Re: INFO: task hung in fuse_reverse_inval_entry

[Miklos Szeredi <miklos@szeredi.hu>]

On Mon, Jul 23, 2018 at 10:11 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
> On Mon, Jul 23, 2018 at 9:59 AM, syzbot
> <syzbot+bb6d800770577a083f8c@syzkaller.appspotmail.com> wrote:
>> Hello,
>>
>> syzbot found the following crash on:
>>
>> HEAD commit:    d72e90f33aa4 Linux 4.18-rc6
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=1324f794400000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=68af3495408deac5
>> dashboard link: https://syzkaller.appspot.com/bug?extid=bb6d800770577a083f8c
>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=11564d1c400000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16fc570c400000
>
>
> Hi fuse maintainers,
>
> We are seeing a bunch of such deadlocks in fuse on syzbot. As far as I
> understand this is mostly working-as-intended (parts about deadlocks
> in Documentation/filesystems/fuse.txt). The intended way to resolve
> this is aborting connections via fusectl, right?

Yes.  Alternative is with "umount -f".

> The doc says "Under
> the fuse control filesystem each connection has a directory named by a
> unique number". The question is: if I start a process and this process
> can mount fuse, how do I kill it? I mean: totally and certainly get
> rid of it right away? How do I find these unique numbers for the
> mounts it created?

It is the device number found in st_dev for the mount.  Other than
doing stat(2) it is possible to find out the device number by reading
/proc/$PID/mountinfo  (third field).

> Taking into account that there is usually no
> operator attached to each server, I wonder if kernel could somehow
> auto-abort fuse on kill?

Depends on what the fuse server is sleeping on.   If it's trying to
acquire an inode lock (e.g. unlink(2)), which is classical way to
deadlock a fuse filesystem, then it will go into an uninterruptible
sleep.  There's no way in which that process can be killed except to
force a release of the offending lock, which can only be done by
aborting the request that is being performed while holding that lock.

Thanks,
Miklos