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Subject: Re: CLANG LTO compatibility issue with DEBUG_INFO_BTF
To:     Jisheng Zhang <Jisheng.Zhang@synaptics.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Kees Cook <keescook@chromium.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        <linux-kernel@vger.kernel.org>
CC:     <bpf@vger.kernel.org>
References: <20210319113730.7ad6a609@xhacker.debian>
From:   Yonghong Song <yhs@fb.com>
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On 3/18/21 8:45 PM, Jisheng Zhang wrote:
> Hi,
> 
> When trying the latest 5.12-rc3 with both LTO_CLANG_THIN and DEBUG_INFO_BTF
> enabled, I met lots of warnings such as:
> 
> ...
> tag__recode_dwarf_type: couldn't find 0x4a7ade5 type for 0x4ab9f88 (subroutine_type)!
> ftype__recode_dwarf_types: couldn't find 0x4a7ade5 type for 0x4ab9fa4 (formal_parameter)!
> ...
> namespace__recode_dwarf_types: couldn't find 0x4a8ff4a type for 0x4aba05c (member)!
> namespace__recode_dwarf_types: couldn't find 0x4a7ae9b type for 0x4aba084 (member)!
> ...
> WARN: multiple IDs found for 'path': 281, 729994 - using 281
> WARN: multiple IDs found for 'task_struct': 421, 730101 - using 421
> ...
> 
> 
> then finally get build error:
> FAILED unresolved symbol vfs_truncate
> 
> 
> Is this a known issue? Do we need to make DEBUG_INFO_BTF depend on !LTO?

This is a known issue for pahole. pahole does not handle dwarf well 
generated with LTO. Bill Wendling from google is looking at the issue 
and I will help look at the issue as well. Since bpf heavily depends
on BTF, at this point, I suggest if you are using bpf, please do not
turn on LTO. Or if you build with LTO, just turn off DEBUG_INFO_BTF
in your config. Thanks!

> 
> pahole version: v1.20
> clang version: 11.0
> 
> 
> Thanks
>