Re: bpf_core_type_id_kernel with qualifier aborts clang compilation

[Yonghong Song <yhs@fb.com>]

On 3/4/21 3:25 AM, Lorenz Bauer wrote:
> Hi Yonghong, Andrii,
> 
> Some more poking at CO-RE. The code below leads to a compiler error:
> 
> struct s {
>      int _1;
>      char _2;
> };
> 
> __section("socket_filter/type_ids") int type_ids() {
>      return bpf_core_type_id_kernel(const struct s);
> }
> 
> Truncated output:
> fatal error: error in backend: Empty type name for BTF_TYPE_ID_REMOTE reloc
> PLEASE submit a bug report to https://bugs.llvm.org/  and include the
> crash backtrace, preprocessed source, and associated run script.
> Stack dump:
> 0.    Program arguments: clang-12 -target bpf -O2 -g -Wall -Werror
> -mlittle-endian -c internal/btf/testdata/relocs.c -o
> internal/btf/testdata/relocs-el.elf
> 1.    <eof> parser at end of file
> 2.    Per-function optimization
> 3.    Running pass 'BPF Preserve Debuginfo Type' on function '@type_ids'
> ...
> clang: error: clang frontend command failed with exit code 70 (use -v
> to see invocation)
> Ubuntu clang version
> 12.0.0-++20210126113614+510b3d4b3e02-1~exp1~20210126104320.178
> Target: bpf
> 
> "volatile" has the same problem. Interestingly, the same code works
> for bpf_core_type_id_local. Is this expected?

First, bpf_core_type_id_local() works as compiler did not check type 
name. for bpf_core_type_id_local(), there is no relocation, libbpf
may need to adjust type id if it tries to do btf dedup, merging, etc.

Second, the above bpf_core_type_id_kernel() failed due to
"const" (or "volatile") modifier. bpf_core_type_id_kernel()
requires a type name as relocation will be performed.
In the current implementation, the btf type is
    const -> struct s
and there is no name for "const", that is why compiler issues
an explicit fatal error:
     fatal error: error in backend: Empty type name for 
BTF_TYPE_ID_REMOTE reloc

To fix the issue, just do not use any modifier,
   bpf_core_type_id_kernel(struct s)
should work fine.

I think in the case, it would be good if the compiler tries
to peel off modifiers and find the ultimate type name
instead of fatal error. I will put a patch on this.

Thanks for reporting!

> 
> Best
> Lorenz
>