linux-kernel - Re: [PATCH] perf core: apply calculated padding to PERF_SAMPLE

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Message-ID: <20230314192740.GQ2017917@hirez.programming.kicks-ass.net>
Date:   Tue, 14 Mar 2023 20:27:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Timo Beckers <timo@...line.eu>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf core: apply calculated padding to PERF_SAMPLE_RAW
 output

On Tue, May 19, 2020 at 03:26:16PM +0200, Timo Beckers wrote:
> Zero the amount of padding bytes determined in perf_prepare_sample().
> This prevents garbage being read from the ring buffer after it has wrapped
> the page boundary at least once.

But it's user garbage, right? And they should be unconsumed anyway.

> Signed-off-by: Timo Beckers <timo@...line.eu>
> ---
>  kernel/events/core.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 80cf996a7f19..d4e0b003ece0 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6807,8 +6807,16 @@ void perf_output_sample(struct perf_output_handle *handle,
>  					break;
>  				frag = frag->next;
>  			} while (1);
> -			if (frag->pad)
> -				__output_skip(handle, NULL, frag->pad);
> +			/*
> +			 * The padding value is determined in
> +			 * perf_prepare_sample() and is not
> +			 * expected to exceed u64.
> +			 */
> +			if (frag->pad) {
> +				u64 zero = 0;
> +
> +				__output_copy(handle, &zero, frag->pad);
> +			}
>  		} else {
>  			struct {
>  				u32	size;
> -- 
> 2.26.2
>