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

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Date:   Fri, 27 Oct 2023 13:33:09 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Xu <dxu@...uu.xyz>
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 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

> Signed-off-by: Daniel Xu <dxu@...uu.xyz>
> ---
>  tools/testing/selftests/bpf/progs/test_tunnel_kern.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> index 3065a716544d..ec7e04e012ae 100644
> --- a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> +++ b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> @@ -6,6 +6,7 @@
>   * modify it under the terms of version 2 of the GNU General Public
>   * License as published by the Free Software Foundation.
>   */
> +#define BPF_NO_PRESERVE_ACCESS_INDEX
>  #include "vmlinux.h"
>  #include <bpf/bpf_helpers.h>
>  #include <bpf/bpf_endian.h>
> --
> 2.42.0
>