On Thu, 17 Nov 2016, Alexei Starovoitov wrote:
> On Thu, Nov 17, 2016 at 09:53:42AM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 16, 2016 at 12:08:52PM -0800, Alexei Starovoitov wrote:
> >
> > > I prefer to avoid 'fixing' things that are not broken.
> > > Note, prog->aux->refcnt already has explicit checks for overflow.
> > > locked_vm is used for resource accounting and not refcnt,
> > > so I don't see issues there either.
> >
> > The idea is to use something along the lines of:
> >
> >
> > http://lkml.kernel.org/r/20161115104608.gh3...@twins.programming.kicks-ass.net
> >
> > for all refcounts in the kernel.
>
> I understand the idea. I'm advocating to fix refcnts
> explicitly the way we did in bpf land instead of leaking memory,
> making processes unkillable and so on.
> If refcnt can be bounds checked, it should be done that way, since
> it's a clean error path without odd side effects.
> Therefore I'm against unconditionally applying refcount to all atomics.
>
> > Also note that your:
> >
> > struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i)
> > {
> > if (atomic_add_return(i, &prog->aux->refcnt) > BPF_MAX_REFCNT) {
> > atomic_sub(i, &prog->aux->refcnt);
> > return ERR_PTR(-EBUSY);
> > }
> > return prog;
> > }
> >
> > is actually broken in the face of an actual overflow. Suppose @i is big
> > enough to wrap refcnt into negative space.
>
> 'i' is not controlled by user. It's a number of nic hw queues
> and BPF_MAX_REFCNT is 32k, so above is always safe.
>
> > Also, the current sentiment is to strongly discourage add/sub operations
> > for refcounts.
>
> I agree with this reasoning as well, but it's not hard and fast rule.
> If we know we can do 'add' safely, we should.
In principle yes. OTOH, history shows that developers have a pretty bad
judgement what is safe and not. They rather copy code from random places,
modify it in creative ways and be done with it.
Thanks,
tglx
