[PATCH] ring-buffer: Fix buffer max_data_size with max_event_size

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>

The maximum ring buffer data size is the maximum size of data that can be
recorded on the ring buffer. Events must be smaller than the sub buffer
data size minus any meta data. This size is checked before trying to
allocate from the ring buffer because the allocation assumes that the size
will fit on the sub buffer.

The maximum size was calculated as the size of a sub buffer page (which is
currently PAGE_SIZE minus the sub buffer header) minus the size of the
meta data of an individual event. But it missed the possible adding of a
time stamp for events that are added long enough apart that the event meta
data can't hold the time delta.

When an event is added that is greater than the current BUF_MAX_DATA_SIZE
minus the size of a time stamp, but still less than or equal to
BUF_MAX_DATA_SIZE, the ring buffer would go into an infinite loop, looking
for a page that can hold the event. Luckily, there's a check for this loop
and after 1000 iterations and a warning is emitted and the ring buffer is
disabled. But this should never happen.

This can happen when a large event is added first, or after a long period
where an absolute timestamp is prefixed to the event, increasing its size
by 8 bytes. This passes the check and then goes into the algorithm that
causes the infinite loop.

Fix this by creating a BUF_MAX_EVENT_SIZE to be used to determine if the
passed in event is too big for the buffer.

Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx> # (on IRC)
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
 kernel/trace/ring_buffer.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 8d2a4f00eca9..a38e5a3c6803 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -378,6 +378,9 @@ static inline bool test_time_stamp(u64 delta)
 /* Max payload is BUF_PAGE_SIZE - header (8bytes) */
 #define BUF_MAX_DATA_SIZE (BUF_PAGE_SIZE - (sizeof(u32) * 2))
 
+/* Events may have a time stamp attached to them */
+#define BUF_MAX_EVENT_SIZE (BUF_MAX_DATA_SIZE - RB_LEN_TIME_EXTEND)
+
 int ring_buffer_print_page_header(struct trace_seq *s)
 {
 	struct buffer_data_page field;
@@ -3810,7 +3813,7 @@ ring_buffer_lock_reserve(struct trace_buffer *buffer, unsigned long length)
 	if (unlikely(atomic_read(&cpu_buffer->record_disabled)))
 		goto out;
 
-	if (unlikely(length > BUF_MAX_DATA_SIZE))
+	if (unlikely(length > BUF_MAX_EVENT_SIZE))
 		goto out;
 
 	if (unlikely(trace_recursive_lock(cpu_buffer)))
@@ -3960,7 +3963,7 @@ int ring_buffer_write(struct trace_buffer *buffer,
 	if (atomic_read(&cpu_buffer->record_disabled))
 		goto out;
 
-	if (length > BUF_MAX_DATA_SIZE)
+	if (length > BUF_MAX_EVENT_SIZE)
 		goto out;
 
 	if (unlikely(trace_recursive_lock(cpu_buffer)))
-- 
2.42.0