linux-kernel - Re: [syzbot] BUG: soft lockup in do_wp

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Date:   Thu, 25 Mar 2021 19:29:19 +0000
From:   Will Deacon <will@...nel.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+0b036374a865ba0efa8e@...kaller.appspotmail.com>,
        kernel-team@...roid.com, Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [syzbot] BUG: soft lockup in do_wp_page (4)

On Thu, Mar 25, 2021 at 08:24:53PM +0100, Dmitry Vyukov wrote:
> On Thu, Mar 25, 2021 at 8:10 PM Will Deacon <will@...nel.org> wrote:
> > On Thu, Mar 25, 2021 at 07:34:54PM +0100, Dmitry Vyukov wrote:
> > > On Thu, Mar 25, 2021 at 7:20 PM Will Deacon <will@...nel.org> wrote:
> > > > On Thu, Mar 18, 2021 at 08:34:16PM +0100, Dmitry Vyukov wrote:
> > > > > On Thu, Mar 18, 2021 at 8:31 PM syzbot
> > > > > <syzbot+0b036374a865ba0efa8e@...kaller.appspotmail.com> wrote:
> > > > > commit cae118b6acc309539b33339e846cbb19187c164c
> > > > > Author: Will Deacon
> > > > > Date:   Wed Mar 3 13:49:27 2021 +0000
> > > > >     arm64: Drop support for CMDLINE_EXTEND
> > > > >
> > > > > syzbot passes lots of critical things in CONFIG_CMDLINE:
> > > > > https://github.com/google/syzkaller/blob/c3c81c94865791469d376eba84f4a2d7763d3f71/dashboard/config/linux/upstream-arm64-kasan.config#L495
> > > > > but also wants the bootloader args to be appended.
> > > > > What is the way to do it now?
> > > >
> > > > For now, there isn't a way to do it with CONFIG_CMDLINE, so I think you can
> > > > either:
> > > >
> > > >   * Revert my patch for your kernels
> > > >   * Pass the arguments via QEMU's -append option
> > > >   * Take a look at one of the series which should hopefully add this
> > > >     functionality back (but with well-defined semantics) [1] [2]
> > >
> > > Unfortunately none of these work for syzbot (and I assume other
> > > testing environments).
> > >
> > > syzbot does not support custom patches by design:
> > > http://bit.do/syzbot#no-custom-patches
> > > As any testing system, it tests the official trees.
> > >
> > > It's not humans who start these VMs, so it's not as easy as changing
> > > the command line after typing...
> > > There is no support for passing args specifically to qemu, syzkaller
> > > support not just qemu, so these things are specifically localized in
> > > the config. Additionally there is an issue of communicating all these
> > > scattered details to developers in bug reports. Currently syzbot
> > > reports the kernel config and it as well captures command line.
> > >
> > > Could you revert the patch? Is there any point in removing the
> > > currently supported feature before the new feature lands?
> >
> > Well, we only just merged it (in 5.10 I think?), and the semantics of the
> > new version will be different, so I really don't see the value in supporting
> > both (even worse, Android has its own implementation which is different
> > again). The timeline was: we merged CMDLINE_EXTEND, then we noticed it was
> > broken, my fixes were rejected, so we removed the feature rather than
> > support the broken version. In the relatively small window while it was
> > merged, syzbot started using it :(
> 
> I didn't realize it was just introduced :)
> We used CMDLINE_EXTEND on x86, and I looked for a similar option for
> arm64 and found it.
> 
> > So I really think the best bet is to wait until the patches are sorted out.
> > I think Christophe is about to spin a new version, and I reviewed his last
> > copy, so I don't see this being far off,
> 
> If it's expected to be merged soon, let's wait.

Thanks, and knowing that we have a keen user helps to prioritise the review
:)

Will