linux-kernel - Re: KCSAN: data-race in __alloc_file / __alloc

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Date:   Mon, 11 Nov 2019 10:50:22 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Marco Elver <elver@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        syzbot <syzbot+3ef049d50587836c0606@...kaller.appspotmail.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrea Parri <parri.andrea@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file

On Mon, Nov 11, 2019 at 10:31 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> Problem is that KASAN/KCSAN stops as soon as one issue is hit,
> regardless of it being a false positive or not.

So mayb e that - together with the known huge base of false positives
- just means that KCSAN needs some more work before it can be used as
a basis for sending out patches.

Maybe the reporting needs to create a hash of the location, and report
once per location? Or something like that.

Maybe KCSAN needs a way to filter out known false positives on a KCSAN
side, without having to change the source for a tool that gives too
much noise?

> If we do not annotate the false positive, the real issues might be
> hidden for years.

I don't think "change the kernel source for a tool that isn't good
enough" is the solution.

> There is no pattern really, only a lot of noise (small ' bugs'  that
> have no real impact)

Yeah, if it hasn't shown any real bugs so far, that just strengthens
the "it needs much fewer false positives to be useful".

KASAN and lockdep can afford to stop after the first problem, because
the problems they report - and the additional annotations you might
want to add - are quality problems and annotations.

                Linus