linux-kernel - Re: [PATCH v1 1/2] perf record: Add 8-byte aligned event type PERF_RECORD

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Message-ID: <Z8AgYUnNkSA_Q36F@google.com>
Date: Thu, 27 Feb 2025 00:20:49 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Andi Kleen <ak@...ux.intel.com>, Chun-Tse Shao <ctshao@...gle.com>,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	mingo@...hat.com, acme@...nel.org, mark.rutland@....com,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	adrian.hunter@...el.com, kan.liang@...ux.intel.com, terrelln@...com,
	leo.yan@....com, dvyukov@...gle.com, james.clark@...aro.org,
	christophe.leroy@...roup.eu, ben.gainey@....com,
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v1 1/2] perf record: Add 8-byte aligned event type
 PERF_RECORD_COMPRESSED2

On Wed, Feb 26, 2025 at 10:20:36PM -0800, Ian Rogers wrote:
> On Wed, Feb 26, 2025 at 10:04 PM Andi Kleen <ak@...ux.intel.com> wrote:
> >
> > On Wed, Feb 26, 2025 at 09:34:06PM -0800, Chun-Tse Shao wrote:
> > > The original PERF_RECORD_COMPRESS is not 8-byte aligned, which can cause
> > > asan runtime error:
> >
> > It seems pointless. Most architectures have cheap unaligned accesses
> > these days.
> >
> > Just disable that error?
> 
> The perf_event_header in perf_event.h is:
> ```
> struct perf_event_header {
> __u32 type;
> __u16 misc;
> __u16 size;
> };
> ```
> so it is assuming at least 4-byte alignment. 8-byte alignment is
> assumed in many places in tools/lib/perf/include/perf/event.h. We pad
> events to ensure the alignment in about 30 places already:
> ```
> $ grep -r PERF_ALIGN tools/perf|grep u64|wc -l
> 32
> ```

I vaguely remember that it needs 8 bytes alignment to deal with partial
mmap-ed data on 32-bit machines so that it can make sure the header is
not across the mmap boundary.

Thanks,
Namhyung


> Having sanitizers I think is a must, if we allow unaligned events we'd
> need to introduce helper functions or memcpys to workaround the
> unaligned undefined behavior. I think the padding is a less worse
> alternative and one that was already picked.
> 
> Thanks,
> Ian