netdev - Re: [PATCH] net: Fix suspicious RCU usage in bpf_sk_reuseport

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Date:   Tue, 16 Aug 2022 22:16:46 +0100
From:   David Howells <dhowells@...hat.com>
To:     Hawkins Jiawei <yin31149@...il.com>
Cc:     dhowells@...hat.com, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org, bpf@...r.kernel.org
Subject: Re: [PATCH] net: Fix suspicious RCU usage in bpf_sk_reuseport_detach()

Hawkins Jiawei <yin31149@...il.com> wrote:

> +__rcu_dereference_sk_user_data_with_flags_check(const struct sock *sk,
> +						uintptr_t flags, bool condition)

That doesn't work.  RCU_LOCKDEP_WARN() relies on anything passing on a
condition down to it to be a macro so that it can vanish the 'condition'
argument without causing an undefined symbol for 'lockdep_is_held' if lockdep
is disabled:

x86_64-linux-gnu-ld: kernel/bpf/reuseport_array.o: in function `bpf_sk_reuseport_detach':
/data/fs/linux-fs/build3/../kernel/bpf/reuseport_array.c:28: undefined reference to `lockdep_is_held'

So either __rcu_dereference_sk_user_data_with_flags_check() has to be a macro,
or we need to go with something like the first version of my patch where I
don't pass the condition through.  Do you have a preference?

David