netdev - Re: [PATCH bpf-next v7 02/10] bpf: Populate kfunc BTF ID sets in struct btf

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Date:   Thu, 13 Jan 2022 22:52:29 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        netfilter-devel <netfilter-devel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Maxim Mikityanskiy <maximmi@...dia.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH bpf-next v7 02/10] bpf: Populate kfunc BTF ID sets in
 struct btf

On Thu, Jan 13, 2022 at 9:22 PM Kumar Kartikeya Dwivedi
<memxor@...il.com> wrote:
>
> On Fri, Jan 14, 2022 at 10:19:50AM IST, Kumar Kartikeya Dwivedi wrote:
> > On Fri, Jan 14, 2022 at 04:02:11AM IST, Alexei Starovoitov wrote:
> > > On Tue, Jan 11, 2022 at 11:34:20PM +0530, Kumar Kartikeya Dwivedi wrote:
> > > > [...]
> > > > + /* Make sure all updates are visible before we go to MODULE_STATE_LIVE,
> > > > +  * pairs with smp_rmb in btf_try_get_module (for success case).
> > > > +  *
> > > > +  * btf_populate_kfunc_set(...)
> > > > +  * smp_wmb()    <-----------.
> > > > +  * mod->state = LIVE        |           if (mod->state == LIVE)
> > > > +  *                          |             atomic_inc_nz(mod)
> > > > +  *                          `--------->   smp_rmb()
> > > > +  *                                        btf_kfunc_id_set_contains(...)
> > > > +  */
> > > > + smp_wmb();
> > >
> > > This comment somehow implies that mod->state = LIVE
> > > and if (mod->state == LIVE && try_mod_get) can race.
> > > That's not the case.
> > > The patch 1 closed the race.
> > > btf_kfunc_id_set_contains() will be called only on LIVE modules.
> > > At that point all __init funcs of the module including register_btf_kfunc_id_set()
> > > have completed.
> > > This smp_wmb/rmb pair serves no purpose.
> > > Unless I'm missing something?
> > >
> >
> > Right, I'm no expert on memory ordering, but even if we closed the race, to me
> > there seems to be no reason why the CPU cannot reorder the stores to tab (or its
> > hook/type slot) with mod->state = LIVE store.
> >
> > Usually, the visibility is handled by whatever lock is used to register the
> > module somewhere in some subsystem, as the next acquirer can see all updates
> > from the previous registration.
> >
> > In this case, we're directly assigning a pointer without holding any locks etc.
> > While it won't be concurrently accessed until module state is LIVE, it is
> > necessary to make all updates visible in the right order (that is, once state is
> > LIVE, everything stored previously in struct btf for module is also visible).
> >
> > Once mod->state = LIVE is visible, we will start accessing kfunc_set_tab, but if
> > previous stores to it were not visible by then, we'll access a badly-formed
> > kfunc_set_tab.
> >
> > For this particular case, you can think of mod->state = LIVE acting as a release
> > store, and the read for mod->state == LIVE acting as an acquire load.
> >
>
> Also, to be more precise, we're now synchronizing using btf_mod->flags, not
> mod->state, so I should atleast update the comment, but the idea is the same.

So the concern is that cpu can execute
mod->state = MODULE_STATE_LIVE;
from kernel/module.c
earlier than stores inside __btf_populate_kfunc_set
that are called from do_one_initcall()
couple lines earlier in kernel/module.c ?
Let's assume cpu is not Intel, since Intel never reorders stores.
(as far as I remember the only weak store ordering architecture
ever produced is Alpha).
But it's not mod->state, it's btf_mod->flags (as you said)
which is done under btf_module_mutex.
and btf_kfunc_id_set_contains() can only do that after
btf_try_get_module() succeeds which is under the same
btf_module_mutex.
So what is the race ?
I think it's important to be concerned about race
conditions, but they gotta be real.
So please spell out a precise scenario if you still believe it's there.