splice: Check for zero count in vfs_splice_read()

Make vfs_splice_read() return immediately if the length is 0.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: linux-block@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/20230522135018.2742245-7-dhowells@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/splice.c

@@ -891,6 +891,8 @@ long vfs_splice_read(struct file *in, loff_t *ppos,
 
 	if (unlikely(!(in->f_mode & FMODE_READ)))
 		return -EBADF;
+	if (!len)
+		return 0;
 
 	/* Don't try to read more the pipe has space for. */
 	p_space = pipe->max_usage - pipe_occupancy(pipe->head, pipe->tail);