[PATCH] Fix double decrement of mqueue_mnt->mnt_count in sys_mq_open

Fixed the refcounting on failure exits in sys_mq_open() and
cleaned the logics up. Rules are actually pretty simple - dentry_open()
expects vfsmount and dentry to be pinned down and it either transfers
them into created struct file or drops them. Old code had been very
confused in that area - if dentry_open() had failed either in do_open()
or do_create(), we ended up dentry and mqueue_mnt dropped twice, once
by dentry_open() cleanup and then by sys_mq_open().

Fix consists of making the rules for do_create() and do_open()
same as for dentry_open() and updating the sys_mq_open() accordingly;
that actually leads to more straightforward code and less work on
normal path.

Signed-off-by: Al Viro <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

ipc/mqueue.c

···
 static struct file *do_create(struct dentry *dir, struct dentry *dentry,
 			int oflag, mode_t mode, struct mq_attr __user *u_attr)
 {
-	struct file *filp;
 	struct mq_attr attr;
 	int ret;
 
-	if (u_attr != NULL) {
+	if (u_attr) {
+		ret = -EFAULT;
 		if (copy_from_user(&attr, u_attr, sizeof(attr)))
-			return ERR_PTR(-EFAULT);
+			goto out;
+		ret = -EINVAL;
 		if (!mq_attr_ok(&attr))
-			return ERR_PTR(-EINVAL);
+			goto out;
 		/* store for use during create */
 		dentry->d_fsdata = &attr;
 	}
···
 	ret = vfs_create(dir->d_inode, dentry, mode, NULL);
 	dentry->d_fsdata = NULL;
 	if (ret)
-		return ERR_PTR(ret);
+		goto out;
 
-	filp = dentry_open(dentry, mqueue_mnt, oflag);
-	if (!IS_ERR(filp))
-		dget(dentry);
+	return dentry_open(dentry, mqueue_mnt, oflag);
 
-	return filp;
+out:
+	dput(dentry);
+	mntput(mqueue_mnt);
+	return ERR_PTR(ret);
 }
 
 /* Opens existing queue */
···
 {
 static int oflag2acc[O_ACCMODE] = { MAY_READ, MAY_WRITE,
 					MAY_READ | MAY_WRITE };
-	struct file *filp;
 
-	if ((oflag & O_ACCMODE) == (O_RDWR | O_WRONLY))
+	if ((oflag & O_ACCMODE) == (O_RDWR | O_WRONLY)) {
+		dput(dentry);
+		mntput(mqueue_mnt);
 		return ERR_PTR(-EINVAL);
+	}
 
-	if (permission(dentry->d_inode, oflag2acc[oflag & O_ACCMODE], NULL))
+	if (permission(dentry->d_inode, oflag2acc[oflag & O_ACCMODE], NULL)) {
+		dput(dentry);
+		mntput(mqueue_mnt);
 		return ERR_PTR(-EACCES);
+	}
 
-	filp = dentry_open(dentry, mqueue_mnt, oflag);
-
-	if (!IS_ERR(filp))
-		dget(dentry);
-
-	return filp;
+	return dentry_open(dentry, mqueue_mnt, oflag);
 }
 
 asmlinkage long sys_mq_open(const char __user *u_name, int oflag, mode_t mode,
···
 
 	if (oflag & O_CREAT) {
 		if (dentry->d_inode) {	/* entry already exists */
-			filp = (oflag & O_EXCL) ? ERR_PTR(-EEXIST) :
-					do_open(dentry, oflag);
+			error = -EEXIST;
+			if (oflag & O_EXCL)
+				goto out;
+			filp = do_open(dentry, oflag);
 		} else {
 			filp = do_create(mqueue_mnt->mnt_root, dentry,
 						oflag, mode, u_attr);
 		}
-	} else
-		filp = (dentry->d_inode) ? do_open(dentry, oflag) :
-					ERR_PTR(-ENOENT);
-
-	dput(dentry);
+	} else {
+		error = -ENOENT;
+		if (!dentry->d_inode)
+			goto out;
+		filp = do_open(dentry, oflag);
+	}
 
 	if (IS_ERR(filp)) {
 		error = PTR_ERR(filp);
···
 	fd_install(fd, filp);
 	goto out_upsem;
 
-out_putfd:
+out:
+	dput(dentry);
 	mntput(mqueue_mnt);
+out_putfd:
 	put_unused_fd(fd);
 out_err:
 	fd = error;