Re: [PATCH v1 1/2] perf record: Add 8-byte aligned event type PERF_RECORD_COMPRESSED2

[Ian Rogers]

On Wed, Feb 26, 2025 at 10:04 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> 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
```
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