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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Fri, 25 Mar 2022 00:38:50 +0800
Message-ID: <CAMZfGtVzUZmz7UObNfEgbxue32_2rivajTbiwZQhGkGhDzLthA@mail.gmail.com>
Subject: Re: [syzbot] general protection fault in list_lru_add
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        syzbot <syzbot+f8c45ccc7d5d45fc5965@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
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On Fri, Mar 25, 2022 at 12:18 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Thu, 24 Mar 2022 at 17:13, Muchun Song <songmuchun@bytedance.com> wrote:
> >
> > On Thu, Mar 24, 2022 at 4:50 PM Dmitry Vyukov <dvyukov@google.com> wrote:
> > >
> > > On Thu, 24 Mar 2022 at 09:44, Muchun Song <songmuchun@bytedance.com> wrote:
> > > >
> > > > On Thu, Mar 24, 2022 at 11:05 AM Linus Torvalds
> > > > <torvalds@linux-foundation.org> wrote:
> > > > >
> > > > > On Wed, Mar 23, 2022 at 7:19 PM Muchun Song <songmuchun@bytedance.com> wrote:
> > > > > >
> > > > > > After this commit, the rules of dentry allocations changed.
> > > > > > The dentry should be allocated by kmem_cache_alloc_lru()
> > > > >
> > > > > Yeah, I looked at that, but I can't find any way there could be other
> > > > > allocations - not only are there strict rules how to initialize
> > > > > everything, but the dentries are free'd using
> > > > >
> > > > >         kmem_cache_free(dentry_cache, dentry);
> > > > >
> > > > > and as a result if they were allocated any other way I would expect
> > > > > things would go south very quickly.
> > > > >
> > > > > The only other thing I could come up with is some breakage in the
> > > > > superblock lifetime so that &dentry->d_sb->s_dentry_lru would have
> > > > > problems, but again, this is *such* core code and not some unusual
> > > > > path, that I would be very very surprised if it wouldn't have
> > > > > triggered other issues long long ago.
> > > > >
> > > > > That's why I'd be more inclined to worry about the list_lru code being
> > > > > somehow broken.
> > > > >
> > > >
> > > > I also have the same concern.  I have been trying for a few hours to
> > > > reproduce this issue, but it didn't oops on my test machine.  And I'll
> > > > continue reproducing this.
> > >
> > > syzbot triggered it 222 times in a day, so it's most likely real:
> > > https://syzkaller.appspot.com/bug?extid=f8c45ccc7d5d45fc5965
> > >
> > > There are 2 reproducers, but they look completely different. May be a race.
> > > You may also try to use syzbot's patch testing feature to get some
> > > additional debug info.
> >
> > Do you know how to tell the syzbot to test the following patch?
> > I found some infos from github, it says "#syz test:", is it like the following?
> > Thanks.
> >
> > #syz test: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
> > master
>
> Yes, this is correct. You can now see the request listed here:
> https://syzkaller.appspot.com/bug?extid=f8c45ccc7d5d45fc5965
>

Cool!.

> but the patch was truncated (probably you email client messed
> whitespaces). In such case it's more reliable to attach the patch as
> text file.

Thanks for your reminder.

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
5abc1e37afa0335c52608d640fd30910b2eeda21

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