Re: CLANG LTO compatibility issue with DEBUG_INFO_BTF

[Yonghong Song]

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