afs: Fix AFS file locking to allow fine grained locks

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>

fs/afs/flock.c

@@ -458,10 +458,6 @@ static int afs_do_setlk(struct file *file, struct file_lock *fl)
 
 	_enter("{%llx:%llu},%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;
@@ -613,10 +609,6 @@ 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;
@@ -644,12 +636,15 @@ 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;