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

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Date:   Thu, 22 Dec 2016 14:24:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
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

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 :-(