fs-util: change chase_symlinks() behaviour in regards to escaping the root dir

Previously, we'd generate an EINVAL error if it is attempted to escape a root
directory with relative ".." symlinks. With this commit this is changed so that
".." from the root directory is a NOP, following the kernel's own behaviour
where /.. is equivalent to /.

As suggested by @keszybz.

diff --git a/src/basic/fs-util.c b/src/basic/fs-util.c
index 39f840581aae92459aa22c3d505c69a53a5f21b3..b97edb552c067c7b10b0c805e3aa7ccd8e2efad1 100644 (file)
--- a/src/basic/fs-util.c
+++ b/src/basic/fs-util.c
@@ -616,8 +616,8 @@ int chase_symlinks(const char *path, const char *_root, char **ret) {
          * symlinks relative to a root directory, instead of the root of the host.
          *
          * Note that "root" primarily matters if we encounter an absolute symlink. It is also used when following
          * symlinks relative to a root directory, instead of the root of the host.
          *
          * Note that "root" primarily matters if we encounter an absolute symlink. It is also used when following

-         * relative symlinks to ensure they cannot be used to "escape" the root directory. (For cases where this is
-         * attempted -EINVAL is returned.). The path parameter passed shall *not* be prefixed by it.
+         * relative symlinks to ensure they cannot be used to "escape" the root directory. The path parameter passed
+         * shall *not* be prefixed by it.

          *
          * Algorithmically this operates on two path buffers: "done" are the components of the path we already
          * processed and resolved symlinks, "." and ".." of. "todo" are the components of the path we still need to
          *
          * Algorithmically this operates on two path buffers: "done" are the components of the path we already
          * processed and resolved symlinks, "." and ".." of. "todo" are the components of the path we still need to


@@ -679,18 +679,20 @@ int chase_symlinks(const char *path, const char *_root, char **ret) {
                         _cleanup_free_ char *parent = NULL;
                         int fd_parent = -1;
 
                         _cleanup_free_ char *parent = NULL;
                         int fd_parent = -1;
 

+                        /* If we already are at the top, then going up will not change anything. This is in-line with
+                         * how the kernel handles this. */

                         if (isempty(done) || path_equal(done, "/"))
                         if (isempty(done) || path_equal(done, "/"))

-                                return -EINVAL;
+                                continue;

 
                         parent = dirname_malloc(done);
                         if (!parent)
                                 return -ENOMEM;
 
 
                         parent = dirname_malloc(done);
                         if (!parent)
                                 return -ENOMEM;
 

-                        /* Don't allow this to leave the root dir */
+                        /* Don't allow this to leave the root dir.  */

                         if (root &&
                             path_startswith(done, root) &&
                             !path_startswith(parent, root))
                         if (root &&
                             path_startswith(done, root) &&
                             !path_startswith(parent, root))

-                                return -EINVAL;
+                                continue;

 
                         free_and_replace(done, parent);
 
 
                         free_and_replace(done, parent);