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

[Sandipan Das]

On 12-10-2026 21:00, Peter Zijlstra wrote:
> 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@xxxxxxxxxx>
>> Closes: https://lore.kernel.org/all/sashiko-outbox-163130@xxxxxxxxxx/
>> Signed-off-by: Sandipan Das <sandipan.das@xxxxxxx>
>> ---
>>  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.

Yes, it is a deviation. I thought of proposing the following but didn't
proceed because I was not sure about the if condition in the second hunk
which considers things other than cpuc->lbr_users for an early return.

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);