When RB_FORCE_8BYTE_ALIGNMENT is true, rb_calculate_event_length()
reserves the space of event->array[0] for placing the data length and
rb_update_event() stores the data length in event->array[0]
accordingly. As a result the whole event length will add extra 4 bytes
for sizeof(event.array[0]) unconditionally.
But ring_buffer_event_length() only subtracts the
sizeof(event->array[0]) for events larger than RB_MAX_SMALL_DATA +
sizeof(event->array[0]). As a result, small events on architectures
with RB_FORCE_8BYTE_ALIGNMENT=true report a data length that is 4
bytes larger than expected.
To fix it, add the RB_FORCE_8BYTE_ALIGNMENT as a condition to subtract
the size of that length field whenever RB_FORCE_8BYTE_ALIGNMENT is
true.
This issue is observed in a riscv64 kernel with
CONFIG_HAVE_64BIT_ALIGNED_ACCESS set to y, when we run ftrace selftest
trace_marker_raw.tc, we get the weird log: for cases where the id is
1..100, the number of data field is 8*N, but once id exceeds 100, the
number of data field becomes 8*N+4:
# 1 buf: 58 00 00 00 80 5e d1 63 (number of data field is 8*1)
...
# a buf: 58 ... (number of data field is 8*2)
...
# 64 buf: 58 ... (number of data field is 8*13)
# 65 buf: 58 ... (number of data field is 8*13+4)
After applying this change, the number of data field keeps being 8*N+4
consistently.
Fixes: 2271048d1b3b ("ring-buffer: Do 8 byte alignment for 64 bit that can not
handle 4 byte align")
Signed-off-by: Hui Wang <[email protected]>
---
kernel/trace/ring_buffer.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 56a328e94395..d9af2bbaf9c0 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -270,7 +270,8 @@ unsigned ring_buffer_event_length(struct ring_buffer_event
*event)
if (event->type_len > RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
return length;
length -= RB_EVNT_HDR_SIZE;
- if (length > RB_MAX_SMALL_DATA + sizeof(event->array[0]))
+ if (length > RB_MAX_SMALL_DATA + sizeof(event->array[0]) ||
+ RB_FORCE_8BYTE_ALIGNMENT)
length -= sizeof(event->array[0]);
return length;
}
--
2.43.0
