Re: [PATCH 1/2] ring-buffer: Fix event length with forced 8-byte alignment
From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Date: 2026-06-08 09:02:49
Also in:
linux-kselftest
On Sun, 7 Jun 2026 15:24:30 +0800 Hui Wang [off-list ref] wrote:
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.
Good catch! This looks good to me. Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org> Thanks,
quoted hunk ↗ jump to hunk
Fixes: 2271048d1b3b ("ring-buffer: Do 8 byte alignment for 64 bit that can not handle 4 byte align") Signed-off-by: Hui Wang <redacted> --- 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
-- Masami Hiramatsu (Google) [off-list ref]