Re: [PATCH 3/3] perf: uapi: Document perf_event_attr::sig_data truncation on 32 bit architectures

[Dmitry Vyukov]

On Mon, 31 Jan 2022 at 11:34, Marco Elver <elver@xxxxxxxxxx> wrote:
>
> Due to the alignment requirements of siginfo_t, as described in
> 3ddb3fd8cdb0 ("signal, perf: Fix siginfo_t by avoiding u64 on 32-bit
> architectures"), siginfo_t::si_perf_data is limited to an unsigned long.
>
> However, perf_event_attr::sig_data is an u64, to avoid having to deal
> with compat conversions. Due to being an u64, it may not immediately be
> clear to users that sig_data is truncated on 32 bit architectures.
>
> Add a comment to explicitly point this out, and hopefully help some
> users save time by not having to deduce themselves what's happening.
>
> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>


> ---
>  include/uapi/linux/perf_event.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 1b65042ab1db..82858b697c05 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -465,6 +465,8 @@ struct perf_event_attr {
>         /*
>          * User provided data if sigtrap=1, passed back to user via
>          * siginfo_t::si_perf_data, e.g. to permit user to identify the event.
> +        * Note, siginfo_t::si_perf_data is long-sized, and sig_data will be
> +        * truncated accordingly on 32 bit architectures.
>          */
>         __u64   sig_data;
>  };
> --
> 2.35.0.rc2.247.g8bbb082509-goog
>