Re: [PATCH v2 2/2] perf/x86/intel/lbr: Limit branch snapshots to valid entries

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

LGTM. Thanks.

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>


On 10/15/2026 7:46 PM, Sandipan Das wrote:
> The branch snapshot callback used by bpf_get_branch_snapshot() always
> copies x86_pmu.lbr_nr entries from cpuc->lbr_entries[], even when the
> last LBR read kept fewer branches. The entries past lbr_stack.nr hold
> invalid branches which end up reaching BPF programs.
>
> Limit copying to lbr_stack.nr entries and also reset lbr_stack.nr when
> intel_pmu_lbr_read() skips reading the registers so that a stale count
> is never reused.
>
> Fixes: c22ac2a3d4bd ("perf: Enable branch record for software events")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://lore.kernel.org/all/sashiko-outbox-163130@kernel.org/
> Signed-off-by: Sandipan Das <sandipan.das@amd.com>
> Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> ---
>  arch/x86/events/intel/core.c | 2 +-
>  arch/x86/events/intel/lbr.c  | 4 +++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 0a34d674df59..5dfb9e9156e8 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -2822,7 +2822,7 @@ __intel_pmu_snapshot_branch_stack(struct perf_branch_entry *entries,
>  	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>  
>  	intel_pmu_lbr_read();
> -	cnt = min_t(unsigned int, cnt, x86_pmu.lbr_nr);
> +	cnt = min_t(unsigned int, cnt, cpuc->lbr_stack.nr);
>  
>  	memcpy(entries, cpuc->lbr_entries, sizeof(struct perf_branch_entry) * cnt);
>  	intel_pmu_enable_all(0);
> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> index 0b446d8912e4..6b070490f461 100644
> --- a/arch/x86/events/intel/lbr.c
> +++ b/arch/x86/events/intel/lbr.c
> @@ -1026,8 +1026,10 @@ void intel_pmu_lbr_read(void)
>  	 * but this simple approach seems to work for now.
>  	 */
>  	if (!cpuc->lbr_users || vlbr_exclude_host() ||
> -	    cpuc->lbr_users == cpuc->lbr_pebs_users)
> +	    cpuc->lbr_users == cpuc->lbr_pebs_users) {
> +		cpuc->lbr_stack.nr = 0;
>  		return;
> +	}
>  
>  	x86_pmu.lbr_read(cpuc);
>