On Mon, 8 Jun 2026 18:02:45 +0900 Masami Hiramatsu (Google) <[email protected]> wrote:
> 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]> This is the patch I meant to reply to. NACK as the test is broken and not the kernel. There's a pending fix already: https://lore.kernel.org/all/[email protected]/ -- Steve
