backend: respect the BOOKMARK_DELETE_DIR flag

Following commit 2e3685f217,
make sure all backends check this flag and return correct error codes.

Normally this is not mandatory, since the kernel looks up
the directory entry to be removed, and fails if the system call
is inappropriate (e.g., calling rmdir() on a regular file).
This happens before FUSE_UNLINK or FUSE_RMDIR is sent to the server.

However, when not in exclusive mode, there is a short window that
TOCTOU problem may occur, which may lead to undesired behavior
(e.g., deletion of a non-empty directory) or even the corruption of
bookmark storage if not properly checked.

Also explain this flag in the user manual.

doc/bookmarkfs.texi

@@ -2808,6 +2808,13 @@ depending on the value of @code{flags}.
 A bit array of the following flags:
 
 @table @code
+@item BOOKMARKFS_BOOKMARK_DELETE_DIR
+Indicates that the object to be removed is a directory.
+
+If the current backend context is not in exclusive mode
+(@pxref{Exclusive Mode}), this information may be incorrect.
+If so, the function should fail with @code{ENOTDIR} or @code{EISDIR}.
+
 @item BOOKMARKFS_BOOKMARK_TYPE_MASK
 The subsystem to operate on.
 

src/backend_chromium.c

@@ -2194,7 +2194,7 @@ bookmark_delete (
     void       *backend_ctx,
     uint64_t    parent_id,
     char const *name,
-    uint32_t    UNUSED_VAR(flags)
+    uint32_t    flags
 ) {
     struct backend_ctx *ctx = backend_ctx;
     debug_assert(!(ctx->flags & BOOKMARKFS_BACKEND_READONLY));
@@ -2223,15 +2223,19 @@ bookmark_delete (
     if (0 != lookup_name(ctx, parent_id, name, strlen(name), &lctx, &entry)) {
         return -ENOENT;
     };
-    json_t *siblings   = parent_entry->children;
-    json_t const *node = entry->node;
 
     // Check if entry can be deleted
-    json_t const *children = json_object_sget(node, "children");
-    if (children != NULL) {
-        if (0 != json_array_size(children)) {
+    if (flags & BOOKMARK_FLAG(DELETE_DIR)) {
+        if (unlikely(entry->children == NULL)) {
+            return -ENOTDIR;
+        }
+        if (0 != json_array_size(entry->children)) {
             return -ENOTEMPTY;
         }
+    } else {
+        if (unlikely(entry->children != NULL)) {
+            return -EISDIR;
+        }
     }
 
     // Remove from maps
@@ -2245,7 +2249,8 @@ bookmark_delete (
     free(entry);
 
     // Remove from store
-    json_array_remove(siblings, json_array_search(siblings, node));
+    json_t *siblings = parent_entry->children;
+    json_array_remove(siblings, json_array_search(siblings, entry->node));
 
     ctx->dirty = DIRTY_LEVEL_DATA;
 

src/backend_firefox.c

@@ -454,7 +454,7 @@ bookmark_do_delete (
         return status;
     }
     if (unlikely((cols.place_id == 0) != is_dir)) {
-        return is_dir ? -ENOTDIR : -EPERM;
+        return is_dir ? -ENOTDIR : -EISDIR;
     }
 
     status = mozbm_delete(ctx, cols.id, is_dir);