[PATCH] configfs: Fix symlink() to a removing item

The rule for configfs symlinks is that symlinks always point to valid
config_items, and prevent the target from being removed. However,
configfs_symlink() only checks that it can grab a reference on the target item,
without ensuring that it remains alive until the symlink is correctly attached.

This patch makes configfs_symlink() fail whenever the target is being removed,
using the CONFIGFS_USET_DROPPING flag set by configfs_detach_prep() and
protected by configfs_dirent_lock.

This patch introduces a similar (weird?) behavior as with mkdir failures making
rmdir fail: if symlink() races with rmdir() of the parent directory (or its
youngest user-created ancestor if parent is a default group) or rmdir() of the
target directory, and then fails in configfs_create(), this can make the racing
rmdir() fail despite the concerned directory having no user-created entry (resp.
no symlink pointing to it or one of its default groups) in the end.
This behavior is fixed in later patches.

Signed-off-by: Louis Rilling <louis.rilling@kerlabs.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/fs/configfs/symlink.c b/fs/configfs/symlink.c
index 0004d18c40ac..c12801a12c34 100644
--- a/fs/configfs/symlink.c
+++ b/fs/configfs/symlink.c
@@ -78,6 +78,12 @@ static int create_link(struct config_item *parent_item,
         if (sl) {
                 sl->sl_target = config_item_get(item);
                 spin_lock(&configfs_dirent_lock);
+                if (target_sd->s_type & CONFIGFS_USET_DROPPING) {
+                        spin_unlock(&configfs_dirent_lock);
+                        config_item_put(item);
+                        kfree(sl);
+                        return -ENOENT;
+                }
                 list_add(&sl->sl_list, &target_sd->s_links);
                 spin_unlock(&configfs_dirent_lock);
                 ret = configfs_create_link(sl, parent_item->ci_dentry,