afs: Fix AFS file locking to allow fine grained locks

[ Upstream commit 68ce801f ]

Fix AFS file locking to allow fine grained locks as some applications, such
as firefox, won't work if they can't take such locks on certain state files
- thereby preventing the use of kAFS to distribute a home directory.

Note that this cannot be made completely functional as the protocol only
has provision for whole-file locks, so there exists the possibility of a
process deadlocking itself by getting a partial read-lock on a file first
and then trying to get a non-overlapping write-lock - but we got the
server's read lock with the first lock, so we're now stuck.

OpenAFS solves this by just granting any partial-range lock directly
without consulting the server - and hoping there's no remote collision.  I
want to implement that in a separate patch and it requires a bit more
thought.

Fixes: 8d6c554126b8 ("AFS: implement file locking")
Reported-by: Jonathan Billings <jsbillings@jsbillings.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/afs/flock.c

@@ -432,10 +432,6 @@ static int afs_do_setlk(struct file *file, struct file_lock *fl)
 
 	_enter("{%x:%u},%u", vnode->fid.vid, vnode->fid.vnode, fl->fl_type);
 
-	/* only whole-file locks are supported */
-	if (fl->fl_start != 0 || fl->fl_end != OFFSET_MAX)
-		return -EINVAL;
-
 	fl->fl_ops = &afs_lock_ops;
 	INIT_LIST_HEAD(&fl->fl_u.afs.link);
 	fl->fl_u.afs.state = AFS_LOCK_PENDING;
@@ -587,10 +583,6 @@ static int afs_do_unlk(struct file *file, struct file_lock *fl)
 	/* Flush all pending writes before doing anything with locks. */
 	vfs_fsync(file, 0);
 
-	/* only whole-file unlocks are supported */
-	if (fl->fl_start != 0 || fl->fl_end != OFFSET_MAX)
-		return -EINVAL;
-
 	ret = posix_lock_file(file, fl, NULL);
 	_leave(" = %d [%u]", ret, vnode->lock_state);
 	return ret;
@@ -618,12 +610,15 @@ static int afs_do_getlk(struct file *file, struct file_lock *fl)
 			goto error;
 
 		lock_count = READ_ONCE(vnode->status.lock_count);
-		if (lock_count > 0)
-			fl->fl_type = F_RDLCK;
-		else
-			fl->fl_type = F_WRLCK;
-		fl->fl_start = 0;
-		fl->fl_end = OFFSET_MAX;
+		if (lock_count != 0) {
+			if (lock_count > 0)
+				fl->fl_type = F_RDLCK;
+			else
+				fl->fl_type = F_WRLCK;
+			fl->fl_start = 0;
+			fl->fl_end = OFFSET_MAX;
+			fl->fl_pid = 0;
+		}
 	}
 
 	ret = 0;