Re: [Patch v4 4/5] perf/x86/intel/lbr: Limit branch snapshots to valid entries

[sashiko-bot@kernel.org]

> 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
> no active LBR events or LBR hardware is owned by guest. So the stale or
> guest LBR stack data can't be leaked.
> 
> For the case where all LBR events are PEBS events, hardware LBR reads are
> currently skipped; that can leave stale LBR stack data from previous PEBS
> sampling in place and may be inconsistent with the intended semantics of
> bpf_get_branch_snapshot(). This will be addressed in a follow-up patch.
> 
> 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/
> Co-developed-by: Sandipan Das <sandipan.das@amd.com>
> Signed-off-by: Sandipan Das <sandipan.das@amd.com>
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261021050711.4043703-1-dapeng1.mi@linux.intel.com?part=4