linux-kernel - [tip: perf/urgent] perf: uapi: Document perf_event_attr::sig

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Message-ID: <164389881239.16921.12788730223046077815.tip-bot2@tip-bot2>
Date:   Thu, 03 Feb 2022 14:33:32 -0000
From:   "tip-bot2 for Marco Elver" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Dmitry Vyukov <dvyukov@...gle.com>, Marco Elver <elver@...gle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: perf/urgent] perf: uapi: Document perf_event_attr::sig_data
 truncation on 32 bit architectures

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     ddecd22878601a606d160680fa85802b75d92eb6
Gitweb:        https://git.kernel.org/tip/ddecd22878601a606d160680fa85802b75d92eb6
Author:        Marco Elver <elver@...gle.com>
AuthorDate:    Mon, 31 Jan 2022 11:34:07 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Wed, 02 Feb 2022 13:11:40 +01:00

perf: uapi: Document perf_event_attr::sig_data truncation on 32 bit architectures

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@...gle.com>
Signed-off-by: Marco Elver <elver@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>
Link: https://lore.kernel.org/r/20220131103407.1971678-3-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 1b65042..82858b6 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;
 };