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

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Due to the alignment requirements of siginfo_t, as described in
3ddb3fd8cd ("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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://lore.kernel.org/r/20220131103407.1971678-3-elver@google.com
This commit is contained in:
Marco Elver 2022-01-31 11:34:07 +01:00 committed by Peter Zijlstra
parent 95d29fa104
commit ddecd22878
1 changed files with 2 additions and 0 deletions
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2
include/uapi/linux/perf_event.h
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@ -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;
};