[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>

2 files changed, 13 insertions, 7 deletions

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index 2495f23e33f4..cb5ea44846af 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -370,6 +370,9 @@ static int configfs_detach_prep(struct dentry *dentry, struct mutex **wait_mutex
         struct configfs_dirent *sd;
         int ret;
 
+        /* Mark that we're trying to drop the group */
+        parent_sd->s_type |= CONFIGFS_USET_DROPPING;
+
         ret = -EBUSY;
         if (!list_empty(&parent_sd->s_links))
                 goto out;
@@ -385,8 +388,6 @@ static int configfs_detach_prep(struct dentry *dentry, struct mutex **wait_mutex
                                         *wait_mutex = &sd->s_dentry->d_inode->i_mutex;
                                 return -EAGAIN;
                         }
-                        /* Mark that we're trying to drop the group */
-                        sd->s_type |= CONFIGFS_USET_DROPPING;
 
                         /*
                          * Yup, recursive.  If there's a problem, blame
@@ -414,12 +415,11 @@ static void configfs_detach_rollback(struct dentry *dentry)
         struct configfs_dirent *parent_sd = dentry->d_fsdata;
         struct configfs_dirent *sd;
 
-        list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
-                if (sd->s_type & CONFIGFS_USET_DEFAULT) {
+        parent_sd->s_type &= ~CONFIGFS_USET_DROPPING;
+
+        list_for_each_entry(sd, &parent_sd->s_children, s_sibling)
+                if (sd->s_type & CONFIGFS_USET_DEFAULT)
                         configfs_detach_rollback(sd->s_dentry);
-                        sd->s_type &= ~CONFIGFS_USET_DROPPING;
-                }
-        }
 }
 
 static void detach_attrs(struct config_item * item)
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,