ring-buffer: Have saved event hold the entire event

For the ring buffer iterator (non-consuming read), the event needs to be
copied into the iterator buffer to make sure that a writer does not
overwrite it while the user is reading it. If a write happens during the
copy, the buffer is simply discarded.

But the temp buffer itself was not big enough. The allocation of the
buffer was only BUF_MAX_DATA_SIZE, which is the maximum data size that can
be passed into the ring buffer and saved. But the temp buffer needs to
hold the meta data as well. That would be BUF_PAGE_SIZE and not
BUF_MAX_DATA_SIZE.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: [email protected]
Cc: Masami Hiramatsu <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Fixes: 785888c ("ring-buffer: Have rb_iter_head_event() handle concurrent writer")
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

kernel/trace/ring_buffer.c

@@ -2409,7 +2409,7 @@ rb_iter_head_event(struct ring_buffer_iter *iter)
 	 */
 	barrier();
 
-	if ((iter->head + length) > commit || length > BUF_MAX_DATA_SIZE)
+	if ((iter->head + length) > commit || length > BUF_PAGE_SIZE)
 		/* Writer corrupted the read? */
 		goto reset;
 
@@ -5118,7 +5118,8 @@ ring_buffer_read_prepare(struct trace_buffer *buffer, int cpu, gfp_t flags)
 	if (!iter)
 		return NULL;
 
-	iter->event = kmalloc(BUF_MAX_DATA_SIZE, flags);
+	/* Holds the entire event: data and meta data */
+	iter->event = kmalloc(BUF_PAGE_SIZE, flags);
 	if (!iter->event) {
 		kfree(iter);
 		return NULL;