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Subject: Re: [PATCH ipsec-next v1 6/7] bpf: selftests: test_tunnel: Disable
 CO-RE relocations
From:   Eduard Zingerman <eddyz87@gmail.com>
To:     Yonghong Song <yonghong.song@linux.dev>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Daniel Xu <dxu@dxuuu.xyz>, Shuah Khan <shuah@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Steffen Klassert <steffen.klassert@secunet.com>,
        antony.antony@secunet.com, Mykola Lysenko <mykolal@fb.com>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        bpf <bpf@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, devel@linux-ipsec.org,
        Network Development <netdev@vger.kernel.org>
Date:   Sun, 26 Nov 2023 22:14:21 +0200
In-Reply-To: <3ec6c068-7f95-419a-a0ae-a901f95e4838@linux.dev>
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On Sat, 2023-11-25 at 20:22 -0800, Yonghong Song wrote:
[...]
> --- a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> +++ b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> @@ -6,7 +6,10 @@
>    * 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
> +#if __has_attribute(preserve_static_offset)
> +struct __attribute__((preserve_static_offset)) erspan_md2;
> +struct __attribute__((preserve_static_offset)) erspan_metadata;
> +#endif
>   #include "vmlinux.h"
[...]
>   int bpf_skb_get_fou_encap(struct __sk_buff *skb_ctx,
> @@ -174,9 +177,13 @@ int erspan_set_tunnel(struct __sk_buff *skb)
>          __u8 hwid =3D 7;
>  =20
>          md.version =3D 2;
> +#if __has_attribute(preserve_static_offset)
>          md.u.md2.dir =3D direction;
>          md.u.md2.hwid =3D hwid & 0xf;
>          md.u.md2.hwid_upper =3D (hwid >> 4) & 0x3;
> +#else
> +       /* Change bit-field store to byte(s)-level stores. */
> +#endif
>   #endif
>  =20
>          ret =3D bpf_skb_set_tunnel_opt(skb, &md, sizeof(md));
>=20
> =3D=3D=3D=3D
>=20
> Eduard, could you double check whether this is a valid use case
> to solve this kind of issue with preserve_static_offset attribute?

Tbh I'm not sure. This test passes with preserve_static_offset
because it suppresses preserve_access_index. In general clang
translates bitfield access to a set of IR statements like:

  C:
    struct foo {
      unsigned _;
      unsigned a:1;
      ...
    };
    ... foo->a ...

  IR:
    %a =3D getelementptr inbounds %struct.foo, ptr %0, i32 0, i32 1
    %bf.load =3D load i8, ptr %a, align 4
    %bf.clear =3D and i8 %bf.load, 1
    %bf.cast =3D zext i8 %bf.clear to i32

With preserve_static_offset the getelementptr+load are replaced by a
single statement which is preserved as-is till code generation,
thus load with align 4 is preserved.

On the other hand, I'm not sure that clang guarantees that load or
stores used for bitfield access would be always aligned according to
verifier expectations.

I think we should check if there are some clang knobs that prevent
generation of unaligned memory access. I'll take a look.