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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 7 Dec 2020 10:40:11 -0800
Message-ID: <CAHk-=wjgG=_-zONkBkKnkOv3uoVRy45hTxx8e-6Ks3j-3TVHKQ@mail.gmail.com>
Subject: Re: [PATCH 3/3] exec: Transform exec_update_mutex into a rw_semaphore
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Bernd Edlinger <bernd.edlinger@hotmail.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Jann Horn <jannh@google.com>,
        Vasiliy Kulikov <segoon@openwall.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Oleg Nesterov <oleg@redhat.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Sargun Dhillon <sargun@sargun.me>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Davidlohr Bueso <dave@stgolabs.net>
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On Mon, Dec 7, 2020 at 1:10 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> > PeterZ, is there something I'm missing?
>
> Like this?
>
>   https://lkml.kernel.org/r/20200828123720.GZ1362448@hirez.programming.kicks-ass.net

Yes, except I think you should remove the old ptrace_may_access() check.

Because otherwise we'll just end up having KCSAN complain about the
unlocked optimistic accesses or something like that.

So do the perfmon_capable() check early - it doesn't need the
exec_update_mutex - and then just do the ptrace_may_access() one late.

I don't see any point at all in checking privileges twice, and I do
see real downsides. Not just that KCSAN issue, but also lack of
coverage (ie the second check will then effectively never be tested,
which is bad too).

               Linus