}
/*
- * Using fork to set umask to 0777 in the child process (not
- * multi-thread safe).
+ * 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(struct sock_info *sock_info, size_t mmap_size)
{
int wait_shm_fd, ret;
- int read_mode;
pid_t pid;
/*
- * At this point, we should be able to open it for
- * reading. If it fails, then it's because there is
- * something wrong: bail out in that case.
+ * Try to open read-only.
*/
- read_mode = S_IRUSR | S_IRGRP;
- if (sock_info->global)
- read_mode |= S_IROTH;
-
- /*
- * Try to open read-only. If it is set read-only, it
- * means the shm size has been already set with
- * ftruncate. Note: all processes creating shm need to
- * call ftruncate on the shm before restricting its
- * access rights to read-only. The shm should never be
- * unlinked. It a rogue process try to create a non-accessible
- * shm or to unlink it, the worse-case scenario is that we don't
- * use the shm wakeup method and sleep/retry instead.
- */
- wait_shm_fd = shm_open(sock_info->wait_shm_path,
- O_RDONLY, read_mode);
+ wait_shm_fd = shm_open(sock_info->wait_shm_path, O_RDONLY, 0);
if (wait_shm_fd >= 0) {
goto end;
} else if (wait_shm_fd < 0 && errno != ENOENT) {
/*
- * Real-only open did not work. It's a failure
- * that prohibits using shm.
+ * Real-only open did not work, and it's not because the
+ * entry was not present. It's a failure that prohibits
+ * using shm.
*/
ERR("Error opening shm %s", sock_info->wait_shm_path);
goto end;
}
-
/*
* If the open failed because the file did not exist, try
* creating it ourself.
/*
* Try to open read-only again after creation.
*/
- wait_shm_fd = shm_open(sock_info->wait_shm_path,
- O_RDONLY, read_mode);
+ wait_shm_fd = shm_open(sock_info->wait_shm_path, O_RDONLY, 0);
if (wait_shm_fd < 0) {
/*
* Real-only open did not work. It's a failure
int create_mode;
/* Child */
- create_mode = S_IRUSR | S_IRGRP | S_IWUSR | S_IWGRP;
+ create_mode = S_IRUSR | S_IWUSR | S_IRGRP;
if (sock_info->global)
- create_mode |= S_IROTH | S_IWOTH;
+ create_mode |= S_IROTH | S_IWGRP | S_IWOTH;
/*
* We're alone in a child process, so we can modify the
* process-wide umask.
*/
- umask(create_mode);
+ umask(~create_mode);
/*
- * First try creating shm (or get rw access). We need to start
- * by this because of the ftruncate vs concurrent map race.
- * We need to give write access to everyone because of the
- * ftruncate vs mmap race too. We don't do an exclusive
- * open, because we allow other processes to
- * create+ftruncate it concurrently.
+ * 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(sock_info->wait_shm_path,
O_RDWR | O_CREAT, create_mode);
PERROR("ftruncate");
exit(EXIT_FAILURE);
}
- ret = fchmod(wait_shm_fd, read_mode);
- if (ret) {
- PERROR("fchmod");
- exit(EXIT_FAILURE);
- }
exit(EXIT_SUCCESS);
}
- if (errno != EACCES) {
+ /*
+ * For local shm, we need to have rw access to accept
+ * opening it: this means the local sessiond will be
+ * able to wake us up. For global shm, we open it even
+ * if rw access is not granted, because the root.root
+ * sessiond will be able to override all rights and wake
+ * us up.
+ */
+ if (!sock_info->global && errno != EACCES) {
ERR("Error opening shm %s", sock_info->wait_shm_path);
exit(EXIT_FAILURE);
}
/*
- * The shm exists, but we cannot open it RW. It means it
- * has already been setup and ftruncated, so we can
- * let the child exit.
+ * The shm exists, but we cannot open it RW. Report
+ * success.
*/
exit(EXIT_SUCCESS);
} else {
return -1;
}
end:
+ if (wait_shm_fd >= 0 && !sock_info->global) {
+ struct stat statbuf;
+
+ /*
+ * Ensure that our user is the owner of the shm file for
+ * local shm. If we do not own the file, it means our
+ * sessiond will not have access to wake us up (there is
+ * probably a rogue process trying to fake our
+ * sessiond). Fallback to polling method in this case.
+ */
+ ret = fstat(wait_shm_fd, &statbuf);
+ if (ret) {
+ PERROR("fstat");
+ goto error_close;
+ }
+ if (statbuf.st_uid != getuid())
+ goto error_close;
+ }
return wait_shm_fd;
+
+error_close:
+ ret = close(wait_shm_fd);
+ if (ret) {
+ PERROR("Error closing fd");
+ }
+ return -1;
}
static
}
wait_shm_mmap = mmap(NULL, mmap_size, PROT_READ,
MAP_SHARED, wait_shm_fd, 0);
- if (wait_shm_mmap == MAP_FAILED) {
- PERROR("mmap");
- goto error;
- }
/* close shm fd immediately after taking the mmap reference */
ret = close(wait_shm_fd);
if (ret) {
- ERR("Error closing fd");
+ PERROR("Error closing fd");
+ }
+ if (wait_shm_mmap == MAP_FAILED) {
+ DBG("mmap error (can be caused by race with sessiond). Fallback to poll mode.");
+ goto error;
}
return wait_shm_mmap;
projects / lttng-ust.git / commitdiff
