netdev - Re: [PATCH ipsec-next v2 3/6] libbpf: Add BPF_CORE_WRITE

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Date: Tue, 28 Nov 2023 19:59:01 +0200
From: Eduard Zingerman <eddyz87@...il.com>
To: Daniel Xu <dxu@...uu.xyz>, ndesaulniers@...gle.com, andrii@...nel.org, 
	nathan@...nel.org, daniel@...earbox.net, ast@...nel.org, 
	steffen.klassert@...unet.com, antony.antony@...unet.com, 
	alexei.starovoitov@...il.com, yonghong.song@...ux.dev
Cc: martin.lau@...ux.dev, song@...nel.org, john.fastabend@...il.com, 
	kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org, 
	trix@...hat.com, bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev, devel@...ux-ipsec.org, netdev@...r.kernel.org
Subject: Re: [PATCH ipsec-next v2 3/6] libbpf: Add BPF_CORE_WRITE_BITFIELD()
 macro

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.
> 
> Two alternatives to this approach are:
> 
> 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.
> 
> (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.
> 
> (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.
> 
> [0]: https://reviews.llvm.org/D133361
> From: Eduard Zingerman <eddyz87@...il.com>
> 
> Signed-off-by: Daniel Xu <dxu@...uu.xyz>
> ---

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