linux-kernel - Re: memory leak in vq_meta

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Message-ID: <CACT4Y+b5H4jvY34iT2K0m6a2HCpzgKd3dtv+YFsApp=-18B+pw@mail.gmail.com>
Date:   Fri, 26 Jul 2019 17:20:55 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     syzbot <syzbot+a871c1e6ea00685e73d7@...kaller.appspotmail.com>,
        alexandre.belloni@...e-electrons.com,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, nicolas.ferre@...el.com,
        Rob Herring <robh@...nel.org>, sre@...nel.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: memory leak in vq_meta_prefetch

On Fri, Jul 26, 2019 at 3:00 PM Catalin Marinas <catalin.marinas@....com> wrote:
>
> On Wed, Jul 24, 2019 at 12:18:07PM -0700, syzbot wrote:
> > syzbot found the following crash on:
> >
> > HEAD commit:    c6dd78fc Merge branch 'x86-urgent-for-linus' of git://git...
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15fffef4600000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=8de7d700ea5ac607
> > dashboard link: https://syzkaller.appspot.com/bug?extid=a871c1e6ea00685e73d7
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=127b0334600000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12609e94600000
> >
> > The bug was bisected to:
> >
> > commit 0e5f7d0b39e1f184dc25e3adb580c79e85332167
> > Author: Nicolas Ferre <nicolas.ferre@...el.com>
> > Date:   Wed Mar 16 13:19:49 2016 +0000
> >
> >     ARM: dts: at91: shdwc binding: add new shutdown controller documentation
>
> That's another wrong commit identification (a documentation patch should
> not cause a memory leak).
>
> I don't really think kmemleak, with its relatively high rate of false
> positives, is suitable for automated testing like syzbot. You could

Hi Catalin,

Do you mean automated testing in general, or bisection only?
The wrong commit identification is related to bisection only, but you
generalized it to automated testing in general. So which exactly you
mean?


> reduce the false positives if you add support for scanning in
> stop_machine(). Otherwise, in order to avoid locking the kernel for long
> periods, kmemleak runs concurrently with other threads (even on the
> current CPU) and under high load, pointers are missed (e.g. they are in
> CPU registers rather than stack).