Re: [PATCH ipsec-next v2 3/6] libbpf: Add BPF_CORE_WRITE_BITFIELD() macro

[Daniel Xu]

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@xxxxxxxxx>
> > 
> > Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> > ---
> 
> Could you please also add a selftest (or several) using __retval()
> annotation for this macro?

Sure, I'll take a look.

Thanks,
Daniel