#include <sys/prctl.h>
#include <sys/mman.h>
#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
}
}
+/*
+ * Using fork to set umask to 0777 in the child process (not
+ * multi-thread safe).
+ */
static
-char *get_map_shm(struct sock_info *sock_info)
+int get_wait_shm(struct sock_info *sock_info, size_t mmap_size)
{
- size_t mmap_size = sysconf(_SC_PAGE_SIZE);
int wait_shm_fd, ret;
- char *wait_shm_mmap;
- int mode;
-
- mode = S_IRUSR | S_IRGRP;
- if (sock_info->global)
- mode |= S_IROTH;
+ int read_mode;
+ pid_t pid;
/*
- * Get existing (read-only) shm, or open new shm.
- * First try to open read-only.
+ * 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.
*/
- wait_shm_fd = shm_open(sock_info->wait_shm_path,
- O_RDONLY, mode);
- if (wait_shm_fd >= 0)
- goto got_shm;
+ read_mode = S_IRUSR | S_IRGRP;
+ if (sock_info->global)
+ read_mode |= S_IROTH;
+
/*
- * Real-only open did not work. If it is because it did
- * not exist, try creating it. Else it's a failure that
- * prohibits using shm.
+ * 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.
*/
- if (errno != ENOENT) {
- ERR("Error opening shm %s", sock_info->wait_shm_path);
- goto error;
- }
wait_shm_fd = shm_open(sock_info->wait_shm_path,
- O_RDWR | O_CREAT | O_EXCL, mode | S_IWUSR);
- if (wait_shm_fd >= 0)
- goto created_shm;
- if (errno != EEXIST) {
+ O_RDONLY, read_mode);
+ 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.
+ */
ERR("Error opening shm %s", sock_info->wait_shm_path);
- goto error;
+ goto end;
}
+
/*
- * If another process beat us to create the shm, we are
- * pretty certain the shm is available for us in
- * read-only mode.
+ * If the open failed because the file did not exist, try
+ * creating it ourself.
*/
- wait_shm_fd = shm_open(sock_info->wait_shm_path,
- O_RDWR | O_CREAT | O_EXCL, mode);
- if (wait_shm_fd >= 0)
- goto got_shm;
- else
- goto error;
+ pid = fork();
+ if (pid > 0) {
+ int status;
-created_shm:
- ret = ftruncate(wait_shm_fd, mmap_size);
- if (ret) {
- PERROR("ftruncate");
- ret = close(wait_shm_fd);
- if (ret) {
- ERR("Error closing fd");
+ /*
+ * 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) {
+ wait_shm_fd = -1;
+ goto end;
}
- wait_shm_fd = -1;
- goto error;
+ /*
+ * Try to open read-only again after creation.
+ */
+ wait_shm_fd = shm_open(sock_info->wait_shm_path,
+ O_RDONLY, read_mode);
+ if (wait_shm_fd < 0) {
+ /*
+ * Real-only open did not work. It's a failure
+ * that prohibits using shm.
+ */
+ ERR("Error opening shm %s", sock_info->wait_shm_path);
+ goto end;
+ }
+ goto end;
+ } else if (pid == 0) {
+ int create_mode;
+
+ /* Child */
+ create_mode = S_IRUSR | S_IRGRP | S_IWUSR | S_IWGRP;
+ if (sock_info->global)
+ create_mode |= S_IROTH | S_IWOTH;
+ /*
+ * We're alone in a child process, so we can modify the
+ * process-wide umask.
+ */
+ 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.
+ */
+ wait_shm_fd = shm_open(sock_info->wait_shm_path,
+ O_RDWR | O_CREAT, create_mode);
+ if (wait_shm_fd >= 0) {
+ ret = ftruncate(wait_shm_fd, mmap_size);
+ if (ret) {
+ 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) {
+ 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.
+ */
+ exit(EXIT_SUCCESS);
+ } else {
+ return -1;
}
- /* Drop write access ASAP */
- ret = chmod(sock_info->wait_shm_path, mode);
- if (ret) {
- PERROR("chmod");
+end:
+ return wait_shm_fd;
+}
+
+static
+char *get_map_shm(struct sock_info *sock_info)
+{
+ size_t mmap_size = sysconf(_SC_PAGE_SIZE);
+ int wait_shm_fd, ret;
+ char *wait_shm_mmap;
+
+ wait_shm_fd = get_wait_shm(sock_info, mmap_size);
+ if (wait_shm_fd < 0) {
+ goto error;
}
-got_shm:
wait_shm_mmap = mmap(NULL, mmap_size, PROT_READ,
MAP_SHARED, wait_shm_fd, 0);
if (wait_shm_mmap == MAP_FAILED) {
projects / lttng-ust.git / commitdiff
