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* [PATCH v1] perf/core: Restore header fields in sideband output callbacks
@ 2026-09-29  1:42 Ian Rogers
  2026-09-29 12:04 ` Peter Zijlstra
  0 siblings, 1 reply; 2+ messages in thread
From: Ian Rogers @ 2026-09-29  1:42 UTC (permalink / raw)
  To: Peter Zijlstra, Ingo Molnar, Arnaldo Carvalho de Melo, Namhyung Kim
  Cc: Mark Rutland, Alexander Shishkin, Jiri Olsa, Ian Rogers,
	Adrian Hunter, James Clark, Song Liu, Alexei Starovoitov,
	linux-perf-users, linux-kernel, bpf, stable

perf_iterate_sb() invokes its callback for each matching perf_event on
the CPU and task context, passing a shared caller-allocated event
structure.

perf_event_header__init_id() increments header->size by
event->id_header_size. Unlike perf_event_task_output(),
perf_event_comm_output(), perf_event_namespaces_output(),
perf_event_cgroup_output(), perf_event_mmap_output(), and
perf_callchain_deferred_output(), three sideband callbacks failed to
save and restore header.size around perf_event_header__init_id():
- perf_event_ksymbol_output()
- perf_event_bpf_output()
- perf_event_text_poke_output()

When multiple events with attr.ksymbol, attr.bpf_event, or
attr.text_poke and sample_id_all are active on the same CPU, each
subsequent event receives a record whose header.size is inflated by all
preceding events' id_header_size values while only a single id_sample is
written, leaving uninitialized ring-buffer bytes at the end of the
record and causing userspace perf to fail with -EFAULT ("Bad address")
when parsing the sample_id trailer.

Similarly, perf_event_mmap_output() sets PERF_RECORD_MISC_MMAP_BUILD_ID
in mmap_event->event_id.header.misc when event->attr.build_id is
enabled, but only saved and restored header.size and header.type. If an
event with attr.build_id is followed by an event with attr.mmap2 and
!attr.build_id, the second event receives PERF_RECORD_MISC_MMAP_BUILD_ID
in header.misc while its payload contains maj/min/ino/ino_generation
instead of a build ID.

Save and restore header.size in the ksymbol, bpf, and text_poke output
callbacks, and save and restore header.misc in perf_event_mmap_output().

Fixes: 76193a94522f ("perf, bpf: Introduce PERF_RECORD_KSYMBOL")
Fixes: 6ee52e2a3fe4 ("perf, bpf: Introduce PERF_RECORD_BPF_EVENT")
Fixes: e17d43b93e54 ("perf: Add perf text poke event")
Fixes: 88a16a130933 ("perf: Add build id data in mmap2 event")
Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
---
 kernel/events/core.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 33210aff3ee6..ea3697295bd4 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9029,6 +9029,11 @@ static void perf_iterate_sb_cpu(perf_iterate_f output, void *data)
  *
  * For new callers; ensure that account_pmu_sb_event() includes
  * your event, otherwise it might not get delivered.
+ *
+ * Note: @data is shared across all @output calls, so any fields modified
+ * incrementally or conditionally (e.g. header.size via
+ * perf_event_header__init_id()) must be saved and restored by @output,
+ * or unconditionally re-initialized on each call.
  */
 static void
 perf_iterate_sb(perf_iterate_f output, void *data,
@@ -9723,6 +9728,7 @@ static void perf_event_mmap_output(struct perf_event *event,
 	struct perf_sample_data sample;
 	int size = mmap_event->event_id.header.size;
 	u32 type = mmap_event->event_id.header.type;
+	u16 misc = mmap_event->event_id.header.misc;
 	bool use_build_id;
 	int ret;
 
@@ -9780,6 +9786,7 @@ static void perf_event_mmap_output(struct perf_event *event,
 out:
 	mmap_event->event_id.header.size = size;
 	mmap_event->event_id.header.type = type;
+	mmap_event->event_id.header.misc = misc;
 }
 
 static void perf_event_mmap_event(struct perf_mmap_event *mmap_event)
@@ -10244,6 +10251,7 @@ static void perf_event_ksymbol_output(struct perf_event *event, void *data)
 	struct perf_ksymbol_event *ksymbol_event = data;
 	struct perf_output_handle handle;
 	struct perf_sample_data sample;
+	u16 header_size = ksymbol_event->event_id.header.size;
 	int ret;
 
 	if (!perf_event_ksymbol_match(event))
@@ -10254,13 +10262,15 @@ static void perf_event_ksymbol_output(struct perf_event *event, void *data)
 	ret = perf_output_begin(&handle, &sample, event,
 				ksymbol_event->event_id.header.size);
 	if (ret)
-		return;
+		goto out;
 
 	perf_output_put(&handle, ksymbol_event->event_id);
 	__output_copy(&handle, ksymbol_event->name, ksymbol_event->name_len);
 	perf_event__output_id_sample(event, &handle, &sample);
 
 	perf_output_end(&handle);
+out:
+	ksymbol_event->event_id.header.size = header_size;
 }
 
 void perf_event_ksymbol(u16 ksym_type, u64 addr, u32 len, bool unregister,
@@ -10334,6 +10344,7 @@ static void perf_event_bpf_output(struct perf_event *event, void *data)
 	struct perf_bpf_event *bpf_event = data;
 	struct perf_output_handle handle;
 	struct perf_sample_data sample;
+	u16 header_size = bpf_event->event_id.header.size;
 	int ret;
 
 	if (!perf_event_bpf_match(event))
@@ -10344,12 +10355,14 @@ static void perf_event_bpf_output(struct perf_event *event, void *data)
 	ret = perf_output_begin(&handle, &sample, event,
 				bpf_event->event_id.header.size);
 	if (ret)
-		return;
+		goto out;
 
 	perf_output_put(&handle, bpf_event->event_id);
 	perf_event__output_id_sample(event, &handle, &sample);
 
 	perf_output_end(&handle);
+out:
+	bpf_event->event_id.header.size = header_size;
 }
 
 static void perf_event_bpf_emit_ksymbols(struct bpf_prog *prog,
@@ -10496,6 +10509,7 @@ static void perf_event_text_poke_output(struct perf_event *event, void *data)
 	struct perf_text_poke_event *text_poke_event = data;
 	struct perf_output_handle handle;
 	struct perf_sample_data sample;
+	u16 header_size = text_poke_event->event_id.header.size;
 	u64 padding = 0;
 	int ret;
 
@@ -10507,7 +10521,7 @@ static void perf_event_text_poke_output(struct perf_event *event, void *data)
 	ret = perf_output_begin(&handle, &sample, event,
 				text_poke_event->event_id.header.size);
 	if (ret)
-		return;
+		goto out;
 
 	perf_output_put(&handle, text_poke_event->event_id);
 	perf_output_put(&handle, text_poke_event->old_len);
@@ -10522,6 +10536,8 @@ static void perf_event_text_poke_output(struct perf_event *event, void *data)
 	perf_event__output_id_sample(event, &handle, &sample);
 
 	perf_output_end(&handle);
+out:
+	text_poke_event->event_id.header.size = header_size;
 }
 
 void perf_event_text_poke(const void *addr, const void *old_bytes,
-- 
2.56.0.rc1.315.gc6ed9934b7-goog


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