libzfs{,_core}: set O_CLOEXEC on persistent (ZFS_DEV and MNTTAB) fds

These were fd 3, 4, and 5 by the time zfs change-key hit
execute_key_fob()

glibc appends "e" to setmntent() mode, but musl's just returns fopen()

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Ahelenia Ziemiańska <[email protected]>
Closes #11866

Author: nabijaczleweli

lib/libzfs/libzfs_dataset.c

@@ -813,7 +813,7 @@ libzfs_mnttab_update(libzfs_handle_t *hdl)
 	struct mnttab entry;
 
 	/* Reopen MNTTAB to prevent reading stale data from open file */
-	if (freopen(MNTTAB, "r", hdl->libzfs_mnttab) == NULL)
+	if (freopen(MNTTAB, "re", hdl->libzfs_mnttab) == NULL)
 		return (ENOENT);
 
 	while (getmntent(hdl->libzfs_mnttab, &entry) == 0) {
@@ -884,7 +884,7 @@ libzfs_mnttab_find(libzfs_handle_t *hdl, const char *fsname,
 			libzfs_mnttab_fini(hdl);
 
 		/* Reopen MNTTAB to prevent reading stale data from open file */
-		if (freopen(MNTTAB, "r", hdl->libzfs_mnttab) == NULL)
+		if (freopen(MNTTAB, "re", hdl->libzfs_mnttab) == NULL)
 			return (ENOENT);
 
 		srch.mnt_special = (char *)fsname;

lib/libzfs/libzfs_mount.c

@@ -1496,7 +1496,7 @@ zpool_disable_datasets(zpool_handle_t *zhp, boolean_t force)
 	namelen = strlen(zhp->zpool_name);
 
 	/* Reopen MNTTAB to prevent reading stale data from open file */
-	if (freopen(MNTTAB, "r", hdl->libzfs_mnttab) == NULL)
+	if (freopen(MNTTAB, "re", hdl->libzfs_mnttab) == NULL)
 		return (ENOENT);
 
 	used = alloc = 0;

lib/libzfs/libzfs_util.c

@@ -1024,15 +1024,15 @@ libzfs_init(void)
 		return (NULL);
 	}
 
-	if ((hdl->libzfs_fd = open(ZFS_DEV, O_RDWR|O_EXCL)) < 0) {
+	if ((hdl->libzfs_fd = open(ZFS_DEV, O_RDWR|O_EXCL|O_CLOEXEC)) < 0) {
 		free(hdl);
 		return (NULL);
 	}
 
 #ifdef HAVE_SETMNTENT
-	if ((hdl->libzfs_mnttab = setmntent(MNTTAB, "r")) == NULL) {
+	if ((hdl->libzfs_mnttab = setmntent(MNTTAB, "re")) == NULL) {
 #else
-	if ((hdl->libzfs_mnttab = fopen(MNTTAB, "r")) == NULL) {
+	if ((hdl->libzfs_mnttab = fopen(MNTTAB, "re")) == NULL) {
 #endif
 		(void) close(hdl->libzfs_fd);
 		free(hdl);

lib/libzfs_core/libzfs_core.c

@@ -137,7 +137,7 @@ libzfs_core_init(void)
 {
 	(void) pthread_mutex_lock(&g_lock);
 	if (g_refcount == 0) {
-		g_fd = open(ZFS_DEV, O_RDWR);
+		g_fd = open(ZFS_DEV, O_RDWR|O_CLOEXEC);
 		if (g_fd < 0) {
 			(void) pthread_mutex_unlock(&g_lock);
 			return (errno);