On Sun, 7 Jun 2026 15:24:30 +0800 Hui Wang <[email protected]> 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) <[email protected]> Thanks, > 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 > > -- Masami Hiramatsu (Google) <[email protected]>
