Messages in this thread | | | Date | Thu, 22 Dec 2016 14:24:49 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v4 1/3] perf: add PERF_RECORD_NAMESPACES to include namespaces related info |
| |
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 :-(
| |