Re: [PATCH v1] perf record: Ensure space for lost samples

[Namhyung Kim <namhyung@kernel.org>]

On Fri, Jun 07, 2024 at 01:47:27PM -0700, Ian Rogers wrote:
> Previous allocation didn't account for sample ID written after the
> lost samples event. Switch from malloc/free to a stack allocation.
> 
> Reported-by: Milian Wolff <milian.wolff@kdab.com>
> Closes: https://lore.kernel.org/linux-perf-users/23879991.0LEYPuXRzz@milian-workstation/
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/lib/perf/include/perf/event.h |  3 +++
>  tools/perf/builtin-record.c         | 37 ++++++++++-------------------
>  2 files changed, 15 insertions(+), 25 deletions(-)
> 
> diff --git a/tools/lib/perf/include/perf/event.h b/tools/lib/perf/include/perf/event.h
> index ae64090184d3..8f3cefef9069 100644
> --- a/tools/lib/perf/include/perf/event.h
> +++ b/tools/lib/perf/include/perf/event.h
> @@ -77,6 +77,9 @@ struct perf_record_lost_samples {
>  	__u64			 lost;
>  };
>  
> +#define MAX_ID_HDR_ENTRIES  6
> +#define PERF_RECORD_MAX_LOST_SAMPLE_AND_ID_SIZE \
> +	(sizeof(struct perf_record_lost_samples) + MAX_ID_HDR_ENTRIES * sizeof(__u64))
>  /*
>   * PERF_FORMAT_ENABLED | PERF_FORMAT_RUNNING | PERF_FORMAT_ID | PERF_FORMAT_LOST
>   */
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 66a3de8ac661..1615a1723fb9 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -1926,7 +1926,10 @@ static void __record__save_lost_samples(struct record *rec, struct evsel *evsel,
>  static void record__read_lost_samples(struct record *rec)
>  {
>  	struct perf_session *session = rec->session;
> -	struct perf_record_lost_samples *lost = NULL;
> +	union {
> +		struct perf_record_lost_samples lost;
> +		char lost_and_sample_id[PERF_RECORD_MAX_LOST_SAMPLE_AND_ID_SIZE];
> +	} lost;

Can we make it struct and keep the sample_id part only?

Other than that, looks good to me.

Thanks,
Namhyung

>  	struct evsel *evsel;
>  
>  	/* there was an error during record__open */
> @@ -1951,20 +1954,13 @@ static void record__read_lost_samples(struct record *rec)
>  
>  				if (perf_evsel__read(&evsel->core, x, y, &count) < 0) {
>  					pr_debug("read LOST count failed\n");
> -					goto out;
> +					return;
>  				}
>  
>  				if (count.lost) {
> -					if (!lost) {
> -						lost = zalloc(sizeof(*lost) +
> -							      session->machines.host.id_hdr_size);
> -						if (!lost) {
> -							pr_debug("Memory allocation failed\n");
> -							return;
> -						}
> -						lost->header.type = PERF_RECORD_LOST_SAMPLES;
> -					}
> -					__record__save_lost_samples(rec, evsel, lost,
> +					memset(&lost, 0, sizeof(lost));
> +					lost.lost.header.type = PERF_RECORD_LOST_SAMPLES;
> +					__record__save_lost_samples(rec, evsel, &lost.lost,
>  								    x, y, count.lost, 0);
>  				}
>  			}
> @@ -1972,21 +1968,12 @@ static void record__read_lost_samples(struct record *rec)
>  
>  		lost_count = perf_bpf_filter__lost_count(evsel);
>  		if (lost_count) {
> -			if (!lost) {
> -				lost = zalloc(sizeof(*lost) +
> -					      session->machines.host.id_hdr_size);
> -				if (!lost) {
> -					pr_debug("Memory allocation failed\n");
> -					return;
> -				}
> -				lost->header.type = PERF_RECORD_LOST_SAMPLES;
> -			}
> -			__record__save_lost_samples(rec, evsel, lost, 0, 0, lost_count,
> +			memset(&lost, 0, sizeof(lost));
> +			lost.lost.header.type = PERF_RECORD_LOST_SAMPLES;
> +			__record__save_lost_samples(rec, evsel, &lost.lost, 0, 0, lost_count,
>  						    PERF_RECORD_MISC_LOST_SAMPLES_BPF);
>  		}
>  	}
> -out:
> -	free(lost);
>  }
>  
>  static volatile sig_atomic_t workload_exec_errno;
> @@ -3198,7 +3185,7 @@ static int switch_output_setup(struct record *rec)
>  	unsigned long val;
>  
>  	/*
> -	 * If we're using --switch-output-events, then we imply its 
> +	 * If we're using --switch-output-events, then we imply its
>  	 * --switch-output=signal, as we'll send a SIGUSR2 from the side band
>  	 *  thread to its parent.
>  	 */
> -- 
> 2.45.2.505.gda0bf45e8d-goog
>