path-util: Fix path_is_mount_point for parent mount points in symlink mode

When we have a structure like this:

  /bin -> /usr/bin
  /usr is a mount point

Then path_is_mount_point("/bin", AT_SYMLINK_FOLLOW) needs to look at the pair
/usr/bin and /usr, not at the pair / and /usr/bin, as the latter have different
mount IDs. But we only want to consider the base name, not any parent.

Thus we have to resolve the given path first to get the real parent when
allowing symlinks.

Bug: https://github.com/systemd/systemd/issues/61

2 files changed, 92 insertions, 5 deletions

diff --git a/src/shared/path-util.c b/src/shared/path-util.c
index be50a1865d..537705446a 100644
--- a/src/shared/path-util.c
+++ b/src/shared/path-util.c
@@ -640,7 +640,7 @@ fallback_fstat:
 /* flags can be AT_SYMLINK_FOLLOW or 0 */
 int path_is_mount_point(const char *t, int flags) {
         _cleanup_close_ int fd = -1;
-        _cleanup_free_ char *parent = NULL;
+        _cleanup_free_ char *canonical = NULL, *parent = NULL;
         int r;
 
         assert(t);
@@ -648,7 +648,17 @@ int path_is_mount_point(const char *t, int flags) {
         if (path_equal(t, "/"))
                 return 1;
 
-        r = path_get_parent(t, &parent);
+        /* we need to resolve symlinks manually, we can't just rely on
+         * fd_is_mount_point() to do that for us; if we have a structure like
+         * /bin -> /usr/bin/ and /usr is a mount point, then the parent that we
+         * look at needs to be /usr, not /. */
+        if (flags & AT_SYMLINK_FOLLOW) {
+                canonical = canonicalize_file_name(t);
+                if (!canonical)
+                        return -errno;
+        }
+
+        r = path_get_parent(canonical ?: t, &parent);
         if (r < 0)
                 return r;
 
@@ -656,7 +666,7 @@ int path_is_mount_point(const char *t, int flags) {
         if (fd < 0)
                 return -errno;
 
-        return fd_is_mount_point(fd, basename(t), flags);
+        return fd_is_mount_point(fd, basename(canonical ?: t), flags);
 }
 
 int path_is_read_only_fs(const char *path) {
diff --git a/src/test/test-path-util.c b/src/test/test-path-util.c
index 0045ae6824..fce4e81a09 100644
--- a/src/test/test-path-util.c
+++ b/src/test/test-path-util.c
@@ -312,9 +312,11 @@ static void test_prefix_root(void) {
 }
 
 static void test_path_is_mount_point(void) {
-        int fd, rt, rf, rlt, rlf;
+        int fd;
         char tmp_dir[] = "/tmp/test-path-is-mount-point-XXXXXX";
         _cleanup_free_ char *file1 = NULL, *file2 = NULL, *link1 = NULL, *link2 = NULL;
+        _cleanup_free_ char *dir1 = NULL, *dir1file = NULL, *dirlink1 = NULL, *dirlink1file = NULL;
+        _cleanup_free_ char *dir2 = NULL, *dir2file = NULL;
 
         assert_se(path_is_mount_point("/", AT_SYMLINK_FOLLOW) > 0);
         assert_se(path_is_mount_point("/", 0) > 0);
@@ -328,6 +330,19 @@ static void test_path_is_mount_point(void) {
         assert_se(path_is_mount_point("/sys", AT_SYMLINK_FOLLOW) > 0);
         assert_se(path_is_mount_point("/sys", 0) > 0);
 
+        /* we'll create a hierarchy of different kinds of dir/file/link
+         * layouts:
+         *
+         * <tmp>/file1, <tmp>/file2
+         * <tmp>/link1 -> file1, <tmp>/link2 -> file2
+         * <tmp>/dir1/
+         * <tmp>/dir1/file
+         * <tmp>/dirlink1 -> dir1
+         * <tmp>/dirlink1file -> dirlink1/file
+         * <tmp>/dir2/
+         * <tmp>/dir2/file
+         */
+
         /* file mountpoints */
         assert_se(mkdtemp(tmp_dir) != NULL);
         file1 = path_join(NULL, tmp_dir, "file1");
@@ -352,8 +367,43 @@ static void test_path_is_mount_point(void) {
         assert_se(path_is_mount_point(link1, AT_SYMLINK_FOLLOW) == 0);
         assert_se(path_is_mount_point(link1, 0) == 0);
 
-        /* this test will only work as root */
+        /* directory mountpoints */
+        dir1 = path_join(NULL, tmp_dir, "dir1");
+        assert_se(dir1);
+        assert_se(mkdir(dir1, 0755) == 0);
+        dirlink1 = path_join(NULL, tmp_dir, "dirlink1");
+        assert_se(dirlink1);
+        assert_se(symlink("dir1", dirlink1) == 0);
+        dirlink1file = path_join(NULL, tmp_dir, "dirlink1file");
+        assert_se(dirlink1file);
+        assert_se(symlink("dirlink1/file", dirlink1file) == 0);
+        dir2 = path_join(NULL, tmp_dir, "dir2");
+        assert_se(dir2);
+        assert_se(mkdir(dir2, 0755) == 0);
+
+        assert_se(path_is_mount_point(dir1, AT_SYMLINK_FOLLOW) == 0);
+        assert_se(path_is_mount_point(dir1, 0) == 0);
+        assert_se(path_is_mount_point(dirlink1, AT_SYMLINK_FOLLOW) == 0);
+        assert_se(path_is_mount_point(dirlink1, 0) == 0);
+
+        /* file in subdirectory mountpoints */
+        dir1file = path_join(NULL, dir1, "file");
+        assert_se(dir1file);
+        fd = open(dir1file, O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0664);
+        assert_se(fd > 0);
+        close(fd);
+
+        assert_se(path_is_mount_point(dir1file, AT_SYMLINK_FOLLOW) == 0);
+        assert_se(path_is_mount_point(dir1file, 0) == 0);
+        assert_se(path_is_mount_point(dirlink1file, AT_SYMLINK_FOLLOW) == 0);
+        assert_se(path_is_mount_point(dirlink1file, 0) == 0);
+
+        /* these tests will only work as root */
         if (mount(file1, file2, NULL, MS_BIND, NULL) >= 0) {
+                int rt, rf, rlt, rlf, rl1t, rl1f;
+
+                /* files */
+                /* capture results in vars, to avoid dangling mounts on failure */
                 rf = path_is_mount_point(file2, 0);
                 rt = path_is_mount_point(file2, AT_SYMLINK_FOLLOW);
                 rlf = path_is_mount_point(link2, 0);
@@ -365,6 +415,33 @@ static void test_path_is_mount_point(void) {
                 assert_se(rt == 1);
                 assert_se(rlf == 0);
                 assert_se(rlt == 1);
+
+                /* dirs */
+                dir2file = path_join(NULL, dir2, "file");
+                assert_se(dir2file);
+                fd = open(dir2file, O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0664);
+                assert_se(fd > 0);
+                close(fd);
+
+                assert_se(mount(dir2, dir1, NULL, MS_BIND, NULL) >= 0);
+
+                rf = path_is_mount_point(dir1, 0);
+                rt = path_is_mount_point(dir1, AT_SYMLINK_FOLLOW);
+                rlf = path_is_mount_point(dirlink1, 0);
+                rlt = path_is_mount_point(dirlink1, AT_SYMLINK_FOLLOW);
+                /* its parent is a mount point, but not /file itself */
+                rl1f = path_is_mount_point(dirlink1file, 0);
+                rl1t = path_is_mount_point(dirlink1file, AT_SYMLINK_FOLLOW);
+
+                assert_se(umount(dir1) == 0);
+
+                assert_se(rf == 1);
+                assert_se(rt == 1);
+                assert_se(rlf == 0);
+                assert_se(rlt == 1);
+                assert_se(rl1f == 0);
+                assert_se(rl1t == 0);
+
         } else
                 printf("Skipping bind mount file test: %m\n");