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Subject: Re: [PATCH ipsec-next v2 3/6] libbpf: Add BPF_CORE_WRITE_BITFIELD()
 macro
From:   Eduard Zingerman <eddyz87@gmail.com>
To:     Daniel Xu <dxu@dxuuu.xyz>, ndesaulniers@google.com,
        andrii@kernel.org, nathan@kernel.org, daniel@iogearbox.net,
        ast@kernel.org, steffen.klassert@secunet.com,
        antony.antony@secunet.com, alexei.starovoitov@gmail.com,
        yonghong.song@linux.dev
Cc:     martin.lau@linux.dev, song@kernel.org, john.fastabend@gmail.com,
        kpsingh@kernel.org, sdf@google.com, haoluo@google.com,
        jolsa@kernel.org, trix@redhat.com, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        devel@linux-ipsec.org, netdev@vger.kernel.org
Date:   Tue, 28 Nov 2023 19:59:01 +0200
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On Tue, 2023-11-28 at 10:54 -0700, Daniel Xu wrote:
> Similar to reading from CO-RE bitfields, we need a CO-RE aware bitfield
> writing wrapper to make the verifier happy.
>=20
> Two alternatives to this approach are:
>=20
> 1. Use the upcoming `preserve_static_offset` [0] attribute to disable
>    CO-RE on specific structs.
> 2. Use broader byte-sized writes to write to bitfields.
>=20
> (1) is a bit a bit hard to use. It requires specific and
> not-very-obvious annotations to bpftool generated vmlinux.h. It's also
> not generally available in released LLVM versions yet.
>=20
> (2) makes the code quite hard to read and write. And especially if
> BPF_CORE_READ_BITFIELD() is already being used, it makes more sense to
> to have an inverse helper for writing.
>=20
> [0]: https://reviews.llvm.org/D133361
> From: Eduard Zingerman <eddyz87@gmail.com>
>=20
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> ---

Could you please also add a selftest (or several) using __retval()
annotation for this macro?