Re: [PATCH v2 bpf 3/5] bpf: bpf_prog_array_free() should take a generic non-rcu pointer

[Roman Gushchin <guro@fb.com>]

On Tue, Jul 17, 2018 at 12:30:18AM +0200, Daniel Borkmann wrote:
> On 07/13/2018 09:41 PM, Roman Gushchin wrote:
> > bpf_prog_array_free() should take a generic non-rcu pointer
> > as an argument, as freeing the objects assumes that we're
> > holding an exclusive rights on it.
> > 
> > rcu_access_pointer() can be used to convert a __rcu pointer to
> > a generic pointer before passing it to bpf_prog_array_free(),
> > if necessary.
> > 
> > This patch eliminates the following sparse warning:
> > kernel/bpf/core.c:1556:9: warning: incorrect type in argument 1 (different address spaces)
> > kernel/bpf/core.c:1556:9:    expected struct callback_head *head
> > kernel/bpf/core.c:1556:9:    got struct callback_head [noderef] <asn:4>*<noident>
> > 
> > Fixes: 324bda9e6c5a ("bpf: multi program support for cgroup+bpf")
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> > Cc: Alexei Starovoitov <ast@kernel.org>
> > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > ---
> >  drivers/media/rc/bpf-lirc.c |  6 +++---
> >  include/linux/bpf.h         |  2 +-
> >  kernel/bpf/cgroup.c         | 11 ++++++-----
> >  kernel/bpf/core.c           |  5 ++---
> >  kernel/trace/bpf_trace.c    |  8 ++++----
> >  5 files changed, 16 insertions(+), 16 deletions(-)
> > 
> > diff --git a/drivers/media/rc/bpf-lirc.c b/drivers/media/rc/bpf-lirc.c
> > index fcfab6635f9c..509b262aa0dc 100644
> > --- a/drivers/media/rc/bpf-lirc.c
> > +++ b/drivers/media/rc/bpf-lirc.c
> > @@ -135,7 +135,7 @@ static int lirc_bpf_attach(struct rc_dev *rcdev, struct bpf_prog *prog)
> >  		goto unlock;
> >  
> >  	rcu_assign_pointer(raw->progs, new_array);
> > -	bpf_prog_array_free(old_array);
> > +	bpf_prog_array_free(rcu_access_pointer(old_array));
> 
> Taking this one as an example, why can't we already do the rcu_dereference() on the
> 'old_array = raw->progs' where we fetch the old_array initially? Then we also wouldn't
> need the rcu_access_pointer() on bpf_prog_array_free() and yet another rcu_dereference()
> inside the bpf_prog_array_copy() from your later patch?

We can, but then we have to change bpf_prog_array_copy() args annotation,
and also all places, where it's called.
IMO, basically all local variables and function args marked as __rcu
should be not marked as RCU, but fixing them all is beyond this patchset.

> 
> Regarding former, rcu_access_pointer() should also only be used for testing the pointer
> value, but deeper in bpf_prog_array_free() we also deref it, etc.

Hm, but at this moment it's a not "real" rcu pointer.
We're sure that we're owning this pointer.

Btw, we probably have to use rcu_swap_protected() in this place.

Thanks!