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Date:   Thu, 19 Jan 2023 21:48:23 -0800
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
To:     David Vernet <void@manifault.com>
Cc:     bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net,
        andrii@kernel.org, martin.lau@linux.dev, song@kernel.org,
        yhs@meta.com, john.fastabend@gmail.com, kpsingh@kernel.org,
        sdf@google.com, haoluo@google.com, jolsa@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@meta.com, tj@kernel.org
Subject: Re: [PATCH bpf-next 4/8] bpf: Enable cpumasks to be queried and used
 as kptrs
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 <20230119235833.2948341-5-void@manifault.com>
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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.