Messages in this thread | | | From | (Eric W. Biederman) | Date | Fri, 23 Dec 2016 07:20:56 +1300 | Subject | Re: [PATCH v4 1/3] perf: add PERF_RECORD_NAMESPACES to include namespaces related info |
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Peter Zijlstra <peterz@infradead.org> writes:
> On Thu, Dec 22, 2016 at 11:19:17PM +1300, Eric W. Biederman wrote: >> Peter Zijlstra <peterz@infradead.org> writes: >> >> > On Thu, Dec 22, 2016 at 08:21:23PM +1300, Eric W. Biederman wrote: >> >> >> >> And please make the array the last item in the structure so that >> >> expanding or contracting it does not affect the ability to read the rest >> >> of the structure. >> > >> > Sorry, sample_id must be last, because hysterical crud :/ >> > >> > (basically because that was the only way to add a field to records like >> > PERF_RECORD_MMAP which used the record length to determine the >> > filename[] length, yes I know, we won't ever do that again). >> >> Why does historical crud need to affect new records? > > Because now the userspace parser expects sample_id to be the tail > field. Basically decoding a record now looks like: > > if (sample_id_all) { > sample_id = (sample_id *)((char *)record + record->size - sizeof(sample_id)); > record->size -= sizeof(sample_id); > } > /* process record */ > > We could of course create more exceptions.. > >> Totally confused. This looks like a major mess. > > Not major, but yes, its ugly, but its also ABI :-(
Fair enough. I will just avert my eyes now and deal with my own challenges.
Eric
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