[PATCH] stale bdev reference in quotactl

sys_quotacl tries to do a get_super on a struct block_device * to which
it doesn't hold a reference (nor does it actually have to be non-NULL).

As lookup bdev by name is a rather common operation I splitted out a new
helper, lookup_bdev() that does this out of open_bdev_excl and switched
quota.c to use it.  lookup_bdev() holds a proper reference that needs
to be dropped by bdput(), and it's well documented.

fs/block_dev.c

@@ -799,24 +799,20 @@ const char *__bdevname(dev_t dev)
 }
 
 /**
- * open_bdev_excl  -  open a block device by name and set it up for use
+ * lookup_bdev  - lookup a struct block_device by name
  *
  * @path:	special file representing the block device
- * @flags:	%MS_RDONLY for opening read-only
- * @kind:	usage (same as the 4th paramter to blkdev_get)
- * @holder:	owner for exclusion
  *
- * Open the blockdevice described by the special file at @path, claim it
- * for the @holder and properly set it up for @kind usage.
+ * Get a reference to the blockdevice at @path in the current
+ * namespace if possible and return it.  Return ERR_PTR(error)
+ * otherwise.
  */
-struct block_device *open_bdev_excl(const char *path, int flags,
-				    int kind, void *holder)
+struct block_device *lookup_bdev(const char *path)
 {
-	struct inode *inode;
 	struct block_device *bdev;
+	struct inode *inode;
 	struct nameidata nd;
-	mode_t mode = FMODE_READ;
-	int error = 0;
+	int error;
 
 	if (!path || !*path)
 		return ERR_PTR(-EINVAL);
@@ -828,17 +824,44 @@ struct block_device *open_bdev_excl(const char *path, int flags,
 	inode = nd.dentry->d_inode;
 	error = -ENOTBLK;
 	if (!S_ISBLK(inode->i_mode))
-		goto path_release;
+		goto fail;
 	error = -EACCES;
 	if (nd.mnt->mnt_flags & MNT_NODEV)
-		goto path_release;
+		goto fail;
 	error = bd_acquire(inode);
 	if (error)
-		goto path_release;
+		goto fail;
 	bdev = inode->i_bdev;
 
-	/* Done with lookups */
+out:
 	path_release(&nd);
+	return bdev;
+fail:
+	bdev = ERR_PTR(error);
+	goto out;
+}
+
+/**
+ * open_bdev_excl  -  open a block device by name and set it up for use
+ *
+ * @path:	special file representing the block device
+ * @flags:	%MS_RDONLY for opening read-only
+ * @kind:	usage (same as the 4th paramter to blkdev_get)
+ * @holder:	owner for exclusion
+ *
+ * Open the blockdevice described by the special file at @path, claim it
+ * for the @holder and properly set it up for @kind usage.
+ */
+struct block_device *open_bdev_excl(const char *path, int flags,
+				    int kind, void *holder)
+{
+	struct block_device *bdev;
+	mode_t mode = FMODE_READ;
+	int error = 0;
+
+	bdev = lookup_bdev(path);
+	if (IS_ERR(bdev))
+		return bdev;
 
 	if (!(flags & MS_RDONLY))
 		mode |= FMODE_WRITE;
@@ -857,10 +880,6 @@ struct block_device *open_bdev_excl(const char *path, int flags,
 blkdev_put:
 	blkdev_put(bdev, BDEV_FS);
 	return ERR_PTR(error);
-
-path_release:
-	path_release(&nd);
-	return ERR_PTR(error);
 }
 
 /**

fs/quota.c

@@ -102,35 +102,6 @@ static int check_quotactl_valid(struct super_block *sb, int type, int cmd, qid_t
 	return security_quotactl (cmd, type, id, sb);
 }
 
-/* Resolve device pathname to superblock */
-static struct super_block *resolve_dev(const char *path)
-{
-	int ret;
-	mode_t mode;
-	struct nameidata nd;
-	struct block_device *bdev;
-	struct super_block *sb;
-
-	ret = user_path_walk(path, &nd);
-	if (ret)
-		goto out;
-
-	bdev = nd.dentry->d_inode->i_bdev;
-	mode = nd.dentry->d_inode->i_mode;
-	path_release(&nd);
-
-	ret = -ENOTBLK;
-	if (!S_ISBLK(mode))
-		goto out;
-	ret = -ENODEV;
-	sb = get_super(bdev);
-	if (!sb)
-		goto out;
-	return sb;
-out:
-	return ERR_PTR(ret);
-}
-
 /* Copy parameters and call proper function */
 static int do_quotactl(struct super_block *sb, int type, int cmd, qid_t id, caddr_t addr)
 {
@@ -249,21 +220,24 @@ asmlinkage long sys_quotactl(unsigned int cmd, const char *special, qid_t id, ca
 {
 	uint cmds, type;
 	struct super_block *sb = NULL;
-	int ret = -EINVAL;
+	struct block_device *bdev;
+	int ret = -ENODEV;
 
 	cmds = cmd >> SUBCMDSHIFT;
 	type = cmd & SUBCMDMASK;
 
-	if (IS_ERR(sb = resolve_dev(special))) {
-		ret = PTR_ERR(sb);
-		sb = NULL;
-		goto out;
-	}
-	if ((ret = check_quotactl_valid(sb, type, cmds, id)) < 0)
-		goto out;
+	bdev = lookup_bdev(special);
+	if (IS_ERR(bdev))
+		return PTR_ERR(bdev);
+	sb = get_super(bdev);
+	bdput(bdev);
+
+	if (sb) {
+		ret = check_quotactl_valid(sb, type, cmds, id);
+		if (ret >= 0)
 			ret = do_quotactl(sb, type, cmds, id, addr);
-out:
-	if (sb)
 		drop_super(sb);
+	}
+
 	return ret;
 }

include/linux/fs.h

@@ -1098,6 +1098,7 @@ extern inline const char *bdevname(struct block_device *bdev)
 {
 	return __bdevname(bdev->bd_dev);
 }
+extern struct block_device *lookup_bdev(const char *);
 extern struct block_device *open_bdev_excl(const char *, int, int, void *);
 extern void close_bdev_excl(struct block_device *, int);