[PATCH 2/2] perf/x86: Disable precise sampling for PERF_SAMPLE_STACK_USER

[Dapeng Mi]

PERF_SAMPLE_STACK_USER needs to return the user stack and user registers
to user space when the PMI exits. Since the skid from the PEBS/IBS sample
and PMI delivery, the PEBS/IBS register snapshot (especially IP/SP/BP)
can diverge from the user stack at PMI return. That mismatch breaks DWARF
unwinding.

Precise sampling provides no benefit in this case, so disable PEBS/IBS
precise sampling and allow only PMI-based sampling when
PERF_SAMPLE_STACK_USER is requested.

Reported-by: Gennady Kupava <gennady.kupava@xxxxxxxxx>
Closes: https://lore.kernel.org/all/CAPu-DQqF0aF6=GS8Z6KKWeeX_V5LiXeKU_rJQZC+uGg8zuTPNw@xxxxxxxxxxxxxx/
Cc: Ravi Bangoria <ravi.bangoria@xxxxxxx>
Fixes: c5ebcedb566e ("perf: Add ability to attach user stack dump to sample")
Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
---
 arch/x86/events/amd/ibs.c    |  3 +++
 arch/x86/events/core.c       |  3 +++
 arch/x86/events/perf_event.h | 19 +++++++++++++++++++
 3 files changed, 25 insertions(+)

diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
index 3531f9c23b8c..c67b659d64e2 100644
--- a/arch/x86/events/amd/ibs.c
+++ b/arch/x86/events/amd/ibs.c
@@ -327,6 +327,9 @@ static int perf_ibs_init(struct perf_event *event)
 	if (has_branch_stack(event))
 		return -EOPNOTSUPP;
 
+	if (!x86_pmu_allow_sample_user_stack(event, true))
+		return -EINVAL;
+
 	/* handle exclude_{user,kernel} in the IRQ handler */
 	if (event->attr.exclude_host || event->attr.exclude_guest ||
 	    event->attr.exclude_idle)
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 8b3ea0adb965..ceb5038d7565 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -594,6 +594,9 @@ int x86_pmu_hw_config(struct perf_event *event)
 		/* There's no sense in having PEBS for non sampling events: */
 		if (!is_sampling_event(event))
 			return -EINVAL;
+
+		if (!x86_pmu_allow_sample_user_stack(event, false))
+			return -EINVAL;
 	}
 	/*
 	 * check that PEBS LBR correction does not conflict with
diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
index 71ed5b2acea2..9fbcf2bcef51 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -1274,6 +1274,25 @@ static inline bool is_counter_pair(struct hw_perf_event *hwc)
 	return hwc->flags & PERF_X86_EVENT_PAIR;
 }
 
+static inline bool
+x86_pmu_allow_sample_user_stack(struct perf_event *event, bool ibs)
+{
+	if (!(event->attr.sample_type & PERF_SAMPLE_STACK_USER))
+		return true;
+
+	/*
+	 * PERF_SAMPLE_STACK_USER needs to return the user stack and
+	 * user registers to user space when the PMI exits. Since the skid
+	 * from the PEBS/IBS sample and PMI delivery, the PEBS/IBS register
+	 * snapshot (especially IP/SP/BP) can diverge from the user stack
+	 * at PMI return. That mismatch breaks DWARF unwinding.
+	 *
+	 * Therefore, disable precise sampling for PERF_SAMPLE_STACK_USER
+	 * and permit only PMI-based sampling for this case.
+	 */
+	return !(event->attr.precise_ip || ibs);
+}
+
 static inline void __x86_pmu_enable_event(struct hw_perf_event *hwc,
 					  u64 enable_mask)
 {
-- 
2.34.1