tracing: Remove extra zeroing out of the ring buffer page

BugLink: http://bugs.launchpad.net/bugs/1745069

commit 6b7e633f upstream.

The ring_buffer_read_page() takes care of zeroing out any extra data in the
page that it returns. There's no need to zero it out again from the
consumer. It was removed from one consumer of this function, but
read_buffers_splice_read() did not remove it, and worse, it contained a
nasty bug because of it.

Fixes: 2711ca23 ("ring-buffer: Move zeroing out excess in page to ring buffer code")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

kernel/trace/trace.c

@@ -5754,7 +5754,7 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
 		.spd_release	= buffer_spd_release,
 	};
 	struct buffer_ref *ref;
-	int entries, size, i;
+	int entries, i;
 	ssize_t ret = 0;
 
 #ifdef CONFIG_TRACER_MAX_TRACE
@@ -5805,14 +5805,6 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
 			break;
 		}
 
-		/*
-		 * zero out any left over data, this is going to
-		 * user land.
-		 */
-		size = ring_buffer_page_len(ref->page);
-		if (size < PAGE_SIZE)
-			memset(ref->page + size, 0, PAGE_SIZE - size);
-
 		page = virt_to_page(ref->page);
 
 		spd.pages[i] = page;