linux-kernel - Re: [GIT PULL] perf fixes

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Message-ID: <CAHk-=wi_VHc=Q2JsPbVmCgpKekNJwnbBiYrmvnSSW8aiAkg7nQ@mail.gmail.com>
Date:   Fri, 1 Nov 2019 11:48:48 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [GIT PULL] perf fixes

On Fri, Nov 1, 2019 at 10:48 AM Ingo Molnar <mingo@...nel.org> wrote:
>
> Alexander Shishkin (1):
>       perf/core: Start rejecting the syscall with attr.__reserved_2 set

This seems to quite possibly break existing apps. Is there any reason
to believe that existing users have actually cleared that field?

It's suspect for another reason too: the commit that added that field
just added it to the end of the structure, with the argument that
"aux_watermark will only matter for new AUX-aware code, so the old
code should still be fine".

So by *definition* those old kinds of users would never have cleared
that field, because that field didn't exist.

Honestly, this all shows a worrying complete disregard for backwards
compatibility. Calling this a "fix" is questionable, when it is much
more likely to break some old user.

I've pulled it, but I need people to be aware that this is utter
garbage, and that if anybody ever reports it, this needs to be
immediately reverted.

And the people involved should stop claiming this "fixes" anything,
and should look hard at their random ABI expansions and "fixes".

The original code that said "old users would not be impacted" is
correct. But this "fix" is very very very questionable indeed, and I
get the feeling that somebody doesn't understand what ABI is, when
they claim that this "fixes" anything.

      Linus