate it concurrently.
+ *
+ * A sysctl, fs.protected_regular may prevent the session daemon from
+ * opening a previously created shm when the O_CREAT flag is provided.
+ * Systemd enables this ABI-breaking change by default since v241.
+ *
+ * First, attempt to use the create-or-open semantic that is
+ * desired here. If this fails with EACCES, work around this broken
+ * behaviour and attempt to open the shm without the O_CREAT flag.
+ *
+ * The two attempts are made in this order since applications are
+ * expected to race with the session daemon to create this shm.
+ * Attempting an shm_open() without the O_CREAT flag first could fail
+ * because the file doesn't exist. It could then be created by an
+ * application, which would cause a second try with the O_CREAT flag to
+ * fail with EACCES.
+ *
+ * Note that this introduces a new failure mode where a user could
+ * launch an application (creating the shm) and unlink the shm while
+ * the session daemon is launching, causing the second attempt
+ * to fail. This is not recovered-from as unlinking the shm will
+ * prevent userspace tracing from succeeding anyhow: the sessiond would
+ * use a now-unlinked shm, while the next application would create
+ * a new named shm.
*/
wait_shm_fd = shm_open(shm_path, O_RDWR | O_CREAT, mode);
if (wait_shm_fd < 0) {
- PERROR("shm_open wait shm");
- goto error;
+ if (errno == EACCES) {
+ /* Work around sysctl fs.protected_regular. */
+ DBG("shm_open of %s returned EACCES, this may be caused "
+ "by the fs.protected_regular sysctl. "
+ "Attempting to open the shm without "
+ "creating it.", shm_path);
+ wait_shm_fd = shm_open(shm_path, O_RDWR, mode);
+ }
+ if (wait_shm_fd < 0) {
+ PERROR("Failed to open wait shm at %s", shm_path);
+ goto error;
+ }
}
ret = ftruncate(wait_shm_fd, mmap_size);
*/
char *shm_ust_get_mmap(char *shm_path, int global)
{
- size_t mmap_size = sysconf(_SC_PAGE_SIZE);
+ size_t mmap_size;
int wait_shm_fd, ret;
char *wait_shm_mmap;
+ long sys_page_size;
assert(shm_path);
+ sys_page_size = sysconf(_SC_PAGE_SIZE);
+ if (sys_page_size < 0) {
+ PERROR("sysconf PAGE_SIZE");
+ goto error;
+ }
+ mmap_size = sys_page_size;
+
wait_shm_fd = get_wait_shm(shm_path, mmap_size, global);
if (wait_shm_fd < 0) {
goto error;