linux-kernel - Re: kernel BUG at fs/inode.c:LINE!

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Date:   Wed, 10 Apr 2019 13:40:18 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Ian Kent <raven@...maw.net>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        syzbot <syzbot+5399ed0832693e29f392@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: kernel BUG at fs/inode.c:LINE!

On Wed, Apr 10, 2019 at 12:35 PM Ian Kent <raven@...maw.net> wrote:
>
> On Wed, 2019-04-10 at 10:27 +0200, Dmitry Vyukov wrote:
> > On Wed, Apr 10, 2019 at 2:26 AM Al Viro <viro@...iv.linux.org.uk> wrote:
> > >
> > > On Tue, Apr 09, 2019 at 07:36:00AM -0700, syzbot wrote:
> > > > Bisection is inconclusive: the first bad commit could be any of:
> > >
> > > [snip the useless pile]
> > >
> > > > bisection log:
> > > > https://syzkaller.appspot.com/x/bisect.txt?x=15e1fc2b200000
> > > > start commit:   [unknown
> > > > git tree:       linux-next
> > > > dashboard link:
> > > > https://syzkaller.appspot.com/bug?extid=5399ed0832693e29f392
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=101032b3400000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16534063400000
> > > >
> > > > For information about bisection process see:
> > > > https://goo.gl/tpsmEJ#bisection
> > >
> > > If I'm not misreading the "crash report" there, it has injected an
> > > allocation
> > > failure in dentry allocation in d_make_root() from autofs_fill_super() (
> > >         root_inode = autofs_get_inode(s, S_IFDIR | 0755);
> > >         root = d_make_root(root_inode);
> > > ) which has triggered iput() on the inode passed to d_make_root() (as it
> > > ought
> > > to).  At which point it stepped into some BUG_ON() in fs/inode.c, but I've
> > > no idea which one it is - line numbers do not match anything in linux-next
> > > or in mainline.  Reported line 1566 is
> > >                 if (inode->i_nlink && (inode->i_state & I_DIRTY_TIME)) {
> > > in all of them; as the matter of fact, the diff in fs/inode.c between
> > > -next and mainline is empty.
> > >
> > > There is a BUG_ON() several lines prior, and in 4.20 it used to be line
> > > 1566,
> > > so _probably_ that's what it is.  With that assumption, it's
> > >         BUG_ON(inode->i_state & I_CLEAR);
> > > IOW, we'd got I_CLEAR in the inode passed to d_make_root() there.  Which
> > > should not happen - the inode must have come from new_inode(), which
> > > gets it from new_inode_pseudo(), which zeroes ->i_state.  And I_CLEAR
> > > is set only in clear_inode().  For autofs inodes that can come only
> > > from autofs_evict_inode(), called as ->evict() from evict_inode().
> > > Which should never ever be called for inode with positive ->i_count...
> > >
> > > It might be memory corruption; it might be a dangling inode pointer
> > > somewhere, it might be something else.
> > >
> > > To get any further we really need a confirmation of the identity of
> > > triggered BUG_ON().
> > >
> > > As an aside, your "sample crash reports" would've been much more useful if
> > > they went with commit SHA1 in question, especially when they contain line
> > > numbers.
> >
> > Hi Al,
> >
> > This is the commit for matching lines:
> >
> > > HEAD commit:    d14b746c6c1c Add linux-next specific files for 20181214
>
> Are you sure?
> what does 20181214 mean?

Yes, I just copy-pasted from the report. "d14b746c6c1c" is the commit
hash. "Add linux-next specific files for 20181214" is the commit
subject.


> Looking at current next code (and several branches) it doesn't
> appear the problem is possible?
>
> > > git tree:       linux-next
>
> But which branch is it really, master (which doesn't match the
> line numbers btw)?

This is d14b746c6c1c commit hash. I don't know if there is a branch
with HEAD pointing to this commit or not, but it seems unimportant.
Tree+commit is the identity of code state.


> > fs/inode.c:1566 points to:
> >
> > void iput(struct inode *inode)
> > {
> >     ...
> >     BUG_ON(inode->i_state & I_CLEAR);
> >
> >
> > The dashboard page provides kernel git repository and commit for each crash.
>
> Those links don't seem to make sense to me ...
>
> Help me out here!

There is git repo name provided and commit hash. It's meant to be
self-explanatory. What exactly is unclear?