netdev - Re: [PATCH v5 bpf-next 6/6] bpf: Refactor RCU enforcement in the verifier.

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Date:   Fri, 3 Mar 2023 08:22:52 -0600
From:   David Vernet <void@...ifault.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     davem@...emloft.net, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...nel.org, davemarchevsky@...a.com, tj@...nel.org,
        memxor@...il.com, netdev@...r.kernel.org, bpf@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH v5 bpf-next 6/6] bpf: Refactor RCU enforcement in the
 verifier.

On Thu, Mar 02, 2023 at 08:14:46PM -0800, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> bpf_rcu_read_lock/unlock() are only available in clang compiled kernels. Lack
> of such key mechanism makes it impossible for sleepable bpf programs to use RCU
> pointers.
> 
> Allow bpf_rcu_read_lock/unlock() in GCC compiled kernels (though GCC doesn't
> support btf_type_tag yet) and allowlist certain field dereferences in important
> data structures like tast_struct, cgroup, socket that are used by sleepable
> programs either as RCU pointer or full trusted pointer (which is valid outside
> of RCU CS). Use BTF_TYPE_SAFE_RCU and BTF_TYPE_SAFE_TRUSTED macros for such
> tagging. They will be removed once GCC supports btf_type_tag.
> 
> With that refactor check_ptr_to_btf_access(). Make it strict in enforcing
> PTR_TRUSTED and PTR_UNTRUSTED while deprecating old PTR_TO_BTF_ID without
> modifier flags. There is a chance that this strict enforcement might break
> existing programs (especially on GCC compiled kernels), but this cleanup has to
> start sooner than later. Note PTR_TO_CTX access still yields old deprecated
> PTR_TO_BTF_ID. Once it's converted to strict PTR_TRUSTED or PTR_UNTRUSTED the
> kfuncs and helpers will be able to default to KF_TRUSTED_ARGS. KF_RCU will
> remain as a weaker version of KF_TRUSTED_ARGS where obj refcnt could be 0.
> 
> Adjust rcu_read_lock selftest to run on gcc and clang compiled kernels.
> 
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>

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