Re: [PATCH ipsec-next v1 6/7] bpf: selftests: test_tunnel: Disable CO-RE relocations

[Eduard Zingerman <eddyz87@gmail.com>]

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 = 7;
>   
>          md.version = 2;
> +#if __has_attribute(preserve_static_offset)
>          md.u.md2.dir = direction;
>          md.u.md2.hwid = hwid & 0xf;
>          md.u.md2.hwid_upper = (hwid >> 4) & 0x3;
> +#else
> +       /* Change bit-field store to byte(s)-level stores. */
> +#endif
>   #endif
>   
>          ret = bpf_skb_set_tunnel_opt(skb, &md, sizeof(md));
> 
> ====
> 
> 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 = getelementptr inbounds %struct.foo, ptr %0, i32 0, i32 1
    %bf.load = load i8, ptr %a, align 4
    %bf.clear = and i8 %bf.load, 1
    %bf.cast = 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.