Re: [syzbot] WARNING in mntput_no_expire (2)

Al Viro
2021-04-05T18:23:57Z

On Mon, Apr 05, 2021 at 07:08:01PM +0200, Christian Brauner wrote:

> Ah dentry count of -127 looks... odd.

dead + 1...

void lockref_mark_dead(struct lockref *lockref)
{
assert_spin_locked(&lockref->lock);
lockref->count = -128;
}

IOW, a leaked (uncounted) reference to dentry, that got dget() called on
it after dentry had been freed.

IOW, current->fs->pwd.dentry happens to point to an already freed
struct dentry here. Joy...

Could you slap

spin_lock(&current->fs->lock);
WARN_ON(d_count(current->fs->pwd.dentry) < 0);
spin_unlock(&current->fs->lock);

before and after calls of io_issue_sqe() and see if it triggers? We definitely
are seeing buggered dentry refcounting here.