path-util: Fix path_is_mount_point for files

Commits 27cc6f166 and f25afeb broke path_is_mount_point() for files (such as
/etc/machine-id → /run/machine-id bind mounts) as with the factorization of
fd_is_mount_point() we lost the parent directory. We cannot determine that from
an fd only as openat(fd, "..") only works for directory fds.

Change fd_is_mount_point() to behave like openat(): It now takes a file
descriptor of the containing directory, a file name in it, and flags (which can
be 0 or AT_SYMLINK_FOLLOW). Unlike name_to_handle_at() or openat(), fstatat()
only accepts the inverse flag AT_SYMLINK_NOFOLLOW and complains with EINVAL
about AT_SYMLINK_FOLLOW; so we need to transform the flags for that fallback.

Adjust rm_rf_children() accordingly (only other caller of fd_is_mount_point()
aside from path_is_mount_point()).

Add test cases for files, links, and file bind mounts (the latter will only
work when running as root). Split out a new test_path_is_mount_point() test
case function as it got significantly larger now.

3 files changed, 24 insertions, 11 deletions

diff --git a/src/shared/path-util.c b/src/shared/path-util.c
index 7090989fcb..8be479cd7f 100644
--- a/src/shared/path-util.c
+++ b/src/shared/path-util.c
@@ -509,7 +509,7 @@ static int fd_fdinfo_mnt_id(int fd, const char *filename, int flags, int *mnt_id
         return safe_atoi(p, mnt_id);
 }
 
-int fd_is_mount_point(int fd) {
+int fd_is_mount_point(int fd, const char *filename, int flags) {
         union file_handle_union h = FILE_HANDLE_INIT, h_parent = FILE_HANDLE_INIT;
         int mount_id = -1, mount_id_parent = -1;
         bool nosupp = false, check_st_dev = true;
@@ -517,6 +517,7 @@ int fd_is_mount_point(int fd) {
         int r;
 
         assert(fd >= 0);
+        assert(filename);
 
         /* First we will try the name_to_handle_at() syscall, which
          * tells us the mount id and an opaque file "handle". It is
@@ -541,7 +542,7 @@ int fd_is_mount_point(int fd) {
          * subvolumes have different st_dev, even though they aren't
          * real mounts of their own. */
 
-        r = name_to_handle_at(fd, "", &h.handle, &mount_id, AT_EMPTY_PATH);
+        r = name_to_handle_at(fd, filename, &h.handle, &mount_id, flags);
         if (r < 0) {
                 if (errno == ENOSYS)
                         /* This kernel does not support name_to_handle_at()
@@ -558,7 +559,7 @@ int fd_is_mount_point(int fd) {
                         return -errno;
         }
 
-        r = name_to_handle_at(fd, "..", &h_parent.handle, &mount_id_parent, 0);
+        r = name_to_handle_at(fd, "", &h_parent.handle, &mount_id_parent, AT_EMPTY_PATH);
         if (r < 0) {
                 if (errno == EOPNOTSUPP) {
                         if (nosupp)
@@ -593,13 +594,13 @@ int fd_is_mount_point(int fd) {
         return mount_id != mount_id_parent;
 
 fallback_fdinfo:
-        r = fd_fdinfo_mnt_id(fd, "", AT_EMPTY_PATH, &mount_id);
+        r = fd_fdinfo_mnt_id(fd, filename, flags, &mount_id);
         if (r == -EOPNOTSUPP)
                 goto fallback_fstat;
         if (r < 0)
                 return r;
 
-        r = fd_fdinfo_mnt_id(fd, "..", 0, &mount_id_parent);
+        r = fd_fdinfo_mnt_id(fd, "", AT_EMPTY_PATH, &mount_id_parent);
         if (r < 0)
                 return r;
 
@@ -615,10 +616,16 @@ fallback_fdinfo:
         check_st_dev = false;
 
 fallback_fstat:
-        if (fstatat(fd, "", &a, AT_EMPTY_PATH) < 0)
+        /* yay for fstatat() taking a different set of flags than the other
+         * _at() above */
+        if (flags & AT_SYMLINK_FOLLOW)
+                flags &= ~AT_SYMLINK_FOLLOW;
+        else
+                flags |= AT_SYMLINK_NOFOLLOW;
+        if (fstatat(fd, filename, &a, flags) < 0)
                 return -errno;
 
-        if (fstatat(fd, "..", &b, 0) < 0)
+        if (fstatat(fd, "", &b, AT_EMPTY_PATH) < 0)
                 return -errno;
 
         /* A directory with same device and inode as its parent? Must
@@ -632,17 +639,23 @@ fallback_fstat:
 
 int path_is_mount_point(const char *t, bool allow_symlink) {
         _cleanup_close_ int fd = -1;
+        _cleanup_free_ char *parent = NULL;
+        int r;
 
         assert(t);
 
         if (path_equal(t, "/"))
                 return 1;
 
-        fd = openat(AT_FDCWD, t, O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC|(allow_symlink ? 0 : O_PATH));
+        r = path_get_parent(t, &parent);
+        if (r < 0)
+                return r;
+
+        fd = openat(AT_FDCWD, parent, O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC|O_PATH);
         if (fd < 0)
                 return -errno;
 
-        return fd_is_mount_point(fd);
+        return fd_is_mount_point(fd, basename(t), (allow_symlink ? AT_SYMLINK_FOLLOW : 0));
 }
 
 int path_is_read_only_fs(const char *path) {
diff --git a/src/shared/path-util.h b/src/shared/path-util.h
index 4f45cfd2b7..38ad799ba0 100644
--- a/src/shared/path-util.h
+++ b/src/shared/path-util.h
@@ -53,7 +53,7 @@ char** path_strv_make_absolute_cwd(char **l);
 char** path_strv_resolve(char **l, const char *prefix);
 char** path_strv_resolve_uniq(char **l, const char *prefix);
 
-int fd_is_mount_point(int fd);
+int fd_is_mount_point(int fd, const char *filename, int flags);
 int path_is_mount_point(const char *path, bool allow_symlink);
 int path_is_read_only_fs(const char *path);
 int path_is_os_tree(const char *path);
diff --git a/src/shared/rm-rf.c b/src/shared/rm-rf.c
index a89e8afc2a..bafd483be2 100644
--- a/src/shared/rm-rf.c
+++ b/src/shared/rm-rf.c
@@ -103,7 +103,7 @@ int rm_rf_children(int fd, RemoveFlags flags, struct stat *root_dev) {
                         }
 
                         /* Stop at mount points */
-                        r = fd_is_mount_point(subdir_fd);
+                        r = fd_is_mount_point(fd, de->d_name, 0);
                         if (r < 0) {
                                 if (ret == 0 && r != -ENOENT)
                                         ret = r;