Re: [PATCH] perf core: apply calculated padding to PERF_SAMPLE_RAW output

[Peter Zijlstra <peterz@infradead.org>]

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@incline.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
>