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

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Message-ID: <qcjvpbuhnj3jwbc5gb5c3tmla7scwpxuwgcqgfbvc6ewwjej3j@o7t3f6wacwps>
Date:   Tue, 28 Nov 2023 13:15:41 -0600
From:   Daniel Xu <dxu@...uu.xyz>
To:     Eduard Zingerman <eddyz87@...il.com>
Cc:     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, 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, Nov 28, 2023 at 07:59:01PM +0200, Eduard Zingerman wrote:
> 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?

Sure, I'll take a look.

Thanks,
Daniel