[PATCH 3.2 068/140] ring-buffer: Mask out the info bits when returning buffer page length

Ben Hutchings
2018-02-28T15:47:39Z

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

commit 45d8b80c2ac5d21cd1e2954431fb676bc2b1e099 upstream.

Two info bits were added to the "commit" part of the ring buffer data page
when returned to be consumed. This was to inform the user space readers that
events have been missed, and that the count may be stored at the end of the
page.

What wasn't handled, was the splice code that actually called a function to
return the length of the data in order to zero out the rest of the page
before sending it up to user space. These data bits were returned with the
length making the value negative, and that negative value was not checked.
It was compared to PAGE_SIZE, and only used if the size was less than
PAGE_SIZE. Luckily PAGE_SIZE is unsigned long which made the compare an
unsigned compare, meaning the negative size value did not end up causing a
large portion of memory to be randomly zeroed out.

Fixes: 66a8cb95ed040 ("ring-buffer: Add place holder recording of dropped events")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
kernel/trace/ring_buffer.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -360,6 +360,8 @@ EXPORT_SYMBOL_GPL(ring_buffer_event_data
/* Missed count stored at end */
#define RB_MISSED_STORED (1 << 30)

+#define RB_MISSED_FLAGS (RB_MISSED_EVENTS|RB_MISSED_STORED)
+
struct buffer_data_page {
u64 time_stamp; /* page time stamp */
local_t commit; /* write committed index */
@@ -411,7 +413,9 @@ static void rb_init_page(struct buffer_d
*/
size_t ring_buffer_page_len(void *page)
{
- return local_read(&((struct buffer_data_page *)page)->commit)
+ struct buffer_data_page *bpage = page;
+
+ return (local_read(&bpage->commit) & ~RB_MISSED_FLAGS)
+ BUF_PAGE_HDR_SIZE;
}