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

[Dmitry Vyukov]

On Wed, Apr 10, 2019 at 1:57 PM Ian Kent <raven@xxxxxxxxxx> 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?
>
> I'm unable to find a branch matching the line numbers.
>
> Given that, on the face of it, the scenario is impossible I'm
> seeking clarification on what linux-next to look at for the
> sake of accuracy.
>
> So I'm wondering if this testing done using the master branch
> or one of the daily branches one would use to check for conflicts
> before posting?
>
> Or perhaps the master branch has been updated and the testing was
> done on something different.


Ah, I see. syzbot continuously tests the master branch of linux-next.
So d14b746c6c1c was once the HEAD of the master branch.
I somehow have d14b746c6c1c locally, but if you don't have it you may
need to fetch git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next-history.git
with tags. That's linux-next specifics.