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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Fri, 8 Dec 2023 13:04:22 -0800
Message-ID: <CAADnVQKjAwWLfn-gBxq03qv4SQTt0YguYSDZXj8L6nmBZqU66g@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Jiri Olsa <olsajiri@gmail.com>, Song Liu <song@kernel.org>,
        Song Liu <songliubraving@meta.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        "David S. Miller" <davem@davemloft.net>,
        David Ahern <dsahern@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Yonghong Song <yonghong.song@linux.dev>,
        John Fastabend <john.fastabend@gmail.com>,
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        Hao Luo <haoluo@google.com>, Arnd Bergmann <arnd@arndb.de>,
        Sami Tolvanen <samitolvanen@google.com>,
        Kees Cook <keescook@chromium.org>,
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On Fri, Dec 8, 2023 at 12:56=E2=80=AFPM Peter Zijlstra <peterz@infradead.or=
g> wrote:
>
> On Fri, Dec 08, 2023 at 12:45:51PM -0800, Alexei Starovoitov wrote:
>
> > I mean we don't need to store a pointer to a func in stubs.
> > Can it be, roughly:
> >
> > extern void bpf_tcp_ca_cong_avoid(struct sock *sk, u32 ack, u32 acked);
> > KCFI_MACRO(hash_of_cong_avoid, bpf_tcp_ca_cong_avoid);
> > u32 __array_of_kcfi_hash[] =3D {hash_of_cong_avoid, hash_of_set_state,.=
..};
> >       .bpf_ops_stubs =3D __array_of_kcfi_hash,
>
> But then how do I index this array? The bpf_ops_stubs thing having the
> same layout at the target struct made it easy and we could use 'moff'
> for both.
>
> That also remains working if someone adds a member to the struct or
> moves some members around.

I was thinking to just moff/2 assuming u32 array will have the same order,
but I missed the fact that init() and release() in tcp_congestion_ops
come after some variables.
And you're right it's more fragile when things change in tcp_congestion_ops=
.
Storing u32 hash as (void *) into function pointers is just ugly.
Let's go with your initial approach.