linux-kernel - Re: [PATCH v4 1/3] perf: add PERF_RECORD_NAMESPACES to include namespaces related info

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Date:   Fri, 23 Dec 2016 07:20:56 +1300
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Hari Bathini <hbathini@...ux.vnet.ibm.com>, ast@...com,
        lkml <linux-kernel@...r.kernel.org>, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, mingo@...hat.com,
        daniel@...earbox.net, rostedt@...dmis.org,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        sargun@...gun.me, Aravinda Prasad <aravinda@...ux.vnet.ibm.com>,
        brendan.d.gregg@...il.com
Subject: Re: [PATCH v4 1/3] perf: add PERF_RECORD_NAMESPACES to include namespaces related info

Peter Zijlstra <peterz@...radead.org> writes:

> On Thu, Dec 22, 2016 at 11:19:17PM +1300, Eric W. Biederman wrote:
>> Peter Zijlstra <peterz@...radead.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