[PATCH 1/1] perf: fix reset interface potential failure

[Masahiko, Yamada]

There is a potential bug where PERF_EVENT_IOC_RESET
does not work when accessing PMU registers directly
from userspace in the perf_event interface.
we have created a patch on the kernel
that fixes a potential perf_event reset failure.

The motivation is to initialize pc->offset.
The perf_mmap__read_self function in tools/lib/perf/mmap.c is set by:.
cnt = READ_ONCE(pc->offset);
The pc->offset value is set in the following process
in the perf_event_update_userpage function:.
userpg->offset -= local64_read(&event->hw.prev_count);
hw->prev_count is set in the armpmu_event_set_period function
in drivers/perf/arm_pmu.c and in the x86_perf_event_set_period function
in arch/x86/events/core.c as follows:.
local64_set(&hwc->prev_count, (u64)-left);

Therefore, this patch was created to initialize hwc->prev_count
during reset processing.

Signed-off-by: Masahiko, Yamada <yamada.masahiko@xxxxxxxxxxx>
---
 kernel/events/core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4ec3717003d5..296549755a9c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5468,8 +5468,13 @@ static __poll_t perf_poll(struct file *file, poll_table *wait)
 
 static void _perf_event_reset(struct perf_event *event)
 {
+	struct hw_perf_event *hwc = &event->hw;
+	s64 left;
+
 	(void)perf_event_read(event, false);
 	local64_set(&event->count, 0);
+	left = local64_read(&hwc->period_left);
+	local64_set(&hwc->prev_count, (u64)-left);
 	perf_event_update_userpage(event);
 }
 
-- 
2.27.0