From: Mathieu Desnoyers Date: Mon, 29 Aug 2011 17:24:19 +0000 (-0400) Subject: Fix shm open race X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=58d4b2a2484f1d6277c5553a661cf161a9de9af3;p=ust.git Fix shm open race Signed-off-by: Mathieu Desnoyers --- diff --git a/include/lttng-ust-comm.h b/include/lttng-ust-comm.h index f06872b..a5220a4 100644 --- a/include/lttng-ust-comm.h +++ b/include/lttng-ust-comm.h @@ -47,7 +47,7 @@ #define DEFAULT_HOME_CLIENT_UNIX_SOCK "%s/.client-ltt-sessiond" #define DEFAULT_GLOBAL_APPS_WAIT_SHM_PATH "/lttng-ust-apps-wait" -#define DEFAULT_HOME_APPS_WAIT_SHM_PATH "/%u-lttng-ust-apps-wait" +#define DEFAULT_HOME_APPS_WAIT_SHM_PATH "/lttng-ust-apps-wait-%u" /* Queue size of listen(2) */ #define MAX_LISTEN 10 diff --git a/libust/lttng-ust-comm.c b/libust/lttng-ust-comm.c index 3ee862a..53e0b1e 100644 --- a/libust/lttng-ust-comm.c +++ b/libust/lttng-ust-comm.c @@ -24,6 +24,8 @@ #include #include #include +#include +#include #include #include #include @@ -308,72 +310,143 @@ void cleanup_sock_info(struct sock_info *sock_info) } } +/* + * 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) {