9pfs: local: resolve special directories in paths

When using the mapped-file security mode, the creds of a path /foo/bar
are stored in the /foo/.virtfs_metadata/bar file. This is okay for all
paths unless they end with '.' or '..', because we cannot create the
corresponding file in the metadata directory.

This patch ensures that '.' and '..' are resolved in all paths.

The core code only passes path elements (no '/') to the backend, with
the notable exception of the '/' path, which refers to the virtfs root.
This patch preserves the current behavior of converting it to '.' so
that it can be passed to "*at()" syscalls ('/' would mean the host root).

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eric Blake <eblake@redhat.com>

hw/9pfs/9p-local.c

@@ -1134,14 +1134,32 @@ static int local_name_to_path(FsContext *ctx, V9fsPath *dir_path,
     }
 
     if (dir_path) {
-        v9fs_path_sprintf(target, "%s/%s", dir_path->data, name);
-    } else if (strcmp(name, "/")) {
-        v9fs_path_sprintf(target, "%s", name);
+        if (!strcmp(name, ".")) {
+            /* "." relative to "foo/bar" is "foo/bar" */
+            v9fs_path_copy(target, dir_path);
+        } else if (!strcmp(name, "..")) {
+            if (!strcmp(dir_path->data, ".")) {
+                /* ".." relative to the root is "." */
+                v9fs_path_sprintf(target, ".");
+            } else {
+                char *tmp = g_path_get_dirname(dir_path->data);
+                /* Symbolic links are resolved by the client. We can assume
+                 * that ".." relative to "foo/bar" is equivalent to "foo"
+                 */
+                v9fs_path_sprintf(target, "%s", tmp);
+                g_free(tmp);
+            }
+        } else {
+            assert(!strchr(name, '/'));
+            v9fs_path_sprintf(target, "%s/%s", dir_path->data, name);
+        }
+    } else if (!strcmp(name, "/") || !strcmp(name, ".") ||
+               !strcmp(name, "..")) {
+            /* This is the root fid */
+        v9fs_path_sprintf(target, ".");
     } else {
-        /* We want the path of the export root to be relative, otherwise
-         * "*at()" syscalls would treat it as "/" in the host.
-         */
-        v9fs_path_sprintf(target, "%s", ".");
+        assert(!strchr(name, '/'));
+        v9fs_path_sprintf(target, "./%s", name);
     }
     return 0;
 }