Re: [PATCH] perf/x86/amd/lbr: Limit branch snapshots to valid entries

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 07, 2026 at 11:40:02PM +0530, Sandipan Das wrote:
> The branch snapshot callback used by bpf_get_branch_snapshot() copies
> up to x86_pmu.lbr_nr entries from cpuc->lbr_entries[], even when the
> last LBR read kept fewer branches. The entries past lbr_stack.nr are
> invalid, but they end up reaching BPF programs.
> 
> Limit copying to lbr_stack.nr entries and also reset lbr_stack.nr when
> amd_pmu_lbr_read() skips reading the registers, so that a stale count
> is never reused.
> 
> Fixes: a4d18112e531 ("perf/x86/amd: Support capturing LBR from 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>
> ---
>  arch/x86/events/amd/core.c | 2 +-
>  arch/x86/events/amd/lbr.c  | 4 +++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
> index 49b6b8fce566..bf631f60757c 100644
> --- a/arch/x86/events/amd/core.c
> +++ b/arch/x86/events/amd/core.c
> @@ -944,7 +944,7 @@ static int amd_pmu_v2_snapshot_branch_stack(struct perf_branch_entry *entries, u
>  	cpuc = this_cpu_ptr(&cpu_hw_events);
>  
>  	amd_pmu_lbr_read();
> -	cnt = min(cnt, x86_pmu.lbr_nr);
> +	cnt = min(cnt, cpuc->lbr_stack.nr);
>  	memcpy(entries, cpuc->lbr_entries, sizeof(struct perf_branch_entry) * cnt);
>  
>  	amd_pmu_v2_enable_all(0);
> diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
> index 1e1b2f08cb51..a00d3850c4a8 100644
> --- a/arch/x86/events/amd/lbr.c
> +++ b/arch/x86/events/amd/lbr.c
> @@ -165,8 +165,10 @@ void amd_pmu_lbr_read(void)
>  	struct branch_entry entry;
>  	int out = 0, idx, i;
>  
> -	if (!cpuc->lbr_users)
> +	if (!cpuc->lbr_users) {
> +		cpuc->lbr_stack.nr = 0;
>  		return;
> +	}
>  
>  	for (i = 0; i < x86_pmu.lbr_nr; i++) {
>  		entry.from.full	= amd_pmu_lbr_get_from(i);

You now seem to deviate from what the Intel code does. Please clarify.