Commit message: cmd/snap-confine: chown private /tmp parent to root.root

When snap-confine creates a private /tmp directory for a given snap it
first creates a temporary directory in /tmp/ named after the snap, along
with a random name. Inside that directory it creates a /tmp directory
with permissions appropriate for a future /tmp, namely 1777.

Up until recently the that directory was owned by the user who first
invoked snap-confine. Since the directory is reused by all the users on
the system this logic makes no sense.

This patch changes the related logic so that the private /tmp directory
is owned by root, just like the real one.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
CVE: CVE-2019-11502
CWE: CWE-703
Vulnerability description: snap-confine in snapd before 2.38 incorrectly set the ownership of a snap application to the uid and gid of the first calling user. Consequently, that user had unintended access to a private /tmp directory.

Project: snapd
File: mount-support.c

static void sc_bootstrap_mount_namespace(const struct sc_mount_config *config)
{
	char scratch_dir[] = "/tmp/snap.rootfs_XXXXXX";
	char src[PATH_MAX] = { 0 };
	char dst[PATH_MAX] = { 0 };
	if (mkdtemp(scratch_dir) == NULL) {
		die("cannot create temporary directory for the root file system");
	}
	// NOTE: at this stage we just called unshare(CLONE_NEWNS). We are in a new
	// mount namespace and have a private list of mounts.
	debug("scratch directory for constructing namespace: %s", scratch_dir);
	// Make the root filesystem recursively shared. This way propagation events
	// will be shared with main mount namespace.
	sc_do_mount("none", "/", NULL, MS_REC | MS_SHARED, NULL);
	// Bind mount the temporary scratch directory for root filesystem over
	// itself so that it is a mount point. This is done so that it can become
	// unbindable as explained below.
	sc_do_mount(scratch_dir, scratch_dir, NULL, MS_BIND, NULL);
	// Make the scratch directory unbindable.
	//
	// This is necessary as otherwise a mount loop can occur and the kernel
	// would crash. The term unbindable simply states that it cannot be bind
	// mounted anywhere. When we construct recursive bind mounts below this
	// guarantees that this directory will not be replicated anywhere.
	sc_do_mount("none", scratch_dir, NULL, MS_UNBINDABLE, NULL);
	// Recursively bind mount desired root filesystem directory over the
	// scratch directory. This puts the initial content into the scratch space
	// and serves as a foundation for all subsequent operations below.
	//
	// The mount is recursive because it can either be applied to the root
	// filesystem of a core system (aka all-snap) or the core snap on a classic
	// system. In the former case we need recursive bind mounts to accurately
	// replicate the state of the root filesystem into the scratch directory.
	sc_do_mount(config->rootfs_dir, scratch_dir, NULL, MS_REC | MS_BIND,
		    NULL);
	// Make the scratch directory recursively private. Nothing done there will
	// be shared with any peer group, This effectively detaches us from the
	// original namespace and coupled with pivot_root below serves as the
	// foundation of the mount sandbox.
	sc_do_mount("none", scratch_dir, NULL, MS_REC | MS_SLAVE, NULL);
	// Bind mount certain directories from the host filesystem to the scratch
	// directory. By default mount events will propagate in both into and out
	// of the peer group. This way the running application can alter any global
	// state visible on the host and in other snaps. This can be restricted by
	// disabling the "is_bidirectional" flag as can be seen below.
	for (const struct sc_mount * mnt = config->mounts; mnt->path != NULL;
	     mnt++) {
		if (mnt->is_bidirectional && mkdir(mnt->path, 0755) < 0 &&
		    errno != EEXIST) {
			die("cannot create %s", mnt->path);
		}
		sc_must_snprintf(dst, sizeof dst, "%s/%s", scratch_dir,
				 mnt->path);
		if (mnt->is_optional) {
			bool ok = sc_do_optional_mount(mnt->path, dst, NULL,
						       MS_REC | MS_BIND, NULL);
			if (!ok) {
				// If we cannot mount it, just continue.
				continue;
			}
		} else {
			sc_do_mount(mnt->path, dst, NULL, MS_REC | MS_BIND,
				    NULL);
		}
		if (!mnt->is_bidirectional) {
			// Mount events will only propagate inwards to the namespace. This
			// way the running application cannot alter any global state apart
			// from that of its own snap.
			sc_do_mount("none", dst, NULL, MS_REC | MS_SLAVE, NULL);
		}
		if (mnt->altpath == NULL) {
			continue;
		}
		// An alternate path of mnt->path is provided at another location.
		// It should behave exactly the same as the original.
		sc_must_snprintf(dst, sizeof dst, "%s/%s", scratch_dir,
				 mnt->altpath);
		struct stat stat_buf;
		if (lstat(dst, &stat_buf) < 0) {
			die("cannot lstat %s", dst);
		}
		if ((stat_buf.st_mode & S_IFMT) == S_IFLNK) {
			die("cannot bind mount alternate path over a symlink: %s", dst);
		}
		sc_do_mount(mnt->path, dst, NULL, MS_REC | MS_BIND, NULL);
		if (!mnt->is_bidirectional) {
			sc_do_mount("none", dst, NULL, MS_REC | MS_SLAVE, NULL);
		}
	}
	if (config->normal_mode) {
		// Since we mounted /etc from the host filesystem to the scratch directory,
		// we may need to put certain directories from the desired root filesystem
		// (e.g. the core snap) back. This way the behavior of running snaps is not
		// affected by the alternatives directory from the host, if one exists.
		//
		// Fixes the following bugs:
		//  - https://bugs.launchpad.net/snap-confine/+bug/1580018
		//  - https://bugzilla.opensuse.org/show_bug.cgi?id=1028568
		const char *dirs_from_core[] =
		    { "/etc/alternatives", "/etc/ssl", "/etc/nsswitch.conf",
			NULL
		};
		for (const char **dirs = dirs_from_core; *dirs != NULL; dirs++) {
			const char *dir = *dirs;
			struct stat buf;
			if (access(dir, F_OK) == 0) {
				sc_must_snprintf(src, sizeof src, "%s%s",
						 config->rootfs_dir, dir);
				sc_must_snprintf(dst, sizeof dst, "%s%s",
						 scratch_dir, dir);
				if (lstat(src, &buf) == 0
				    && lstat(dst, &buf) == 0) {
					sc_do_mount(src, dst, NULL, MS_BIND,
						    NULL);
					sc_do_mount("none", dst, NULL, MS_SLAVE,
						    NULL);
				}
			}
		}
	}
	// The "core" base snap is special as it contains snapd and friends.
	// Other base snaps do not, so whenever a base snap other than core is
	// in use we need extra provisions for setting up internal tooling to
	// be available.
	//
	// However on a core18 (and similar) system the core snap is not
	// a special base anymore and we should map our own tooling in.
	if (config->distro == SC_DISTRO_CORE_OTHER
	    || !sc_streq(config->base_snap_name, "core")) {
		// when bases are used we need to bind-mount the libexecdir
		// (that contains snap-exec) into /usr/lib/snapd of the
		// base snap so that snap-exec is available for the snaps
		// (base snaps do not ship snapd)

		// dst is always /usr/lib/snapd as this is where snapd
		// assumes to find snap-exec
		sc_must_snprintf(dst, sizeof dst, "%s/usr/lib/snapd",
				 scratch_dir);

		// bind mount the current $ROOT/usr/lib/snapd path,
		// where $ROOT is either "/" or the "/snap/{core,snapd}/current"
		// that we are re-execing from
		char *src = NULL;
		char self[PATH_MAX + 1] = { 0 };
		if (readlink("/proc/self/exe", self, sizeof(self) - 1) < 0) {
			die("cannot read /proc/self/exe");
		}
		// this cannot happen except when the kernel is buggy
		if (strstr(self, "/snap-confine") == NULL) {
			die("cannot use result from readlink: %s", self);
		}
		src = dirname(self);
		// dirname(path) might return '.' depending on path.
		// /proc/self/exe should always point
		// to an absolute path, but let's guarantee that.
		if (src[0] != '/') {
			die("cannot use the result of dirname(): %s", src);
		}

		sc_do_mount(src, dst, NULL, MS_BIND | MS_RDONLY, NULL);
		sc_do_mount("none", dst, NULL, MS_SLAVE, NULL);
	}
	// Bind mount the directory where all snaps are mounted. The location of
	// the this directory on the host filesystem may not match the location in
	// the desired root filesystem. In the "core" and "ubuntu-core" snaps the
	// directory is always /snap. On the host it is a build-time configuration
	// option stored in SNAP_MOUNT_DIR.
	sc_must_snprintf(dst, sizeof dst, "%s/snap", scratch_dir);
	sc_do_mount(SNAP_MOUNT_DIR, dst, NULL, MS_BIND | MS_REC | MS_SLAVE,
		    NULL);
	sc_do_mount("none", dst, NULL, MS_REC | MS_SLAVE, NULL);
	// Create the hostfs directory if one is missing. This directory is a part
	// of packaging now so perhaps this code can be removed later.
	if (access(SC_HOSTFS_DIR, F_OK) != 0) {
		debug("creating missing hostfs directory");
		if (mkdir(SC_HOSTFS_DIR, 0755) != 0) {
			die("cannot perform operation: mkdir %s",
			    SC_HOSTFS_DIR);
		}
	}
	// Ensure that hostfs isgroup owned by root. We may have (now or earlier)
	// created the directory as the user who first ran a snap on a given
	// system and the group identity of that user is visilbe on disk.
	// This was LP:#1665004
	struct stat sb;
	if (stat(SC_HOSTFS_DIR, &sb) < 0) {
		die("cannot stat %s", SC_HOSTFS_DIR);
	}
	if (sb.st_uid != 0 || sb.st_gid != 0) {
		if (chown(SC_HOSTFS_DIR, 0, 0) < 0) {
			die("cannot change user/group owner of %s to root",
			    SC_HOSTFS_DIR);
		}
	}
	// Make the upcoming "put_old" directory for pivot_root private so that
	// mount events don't propagate to any peer group. In practice pivot root
	// has a number of undocumented requirements and one of them is that the
	// "put_old" directory (the second argument) cannot be shared in any way.
	sc_must_snprintf(dst, sizeof dst, "%s/%s", scratch_dir, SC_HOSTFS_DIR);
	sc_do_mount(dst, dst, NULL, MS_BIND, NULL);
	sc_do_mount("none", dst, NULL, MS_PRIVATE, NULL);
	// On classic mount the nvidia driver. Ideally this would be done in an
	// uniform way after pivot_root but this is good enough and requires less
	// code changes the nvidia code assumes it has access to the existing
	// pre-pivot filesystem.
	if (config->distro == SC_DISTRO_CLASSIC) {
		sc_mount_nvidia_driver(scratch_dir);
	}
	// XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
	//                    pivot_root
	// XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
	// Use pivot_root to "chroot" into the scratch directory.
	//
	// Q: Why are we using something as esoteric as pivot_root(2)?
	// A: Because this makes apparmor handling easy. Using a normal chroot
	// makes all apparmor rules conditional.  We are either running on an
	// all-snap system where this would-be chroot didn't happen and all the
	// rules see / as the root file system _OR_ we are running on top of a
	// classic distribution and this chroot has now moved all paths to
	// /tmp/snap.rootfs_*.
	//
	// Because we are using unshare(2) with CLONE_NEWNS we can essentially use
	// pivot_root just like chroot but this makes apparmor unaware of the old
	// root so everything works okay.
	//
	// HINT: If you are debugging this and are trying to see why pivot_root
	// happens to return EINVAL with any changes you may be making, please
	// consider applying
	// misc/0001-Add-printk-based-debugging-to-pivot_root.patch to your tree
	// kernel.
	debug("performing operation: pivot_root %s %s", scratch_dir, dst);
	if (syscall(SYS_pivot_root, scratch_dir, dst) < 0) {
		die("cannot perform operation: pivot_root %s %s", scratch_dir,
		    dst);
	}
	// Unmount the self-bind mount over the scratch directory created earlier
	// in the original root filesystem (which is now mounted on SC_HOSTFS_DIR).
	// This way we can remove the temporary directory we created and "clean up"
	// after ourselves nicely.
	sc_must_snprintf(dst, sizeof dst, "%s/%s", SC_HOSTFS_DIR, scratch_dir);
	sc_do_umount(dst, 0);
	// Remove the scratch directory. Note that we are using the path that is
	// based on the old root filesystem as after pivot_root we cannot guarantee
	// what is present at the same location normally. (It is probably an empty
	// /tmp directory that is populated in another place).
	debug("performing operation: rmdir %s", dst);
	if (rmdir(scratch_dir) < 0) {
		die("cannot perform operation: rmdir %s", dst);
	};
	// Make the old root filesystem recursively slave. This way operations
	// performed in this mount namespace will not propagate to the peer group.
	// This is another essential part of the confinement system.
	sc_do_mount("none", SC_HOSTFS_DIR, NULL, MS_REC | MS_SLAVE, NULL);
	// Detach the redundant hostfs version of sysfs since it shows up in the
	// mount table and software inspecting the mount table may become confused
	// (eg, docker and LP:# 162601).
	sc_must_snprintf(src, sizeof src, "%s/sys", SC_HOSTFS_DIR);
	sc_do_umount(src, UMOUNT_NOFOLLOW | MNT_DETACH);
	// Detach the redundant hostfs version of /dev since it shows up in the
	// mount table and software inspecting the mount table may become confused.
	sc_must_snprintf(src, sizeof src, "%s/dev", SC_HOSTFS_DIR);
	sc_do_umount(src, UMOUNT_NOFOLLOW | MNT_DETACH);
	// Detach the redundant hostfs version of /proc since it shows up in the
	// mount table and software inspecting the mount table may become confused.
	sc_must_snprintf(src, sizeof src, "%s/proc", SC_HOSTFS_DIR);
	sc_do_umount(src, UMOUNT_NOFOLLOW | MNT_DETACH);
}

Project: snapd
File: mount-support.c

static void setup_private_pts(void)
{
	// See https://www.kernel.org/doc/Documentation/filesystems/devpts.txt
	//
	// Ubuntu by default uses devpts 'single-instance' mode where
	// /dev/pts/ptmx is mounted with ptmxmode=0000. We don't want to change
	// the startup scripts though, so we follow the instructions in point
	// '4' of 'User-space changes' in the above doc. In other words, after
	// unshare(CLONE_NEWNS), we mount devpts with -o
	// newinstance,ptmxmode=0666 and then bind mount /dev/pts/ptmx onto
	// /dev/ptmx

	struct stat st;

	// Make sure /dev/pts/ptmx exists, otherwise we are in legacy mode
	// which doesn't provide the isolation we require.
	if (stat("/dev/pts/ptmx", &st) != 0) {
		die("cannot stat /dev/pts/ptmx");
	}
	// Make sure /dev/ptmx exists so we can bind mount over it
	if (stat("/dev/ptmx", &st) != 0) {
		die("cannot stat /dev/ptmx");
	}
	// Since multi-instance, use ptmxmode=0666. The other options are
	// copied from /etc/default/devpts
	sc_do_mount("devpts", "/dev/pts", "devpts", MS_MGC_VAL,
		    "newinstance,ptmxmode=0666,mode=0620,gid=5");
	sc_do_mount("/dev/pts/ptmx", "/dev/ptmx", "none", MS_BIND, 0);
}

Project: snapd
File: mount-support.c

static void sc_make_slave_mount_ns(void)
{
	if (unshare(CLONE_NEWNS) < 0) {
		die("can not unshare mount namespace");
	}
	// In our new mount namespace, recursively change all mounts
	// to slave mode, so we see changes from the parent namespace
	// but don't propagate our own changes.
	sc_do_mount("none", "/", NULL, MS_REC | MS_SLAVE, NULL);
}

Project: snapd
File: mount-support.c

static bool is_mounted_with_shared_option(const char *dir)
{
	struct sc_mountinfo *sm SC_CLEANUP(sc_cleanup_mountinfo) = NULL;
	sm = sc_parse_mountinfo(NULL);
	if (sm == NULL) {
		die("cannot parse /proc/self/mountinfo");
	}
	struct sc_mountinfo_entry *entry = sc_first_mountinfo_entry(sm);
	while (entry != NULL) {
		const char *mount_dir = entry->mount_dir;
		if (sc_streq(mount_dir, dir)) {
			const char *optional_fields = entry->optional_fields;
			if (strstr(optional_fields, "shared:") != NULL) {
				return true;
			}
		}
		entry = sc_next_mountinfo_entry(entry);
	}
	return false;
}

Project: snapd
File: mount-support.c

static void setup_private_mount(const char *snap_name)
{
	char tmpdir[MAX_BUF] = { 0 };

	// Create a 0700 base directory, this is the base dir that is
	// protected from other users.
	//
	// Under that basedir, we put a 1777 /tmp dir that is then bind
	// mounted for the applications to use
	sc_must_snprintf(tmpdir, sizeof(tmpdir), "/tmp/snap.%s_XXXXXX", snap_name);
	if (mkdtemp(tmpdir) == NULL) {
		die("cannot create temporary directory essential for private /tmp");
	}
	// now we create a 1777 /tmp inside our private dir
	mode_t old_mask = umask(0);
	char *d = sc_strdup(tmpdir);
	sc_must_snprintf(tmpdir, sizeof(tmpdir), "%s/tmp", d);
	free(d);

	if (mkdir(tmpdir, 01777) != 0) {
		die("cannot create temporary directory for private /tmp");
	}
	umask(old_mask);

	// chdir to '/' since the mount won't apply to the current directory
	char *pwd = get_current_dir_name();
	if (pwd == NULL)
		die("cannot get current working directory");
	if (chdir("/") != 0)
		die("cannot change directory to '/'");

	// MS_BIND is there from linux 2.4
	sc_do_mount(tmpdir, "/tmp", NULL, MS_BIND, NULL);
	// MS_PRIVATE needs linux > 2.6.11
	sc_do_mount("none", "/tmp", NULL, MS_PRIVATE, NULL);
	// do the chown after the bind mount to avoid potential shenanigans
	if (chown("/tmp/", 0, 0) < 0) {
		die("cannot change ownership of /tmp");
	}
	// chdir to original directory
	if (chdir(pwd) != 0)
		die("cannot change current working directory to the original directory");
	free(pwd);
}

Project: snapd
File: mount-support.c

    get_nextpath(char *path, size_t * offsetp, size_t fulllen)
{
	size_t offset = *offsetp;

	if (offset >= fulllen)
		return NULL;

	while (offset < fulllen && path[offset] != '\0')
		offset++;
	while (offset < fulllen && path[offset] == '\0')
		offset++;

	*offsetp = offset;
	return (offset < fulllen) ? &path[offset] : NULL;
}

Project: snapd
File: mount-support.c

void sc_populate_mount_ns(struct sc_apparmor *apparmor, int snap_update_ns_fd,
			  const char *base_snap_name, const char *snap_name)
{
	// Get the current working directory before we start fiddling with
	// mounts and possibly pivot_root.  At the end of the whole process, we
	// will try to re-locate to the same directory (if possible).
	char *vanilla_cwd SC_CLEANUP(sc_cleanup_string) = NULL;
	vanilla_cwd = get_current_dir_name();
	if (vanilla_cwd == NULL) {
		die("cannot get the current working directory");
	}
	// Classify the current distribution, as claimed by /etc/os-release.
	sc_distro distro = sc_classify_distro();
	// Check which mode we should run in, normal or legacy.
	if (sc_should_use_normal_mode(distro, base_snap_name)) {
		// In normal mode we use the base snap as / and set up several bind mounts.
		const struct sc_mount mounts[] = {
			{"/dev"},	// because it contains devices on host OS
			{"/etc"},	// because that's where /etc/resolv.conf lives, perhaps a bad idea
			{"/home"},	// to support /home/*/snap and home interface
			{"/root"},	// because that is $HOME for services
			{"/proc"},	// fundamental filesystem
			{"/sys"},	// fundamental filesystem
			{"/tmp"},	// to get writable tmp
			{"/var/snap"},	// to get access to global snap data
			{"/var/lib/snapd"},	// to get access to snapd state and seccomp profiles
			{"/var/tmp"},	// to get access to the other temporary directory
			{"/run"},	// to get /run with sockets and what not
			{"/lib/modules",.is_optional = true},	// access to the modules of the running kernel
			{"/usr/src"},	// FIXME: move to SecurityMounts in system-trace interface
			{"/var/log"},	// FIXME: move to SecurityMounts in log-observe interface
#ifdef MERGED_USR
			{"/run/media", true, "/media"},	// access to the users removable devices
#else
			{"/media", true},	// access to the users removable devices
#endif				// MERGED_USR
			{"/run/netns", true},	// access to the 'ip netns' network namespaces
			// The /mnt directory is optional in base snaps to ensure backwards
			// compatibility with the first version of base snaps that was
			// released.
			{"/mnt",.is_optional = true},	// to support the removable-media interface
			{"/var/lib/extrausers",.is_optional = true},	// access to UID/GID of extrausers (if available)
			{},
		};
		char rootfs_dir[PATH_MAX] = { 0 };
		sc_must_snprintf(rootfs_dir, sizeof rootfs_dir,
				 "%s/%s/current/", SNAP_MOUNT_DIR,
				 base_snap_name);
		if (access(rootfs_dir, F_OK) != 0) {
			if (sc_streq(base_snap_name, "core")) {
				// As a special fallback, allow the
				// base snap to degrade from "core" to
				// "ubuntu-core". This is needed for
				// the migration tests.
				base_snap_name = "ubuntu-core";
				sc_must_snprintf(rootfs_dir, sizeof rootfs_dir,
						 "%s/%s/current/",
						 SNAP_MOUNT_DIR,
						 base_snap_name);
				if (access(rootfs_dir, F_OK) != 0) {
					die("cannot locate the core or legacy core snap (current symlink missing?)");
				}
			}
			// If after the special case handling above we are
			// still not ok, die
			if (access(rootfs_dir, F_OK) != 0)
			        die("cannot locate the base snap: %s", base_snap_name);
		}
		struct sc_mount_config normal_config = {
			.rootfs_dir = rootfs_dir,
			.mounts = mounts,
			.distro = distro,
			.normal_mode = true,
			.base_snap_name = base_snap_name,
		};
		sc_bootstrap_mount_namespace(&normal_config);
	} else {
		// In legacy mode we don't pivot and instead just arrange bi-
		// directional mount propagation for two directories.
		const struct sc_mount mounts[] = {
			{"/media", true},
			{"/run/netns", true},
			{},
		};
		struct sc_mount_config legacy_config = {
			.rootfs_dir = "/",
			.mounts = mounts,
			.distro = distro,
			.normal_mode = false,
			.base_snap_name = base_snap_name,
		};
		sc_bootstrap_mount_namespace(&legacy_config);
	}

	// set up private mounts
	// TODO: rename this and fold it into bootstrap
	setup_private_mount(snap_name);

	// set up private /dev/pts
	// TODO: fold this into bootstrap
	setup_private_pts();

	// setup the security backend bind mounts
	sc_call_snap_update_ns(snap_update_ns_fd, snap_name, apparmor);

	// Try to re-locate back to vanilla working directory. This can fail
	// because that directory is no longer present.
	if (chdir(vanilla_cwd) != 0) {
		debug("cannot remain in %s, moving to the void directory",
		      vanilla_cwd);
		if (chdir(SC_VOID_DIR) != 0) {
			die("cannot change directory to %s", SC_VOID_DIR);
		}
		debug("successfully moved to %s", SC_VOID_DIR);
	}
}

Project: snapd
File: mount-support.c

void sc_ensure_shared_snap_mount(void)
{
	if (!is_mounted_with_shared_option("/")
	    && !is_mounted_with_shared_option(SNAP_MOUNT_DIR)) {
		// TODO: We could be more aggressive and refuse to function but since
		// we have no data on actual environments that happen to limp along in
		// this configuration let's not do that yet.  This code should be
		// removed once we have a measurement and feedback mechanism that lets
		// us decide based on measurable data.
		sc_do_mount(SNAP_MOUNT_DIR, SNAP_MOUNT_DIR, "none",
			    MS_BIND | MS_REC, 0);
		sc_do_mount("none", SNAP_MOUNT_DIR, NULL, MS_SHARED | MS_REC,
			    NULL);
	}
}

Project: snapd
File: mount-support.c

    is_subdir(const char *subdir, const char *dir)
{
	size_t dirlen = strlen(dir);
	size_t subdirlen = strlen(subdir);

	// @dir has to be at least as long as @subdir
	if (subdirlen < dirlen)
		return false;
	// @dir has to be a prefix of @subdir
	if (strncmp(subdir, dir, dirlen) != 0)
		return false;
	// @dir can look like "path/" (that is, end with the directory separator).
	// When that is the case then given the test above we can be sure @subdir
	// is a real subdirectory.
	if (dirlen > 0 && dir[dirlen - 1] == '/')
		return true;
	// @subdir can look like "path/stuff" and when the directory separator
	// is exactly at the spot where @dir ends (that is, it was not caught
	// by the test above) then @subdir is a real subdirectory.
	if (subdir[dirlen] == '/' && dirlen > 0)
		return true;
	// If both @dir and @subdir have identical length then given that the
	// prefix check above @subdir is a real subdirectory.
	if (subdirlen == dirlen)
		return true;
	return false;
}

Project: snapd
File: mount-support.c

void sc_setup_user_mounts(struct sc_apparmor *apparmor, int snap_update_ns_fd,
			  const char *snap_name)
{
	debug("%s: %s", __FUNCTION__, snap_name);

	char profile_path[PATH_MAX];
	struct stat st;

	sc_must_snprintf(profile_path, sizeof(profile_path),
			 "/var/lib/snapd/mount/snap.%s.user-fstab", snap_name);
	if (stat(profile_path, &st) != 0) {
		// It is ok for the user fstab to not exist.
		return;
	}

	sc_make_slave_mount_ns();
	sc_call_snap_update_ns_as_user(snap_update_ns_fd, snap_name, apparmor);
}