Re: [PATCH v2 2/2] 9p: Add refcount to p9_req_t

From: Dominique Martinet
Date: Mon Aug 27 2018 - 19:10:20 EST

Tomas Bortoli wrote on Tue, Aug 14, 2018:
> To avoid use-after-free(s), use a refcount to keep track of the
> usable references to any instantiated struct p9_req_t.
> This commit adds p9_req_put(), p9_req_get() and p9_req_try_get() as
> wrappers to kref_put(), kref_get() and kref_get_unless_zero().
> These are used by the client and the transports to keep track of
> valid requests' references.
> p9_free_req() is added back and used as callback by kref_put().
> Add SLAB_TYPESAFE_BY_RCU as it ensures that the memory freed by
> kmem_cache_free() will not be reused for another type until the rcu
> synchronisation period is over, so an address gotten under rcu read
> lock is safe to inc_ref() without corrupting random memory while
> the lock is held.

FWIW, since 4.19-rc1 has been tagged I was going to push this and all
the perrequesites to linux-next, but I've managed to leak some requests
by interrupting them in trans_virtio.
I think I've found why (see below), so I'll push a fixed version after
some more testing and another thorough read -- at some point today, but
this hasn't been 'approved' explicitely so please review! :)

(Jun, I think you'll need to ask again to rename 'req' to 'rreq' if you
think it's important -- I think such a rename should go in a separate
patch anyway, there's plenty of time until the 4.20 merge window)

By "all the prerequesites" I mean this patch "serie":
* 9p: Use a slab for allocating requests
* 9p: Remove p9_idpool
* net/9p: embed fcall in req to round down buffer allocs
* net/9p: add a per-client fcall kmem_cache
* 9p: rename p9_free_req() function
* 9p: Add refcount to p9_req_t

All the other patchs have had some review though, I was just waiting for
the start of this cycle, but if someone has any issue with the above
patches now is a good time to say.

> diff --git a/net/9p/client.c b/net/9p/client.c
> index 7942c0bfcc5b..c9bb5d41afa4 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -716,6 +756,8 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
> err = c->trans_mod->request(c, req);
> if (err < 0) {
> + /* write won't happen */
> + p9_req_put(req);
> if (err != -ERESTARTSYS && err != -EFAULT)
> c->status = Disconnected;
> goto recalc_sigpending;

p9_client_zc_rpc needs the same put if zc_request failed, I'm not sure
why it wasn't here in my draft