linux-kernel - Re: [PATCH bpf-next 4/8] bpf: Enable cpumasks to be queried and used as kptrs

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Message-ID: <20230120055247.obojh465e7pk2rrp@MacBook-Pro-6.local.dhcp.thefacebook.com>
Date:   Thu, 19 Jan 2023 21:52:47 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     David Vernet <void@...ifault.com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...a.com, john.fastabend@...il.com, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...a.com, tj@...nel.org
Subject: Re: [PATCH bpf-next 4/8] bpf: Enable cpumasks to be queried and used
 as kptrs

On Thu, Jan 19, 2023 at 11:50:47PM -0600, David Vernet wrote:
> On Thu, Jan 19, 2023 at 09:48:23PM -0800, Alexei Starovoitov wrote:
> > On Thu, Jan 19, 2023 at 05:58:29PM -0600, David Vernet wrote:
> > > silently check for and ignore these cases at runtime. When we have e.g.
> > > per-argument kfunc flags, it might be helpful to add another KF_CPU-type
> > > flag that specifies that the verifier should validate that it's a valid
> > > CPU.
> > 
> > ...
> > 
> > > +void bpf_cpumask_set_cpu(u32 cpu, struct bpf_cpumask *cpumask)
> > > +{
> > > +	if (!cpu_valid(cpu))
> > > +		return;
> > > +
> > > +	cpumask_set_cpu(cpu, (struct cpumask *)cpumask);
> > > +}
> > 
> > ...
> > 
> > > +void bpf_cpumask_clear_cpu(u32 cpu, struct bpf_cpumask *cpumask)
> > > +{
> > > +	if (!cpu_valid(cpu))
> > > +		return;
> > 
> > I don't think we'll be able to get rid of this with KF_CPU or special suffix.
> > The argument might be a variable and not a constant at the verification time.
> > We would have to allow passing unknown vars otherwise the UX will be too restrictive,
> > so this run-time check would have to stay.
> 
> Makes sense. We'll just leave it as is then and document that passing in
> cpu >= nr_cpus is silently ignored for any kfunc taking a cpu argument.

Eventually we can clean it up with bpf_assert infra.