linux-kernel - Re: [RFC bpf-next 5/6] bpf: selftests: test

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Date:   Sun, 29 Oct 2023 17:22:02 -0600
From:   Daniel Xu <dxu@...uu.xyz>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     ast@...nel.org, andrii@...nel.org, shuah@...nel.org,
        daniel@...earbox.net, steffen.klassert@...unet.com,
        antony.antony@...unet.com, mykolal@...com, martin.lau@...ux.dev,
        song@...nel.org, yonghong.song@...ux.dev, john.fastabend@...il.com,
        kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
        jolsa@...nel.org, bpf@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        devel@...ux-ipsec.org
Subject: Re: [RFC bpf-next 5/6] bpf: selftests: test_tunnel: Disable CO-RE
 relocations

On Fri, Oct 27, 2023 at 01:33:09PM -0700, Andrii Nakryiko wrote:
> On Fri, Oct 27, 2023 at 11:46 AM Daniel Xu <dxu@...uu.xyz> wrote:
> >
> > Switching to vmlinux.h definitions seems to make the verifier very
> > unhappy with bitfield accesses. The error is:
> >
> >     ; md.u.md2.dir = direction;
> >     33: (69) r1 = *(u16 *)(r2 +11)
> >     misaligned stack access off (0x0; 0x0)+-64+11 size 2
> >
> > It looks like disabling CO-RE relocations seem to make the error go
> > away.
> >
> 
> for accessing bitfields libbpf provides
> BPF_CORE_READ_BITFIELD_PROBED() and BPF_CORE_READ_BITFIELD() macros

In this case the code in question is:

        __u8 direction = 0;
        md.u.md2.dir = direction;

IOW the problem is assigning to bitfields, not reading from them.

Is that something that libbpf needs to support as well?

Thanks,
Daniel