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

[Dmitry Vyukov]

On Wed, Apr 10, 2019 at 2:26 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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
> git tree:       linux-next

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.