[PATCH 3/3] perf: uapi: Document perf_event_attr::sig_data truncation on 32 bit architectures

Marco Elver
2022-01-31T10:34:25Z

Due to the alignment requirements of siginfo_t, as described in
3ddb3fd8cdb0 ("signal, perf: Fix siginfo_t by avoiding u64 on 32-bit
architectures"), siginfo_t::si_perf_data is limited to an unsigned long.

However, perf_event_attr::sig_data is an u64, to avoid having to deal
with compat conversions. Due to being an u64, it may not immediately be
clear to users that sig_data is truncated on 32 bit architectures.

Add a comment to explicitly point this out, and hopefully help some
users save time by not having to deduce themselves what's happening.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
---
include/uapi/linux/perf_event.h | 2 ++
1 file changed, 2 insertions(+)

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 1b65042ab1db..82858b697c05 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -465,6 +465,8 @@ struct perf_event_attr {
/*
* User provided data if sigtrap=1, passed back to user via
* siginfo_t::si_perf_data, e.g. to permit user to identify the event.
+ * Note, siginfo_t::si_perf_data is long-sized, and sig_data will be
+ * truncated accordingly on 32 bit architectures.
*/
__u64 sig_data;
};
--
2.35.0.rc2.247.g8bbb082509-goog