Re: [syzbot] WARNING in p9_client_destroy

[Dmitry Vyukov]

On Mon, 25 Jul 2022 at 13:51, <asmadeus@xxxxxxxxxxxxx> wrote:
>
> Vlastimil Babka wrote on Mon, Jul 25, 2022 at 12:15:24PM +0200:
> > On 7/24/22 15:17, syzbot wrote:
> > > syzbot has bisected this issue to:
> > >
> > > commit 7302e91f39a81a9c2efcf4bc5749d18128366945
> > > Author: Marco Elver <elver@xxxxxxxxxx>
> > > Date:   Fri Jan 14 22:03:58 2022 +0000
> > >
> > >     mm/slab_common: use WARN() if cache still has objects on destroy
> >
> > Just to state the obvious, bisection pointed to a commit that added the
> > warning, but the reason for the warning would be that p9 is destroying a
> > kmem_cache without freeing all the objects there first, and that would be
> > true even before the commit.
>
> Probably true from the moment that cache/idr was introduced... I've got
> a couple of fixes in next but given syzcaller claims that's the tree it
> was produced on I guess there can be more such leaks.
> (well, the lines it sent in the backtrace yesterday don't match next,
> but I wouldn't count on it)
>
> If someone wants to have a look please feel free, I would bet the
> problem is just that p9_fd_close() doesn't call or does something
> equivalent to p9_conn_cancel() and there just are some requests that
> haven't been sent yet when the mount is closed..
> But I don't have/can/want to take the time to check right now as I
> consider such a leak harmless enough, someone has to be root or
> equivalent to do 9p mounts in most cases.

FWIW with KASAN we have allocation stacks for each heap object. So
when KASAN is enabled that warning could list all live object
allocation stacks.