Re: [PATCH v3 bpf-next 2/5] bpf: Mark cgroups and dfl_cgrp fields as trusted.

David Vernet
2023-02-28T16:46:04Z

On Mon, Feb 27, 2023 at 08:01:18PM -0800, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.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
> 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@kernel.org>

Acked-by: David Vernet <void@manifault.com>