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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:     Daniel Xu <dxu@dxuuu.xyz>, Yonghong Song <yonghong.song@linux.dev>
Cc:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        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:   Mon, 27 Nov 2023 23:32:35 +0200
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On Mon, 2023-11-27 at 14:45 -0600, Daniel Xu wrote:
[...]
> IIUC uapi structs look the same in BTF as any other struct.

Yes, and all share preserve_access_index attribute because of the way
attribute push/pop directives are generated in vmlinux.h.

> Just wondering, though: will bpftool be able to generate the appropriate
> annotations for uapi structs?=20

The problem is that there is no easy way to identify if structure is
uapi in DWARF (from which BTF is generated).
One way to do this:
- modify pahole to check DW_AT_decl_file for each struct DWARF entry
  and generate some special decl tag in BTF;
- modify bpftool to interpret this tag as a marker to not generate
  preserve_access_index for a structure.

The drawback is that such behavior hardcodes some kernel specific
assumptions both in pahole and in bpftool. It also remains to be seen
if DW_AT_decl_file tags are consistent.

It might be the case that allowing excessive CO-RE relocations is a
better option. (And maybe tweak something about bitfield access
generation to avoid such issues as in this thread).

Thanks,
Eduard