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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

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>

Thanks,

> Fixes: 2271048d1b3b ("ring-buffer: Do 8 byte alignment for 64 bit that can not handle 4 byte align")
> Signed-off-by: Hui Wang <hui.wang@canonical.com>
> ---
>  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) <mhiramat@kernel.org>