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Message-ID: <20251024170543.11201-1-thaumy.love@gmail.com>
Date: Sat, 25 Oct 2025 01:05:43 +0800
From: Thaumy Cheng <thaumy.love@...il.com>
To: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>,
Ian Rogers <irogers@...gle.com>,
Adrian Hunter <adrian.hunter@...el.com>,
Kan Liang <kan.liang@...ux.intel.com>
Cc: linux-perf-users@...r.kernel.org,
linux-kernel@...r.kernel.org,
Thaumy Cheng <thaumy.love@...il.com>
Subject: [PATCH v3] perf/core: Fix missing read event generation on task exit
For events with inherit_stat enabled, a "read" event will be generated
to collect per task event counts on task exit.
The call chain is as follows:
do_exit
-> perf_event_exit_task
-> perf_event_exit_task_context
-> perf_event_exit_event
-> perf_remove_from_context
-> perf_child_detach
-> sync_child_event
-> perf_event_read_event
However, the child event context detaches the task too early in
perf_event_exit_task_context, which causes sync_child_event to never
generate the read event in this case, since child_event->ctx->task is
always set to TASK_TOMBSTONE. Fix that by moving context lock section
backward to ensure ctx->task is not set to TASK_TOMBSTONE before
generating the read event.
Because perf_event_free_task calls perf_event_exit_task_context with
exit = false to tear down all child events from the context, and the
task never lived, accessing the task PID can lead to a use-after-free.
To fix that, let sync_child_event read task from argument and move the
call to the only place it should be triggered to avoid the effect of
setting ctx->task to TASK_TOMESTONE, and add a task parameter to
perf_event_exit_event to trigger the sync_child_event properly when
needed.
This bug can be reproduced by running "perf record -s" and attaching to
any program that generates perf events in its child tasks. If we check
the result with "perf report -T", the last line of the report will leave
an empty table like "# PID TID", which is expected to contain the
per-task event counts by design.
Fixes: ef54c1a476ae ("perf: Rework perf_event_exit_event()")
Signed-off-by: Thaumy Cheng <thaumy.love@...il.com>
---
Changes in v3:
- Fix the bug in a more direct way by moving the call to
sync_child_event and bring back the task param to
perf_event_exit_event.
This approach avoids the event unscheduling issue in v2.
Changes in v2:
- Only trigger read event on task exit.
- Rename perf_event_exit_event to perf_event_detach_event.
- Link to v2: https://lore.kernel.org/all/20250817132742.85154-1-thaumy.love@gmail.com/
Changes in v1:
- Set TASK_TOMBSTONE after the read event is tirggered.
- Link to v1: https://lore.kernel.org/all/20250720000424.12572-1-thaumy.love@gmail.com/
kernel/events/core.c | 23 ++++++++++++++---------
1 file changed, 14 insertions(+), 9 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 177e57c1a362..618e7947c358 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2316,7 +2316,8 @@ static void perf_group_detach(struct perf_event *event)
perf_event__header_size(leader);
}
-static void sync_child_event(struct perf_event *child_event);
+static void sync_child_event(struct perf_event *child_event,
+ struct task_struct *task);
static void perf_child_detach(struct perf_event *event)
{
@@ -2336,7 +2337,6 @@ static void perf_child_detach(struct perf_event *event)
lockdep_assert_held(&parent_event->child_mutex);
*/
- sync_child_event(event);
list_del_init(&event->child_list);
}
@@ -4587,6 +4587,7 @@ static void perf_event_enable_on_exec(struct perf_event_context *ctx)
static void perf_remove_from_owner(struct perf_event *event);
static void perf_event_exit_event(struct perf_event *event,
struct perf_event_context *ctx,
+ struct task_struct *task,
bool revoke);
/*
@@ -4614,7 +4615,7 @@ static void perf_event_remove_on_exec(struct perf_event_context *ctx)
modified = true;
- perf_event_exit_event(event, ctx, false);
+ perf_event_exit_event(event, ctx, ctx->task, false);
}
raw_spin_lock_irqsave(&ctx->lock, flags);
@@ -12437,7 +12438,7 @@ static void __pmu_detach_event(struct pmu *pmu, struct perf_event *event,
/*
* De-schedule the event and mark it REVOKED.
*/
- perf_event_exit_event(event, ctx, true);
+ perf_event_exit_event(event, ctx, ctx->task, true);
/*
* All _free_event() bits that rely on event->pmu:
@@ -13994,14 +13995,13 @@ void perf_pmu_migrate_context(struct pmu *pmu, int src_cpu, int dst_cpu)
}
EXPORT_SYMBOL_GPL(perf_pmu_migrate_context);
-static void sync_child_event(struct perf_event *child_event)
+static void sync_child_event(struct perf_event *child_event,
+ struct task_struct *task)
{
struct perf_event *parent_event = child_event->parent;
u64 child_val;
if (child_event->attr.inherit_stat) {
- struct task_struct *task = child_event->ctx->task;
-
if (task && task != TASK_TOMBSTONE)
perf_event_read_event(child_event, task);
}
@@ -14020,7 +14020,9 @@ static void sync_child_event(struct perf_event *child_event)
static void
perf_event_exit_event(struct perf_event *event,
- struct perf_event_context *ctx, bool revoke)
+ struct perf_event_context *ctx,
+ struct task_struct *task,
+ bool revoke)
{
struct perf_event *parent_event = event->parent;
unsigned long detach_flags = DETACH_EXIT;
@@ -14043,6 +14045,9 @@ perf_event_exit_event(struct perf_event *event,
mutex_lock(&parent_event->child_mutex);
/* PERF_ATTACH_ITRACE might be set concurrently */
attach_state = READ_ONCE(event->attach_state);
+
+ if (attach_state & PERF_ATTACH_CHILD)
+ sync_child_event(event, task);
}
if (revoke)
@@ -14134,7 +14139,7 @@ static void perf_event_exit_task_context(struct task_struct *task, bool exit)
perf_event_task(task, ctx, 0);
list_for_each_entry_safe(child_event, next, &ctx->event_list, event_entry)
- perf_event_exit_event(child_event, ctx, false);
+ perf_event_exit_event(child_event, ctx, exit ? task : NULL, false);
mutex_unlock(&ctx->mutex);
--
2.51.0
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