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

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Message-ID: <CAMB2axPEmykdt2Wcvb49j1iG8b+ZTxvDoRgRYKmJAnTvbLsN9g@mail.gmail.com>
Date: Mon, 17 Nov 2025 12:37:45 -0800
From: Amery Hung <ameryhung@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: 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 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.

>
> The rest looks good.
> If that's the only issue, I can fix it up while applying.