Re: [PATCH 1/2] ring-buffer: Fix event length with forced 8-byte alignment

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 8 Jun 2026 18:02:45 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> On Sun,  7 Jun 2026 15:24:30 +0800
> Hui Wang <hui.wang@canonical.com> 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>

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/20260601023251.1916483-1-dtcccc@linux.alibaba.com/


-- Steve