whitelist: avoid nested whitelist mounts

Check mountids while creating path of a new mount target.
If the mountid differs from the top level directory (tmpfs)
mountid, this proves an earlier whitelist command.

It is important to note though that this check is not exhaustive,
as besides nested whitelist commands there are also nested
top level directories. So a user could run:
firejail --whitelist=/a/b --whitelist=/a/b/c where both
a and b are (whitelist) top level directories. Such a command
may result in b and c sharing the filesystem and hence mountid.
In this case the nested nature of the whitelist commands
will go unnoticed.

A more rigorous version will probably need to apply some
sorting to the whitelist command, possibly by means of
glob(3).

src/firejail/fs_whitelist.c

@@ -55,6 +55,13 @@ static int whitelist_mkpath(const char *parentdir, const char *relpath, mode_t m
 	if (parentfd < 0)
 		errExit("open");
 
+	// top level directory mount id
+	int mountid = get_mount_id(parentfd);
+	if (mountid < 0) {
+		close(parentfd);
+		return -1;
+	}
+
 	// work on a copy of the path
 	char *dup = strdup(relpath);
 	if (!dup)
@@ -95,6 +102,15 @@ static int whitelist_mkpath(const char *parentdir, const char *relpath, mode_t m
 			free(dup);
 			return -1;
 		}
+		// different mount id indicates earlier whitelist mount
+		if (get_mount_id(fd) != mountid) {
+			if (arg_debug || arg_debug_whitelists)
+				printf("Debug %d: whitelisted already\n", __LINE__);
+			close(parentfd);
+			close(fd);
+			free(dup);
+			return -1;
+		}
 		// move on to next path segment
 		close(parentfd);
 		parentfd = fd;