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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 29 Apr 2020 16:22:08 -0700
Message-ID: <CAHk-=wjUZLybZBJgOtD2gng=FS7USrbQQ1-tn5M+UP5DbCWdzw@mail.gmail.com>
Subject: Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Waiman Long <longman@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexey Gladkov <gladkov.alexey@gmail.com>
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On Wed, Apr 29, 2020 at 3:38 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> If you do it properly, with a helper function instead of repeating
> that fragile nasty thing, maybe it will look better to me.

Side note: if it has a special helper function for the "get lock,
repeat if it was invalid", you can do a better job than return
-EAGAIN.

In particular, you can do this

        set_thread_flag(TIF_SIGPENDING);
        return -RESTARTNOINTR;

which will actually restart the system call. So a ptrace() user (or
somebody doing a "write()" to /proc/<pid>/attr/xyz, wouldn't even see
the impossible EAGAIN error.

But that all requires that you have some locking helper routines like

    int lock_exec_creds(struct task_struct *);
    void unlock_exec_guard(struct task_struct *);

because there's no way we put that kind of super-fragile code in
several places. It would be very much one single routine with a *HUGE*
comment on it.

             Linus