cifs: reduce false positives with inode aliasing serverino autodisable

It turns out that not all directory inodes with dentries on the
i_dentry list are unusable here. We only consider them unusable if they
are still hashed or if they have a root dentry attached.

Full disclosure -- this check is inherently racy. There's nothing that
stops someone from slapping a new dentry onto this inode just after
this check, or hashing an existing one that's already attached. So,
this is really a "best effort" thing to work around misbehaving servers.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

1 files changed, 31 insertions, 12 deletions

diff --git a/fs/cifs/inode.c b/fs/cifs/inode.c
index a15b3a9bbff4..dc4c47ab9588 100644
--- a/fs/cifs/inode.c
+++ b/fs/cifs/inode.c
@@ -732,15 +732,9 @@ cifs_find_inode(struct inode *inode, void *opaque)
         if ((inode->i_mode & S_IFMT) != (fattr->cf_mode & S_IFMT))
                 return 0;
 
-        /*
-         * uh oh -- it's a directory. We can't use it since hardlinked dirs are
-         * verboten. Disable serverino and return it as if it were found, the
-         * caller can discard it, generate a uniqueid and retry the find
-         */
-        if (S_ISDIR(inode->i_mode) && !list_empty(&inode->i_dentry)) {
+        /* if it's not a directory or has no dentries, then flag it */
+        if (S_ISDIR(inode->i_mode) && !list_empty(&inode->i_dentry))
                 fattr->cf_flags |= CIFS_FATTR_INO_COLLISION;
-                cifs_autodisable_serverino(CIFS_SB(inode->i_sb));
-        }
 
         return 1;
 }
@@ -754,6 +748,27 @@ cifs_init_inode(struct inode *inode, void *opaque)
         return 0;
 }
 
+/*
+ * walk dentry list for an inode and report whether it has aliases that
+ * are hashed. We use this to determine if a directory inode can actually
+ * be used.
+ */
+static bool
+inode_has_hashed_dentries(struct inode *inode)
+{
+        struct dentry *dentry;
+
+        spin_lock(&dcache_lock);
+        list_for_each_entry(dentry, &inode->i_dentry, d_alias) {
+                if (!d_unhashed(dentry) || IS_ROOT(dentry)) {
+                        spin_unlock(&dcache_lock);
+                        return true;
+                }
+        }
+        spin_unlock(&dcache_lock);
+        return false;
+}
+
 /* Given fattrs, get a corresponding inode */
 struct inode *
 cifs_iget(struct super_block *sb, struct cifs_fattr *fattr)
@@ -769,12 +784,16 @@ retry_iget5_locked:
 
         inode = iget5_locked(sb, hash, cifs_find_inode, cifs_init_inode, fattr);
         if (inode) {
-                /* was there a problematic inode number collision? */
+                /* was there a potentially problematic inode collision? */
                 if (fattr->cf_flags & CIFS_FATTR_INO_COLLISION) {
-                        iput(inode);
-                        fattr->cf_uniqueid = iunique(sb, ROOT_I);
                         fattr->cf_flags &= ~CIFS_FATTR_INO_COLLISION;
-                        goto retry_iget5_locked;
+
+                        if (inode_has_hashed_dentries(inode)) {
+                                cifs_autodisable_serverino(CIFS_SB(sb));
+                                iput(inode);
+                                fattr->cf_uniqueid = iunique(sb, ROOT_I);
+                                goto retry_iget5_locked;
+                        }
                 }
 
                 cifs_fattr_to_inode(inode, fattr);