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.

Marco Elver committed 4 years ago

Commit ddecd221 parent 95d29fa1 file · +2 −0Browse files

Showing the changes to include/uapi (1 of 1 file). Show all

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include/uapi/linux/perf_event.h+2−0

@@ -465,6 +465,8 @@ struct perf_event_attr { · 464 unmodified lines
465	465	/*
466	466	* User provided data if sigtrap=1, passed back to user via
467	467	* siginfo_t::si_perf_data, e.g. to permit user to identify the event.
	468	+ * Note, siginfo_t::si_perf_data is long-sized, and sig_data will be
	469	+ * truncated accordingly on 32 bit architectures.
468	470	*/
469	471	__u64 sig_data;
470	472	};