netdev - Re: [PATCH v3 bpf-next 2/5] bpf: Mark cgroups and dfl

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Date:   Tue, 28 Feb 2023 09:32:38 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     davem@...emloft.net, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...nel.org, void@...ifault.com, davemarchevsky@...a.com,
        memxor@...il.com, netdev@...r.kernel.org, bpf@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH v3 bpf-next 2/5] bpf: Mark cgroups and dfl_cgrp fields as
 trusted.

On Mon, Feb 27, 2023 at 08:01:18PM -0800, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> bpf programs sometimes do:
> bpf_cgrp_storage_get(&map, task->cgroups->dfl_cgrp, ...);
> It is safe to do, because cgroups->dfl_cgrp pointer is set diring init and
                                                              ^
							      u

> never changes. The task->cgroups is also never NULL. It is also set during init
> and will change when task switches cgroups. For any trusted task pointer
> dereference of cgroups and dfl_cgrp should yield trusted pointers. The verifier
> wasn't aware of this. Hence in gcc compiled kernels task->cgroups dereference
> was producing PTR_TO_BTF_ID without modifiers while in clang compiled kernels
> the verifier recognizes __rcu tag in cgroups field and produces
> PTR_TO_BTF_ID | MEM_RCU | MAYBE_NULL.
> Tag cgroups and dfl_cgrp as trusted to equalize clang and gcc behavior.
> When GCC supports btf_type_tag such tagging will done directly in the type.
> 
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun