relay: require non-NULL callbacks in relay_open()

There are no clients passing NULL callbacks, which makes sense as it
wouldn't even create a file.  Require non-NULL callbacks, and throw away
the handling for NULL callbacks.

Link: https://lkml.kernel.org/r/e40642f3b027d2bb6bc851ddb60e0a61ea51f5f8.1606153547.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/relay.c

@@ -291,13 +291,6 @@ static int remove_buf_file_default_callback(struct dentry *dentry)
 	return -EINVAL;
 }
 
-/* relay channel default callbacks */
-static struct rchan_callbacks default_channel_callbacks = {
-	.subbuf_start = subbuf_start_default_callback,
-	.create_buf_file = create_buf_file_default_callback,
-	.remove_buf_file = remove_buf_file_default_callback,
-};
-
 /**
  *	wakeup_readers - wake up readers waiting on a channel
  *	@work: contains the channel buffer
@@ -472,11 +465,6 @@ static void relay_close_buf(struct rchan_buf *buf)
 static void setup_callbacks(struct rchan *chan,
 				   struct rchan_callbacks *cb)
 {
-	if (!cb) {
-		chan->cb = &default_channel_callbacks;
-		return;
-	}
-
 	if (!cb->subbuf_start)
 		cb->subbuf_start = subbuf_start_default_callback;
 	if (!cb->create_buf_file)
@@ -542,6 +530,8 @@ struct rchan *relay_open(const char *base_filename,
 		return NULL;
 	if (subbuf_size > UINT_MAX / n_subbufs)
 		return NULL;
+	if (!cb)
+		return NULL;
 
 	chan = kzalloc(sizeof(struct rchan), GFP_KERNEL);
 	if (!chan)