From mboxrd@z Thu Jan  1 00:00:00 1970
From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Subject: Re: [PATCH bpf-next v6 03/11] bpf: Add write access to tcp_sock and
 sock fields
Date: Fri, 19 Jan 2018 19:54:45 -0800
Message-ID: <20180120035443.sfm54gmdwfqw734s@ast-mbp>
References: <20180120014548.2941040-1-brakmo@fb.com>
 <20180120014548.2941040-4-brakmo@fb.com>
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Cc: netdev <netdev@vger.kernel.org>, Kernel Team <kernel-team@fb.com>,
        Blake Matheny <bmatheny@fb.com>,
        Alexei Starovoitov <ast@fb.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Neal Cardwell <ncardwell@google.com>,
        Yuchung Cheng <ycheng@google.com>
To: Lawrence Brakmo <brakmo@fb.com>
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On Fri, Jan 19, 2018 at 05:45:40PM -0800, Lawrence Brakmo wrote:
> This patch adds a macro, SOCK_OPS_SET_FIELD, for writing to
> struct tcp_sock or struct sock fields. This required adding a new
> field "temp" to struct bpf_sock_ops_kern for temporary storage that
> is used by sock_ops_convert_ctx_access. It is used to store and recover
> the contents of a register, so the register can be used to store the
> address of the sk. Since we cannot overwrite the dst_reg because it
> contains the pointer to ctx, nor the src_reg since it contains the value
> we want to store, we need an extra register to contain the address
> of the sk.
> 
> Also adds the macro SOCK_OPS_GET_OR_SET_FIELD that calls one of the
> GET or SET macros depending on the value of the TYPE field.
> 
> Signed-off-by: Lawrence Brakmo <brakmo@fb.com>

Acked-by: Alexei Starovoitov <ast@kernel.org>

that is really clever way of doing inline writes into fields.
I suspect we will be using this approach in other places.