netdev - Re: [PATCH v2 bpf-next 4/4] bpf: Replace bpf memory allocator with kmalloc

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Message-ID: <CAMB2axM==X6+WJFenbuwTn82=2iRL-5_GCmj5RmK_fsGf07x7w@mail.gmail.com>
Date: Mon, 17 Nov 2025 16:24:56 -0800
From: Amery Hung <ameryhung@...il.com>
To: paulmck@...nel.org
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, bpf <bpf@...r.kernel.org>, 
	Network Development <netdev@...r.kernel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <martin.lau@...nel.org>, 
	Kumar Kartikeya Dwivedi <memxor@...il.com>, KP Singh <kpsingh@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, Song Liu <song@...nel.org>, 
	Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH v2 bpf-next 4/4] bpf: Replace bpf memory allocator with
 kmalloc_nolock() in local storage

On Mon, Nov 17, 2025 at 3:46 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Mon, Nov 17, 2025 at 03:36:08PM -0800, Alexei Starovoitov wrote:
> > On Mon, Nov 17, 2025 at 12:37 PM Amery Hung <ameryhung@...il.com> wrote:
> > >
> > > On Fri, Nov 14, 2025 at 6:01 PM Alexei Starovoitov
> > > <alexei.starovoitov@...il.com> wrote:
> > > >
> > > > On Fri, Nov 14, 2025 at 12:13 PM Amery Hung <ameryhung@...il.com> wrote:
> > > > >
> > > > >
> > > > > -       if (smap->bpf_ma) {
> > > > > +       if (smap->use_kmalloc_nolock) {
> > > > >                 rcu_barrier_tasks_trace();
> > > > > -               if (!rcu_trace_implies_rcu_gp())
> > > > > -                       rcu_barrier();
> > > > > -               bpf_mem_alloc_destroy(&smap->selem_ma);
> > > > > -               bpf_mem_alloc_destroy(&smap->storage_ma);
> > > > > +               rcu_barrier();
> > > >
> > > > Why unconditional rcu_barrier() ?
> > > > It's implied in rcu_barrier_tasks_trace().
> > >
> > > Hmm, I am not sure.
> > >
> > > > What am I missing?
> > >
> > > I hit a UAF in v1 in bpf_selem_free_rcu() when running selftests and
> > > making rcu_barrier() unconditional addressed it. I think the bug was
> > > due to map_free() not waiting for bpf_selem_free_rcu() (an RCU
> > > callback) to finish.
> > >
> > > Looking at rcu_barrier() and rcu_barrier_tasks_trace(), they pass
> > > different rtp to rcu_barrier_tasks_generic() so I think both are
> > > needed to make sure in-flight RCU and RCU tasks trace callbacks are
> > > done.
> > >
> > > Not an expert in RCU so I might be wrong and it was something else.
> >
> > Paul,
> >
> > Please help us here.
> > Does rcu_barrier_tasks_trace() imply rcu_barrier() ?
>
> I am sorry, but no, it does not.

Thanks for the clarification, Paul!

>
> If latency proves to be an issue, one approach is to invoke rcu_barrier()
> and rcu_barrier_tasks_trace() each in its own workqueue handler.  But as
> always, I suggest invoking them one after the other to see if a latency
> problem really exists before adding complexity.
>
> Except that rcu_barrier_tasks_trace() is never invoked by rcu_barrier(),
> only rcu_barrier_tasks() and rcu_barrier_tasks_trace().  So do you really
> mean rcu_barrier()?  Or rcu_barrier_tasks()?

Sorry for the confusion. I misread the code. I was trying to say that
rcu_barrier() and rcu_barrier_tasks_trace() seem to wait on different
callacks but then referring to rcu_barrier_tasks() implementation
wrongly.

>
> Either way, rcu_barrier_tasks() and rcu_barrier_tasks_trace() are also
> independent of each other in the sense that if you need tw wait on
> callbacks from both call_rcu_tasks() and call_rcu_tasks_trace(), you
> need both rcu_barrier_tasks() and rcu_barrier_tasks_trace() to be invoked.
>
>                                                         Thanx, Paul