Re: Kernel 5.5.4 build fail for BPF-selftests with latest LLVM

On Wed, 19 Feb 2020 10:38:45 -0800
Andrii Nakryiko [off-list ref] wrote:

On Wed, Feb 19, 2020 at 10:29 AM Jesper Dangaard Brouer
[off-list ref] wrote:

quoted

On Wed, 19 Feb 2020 09:38:50 -0800
Andrii Nakryiko [off-list ref] wrote:
 

quoted

On Wed, Feb 19, 2020 at 9:04 AM Jesper Dangaard Brouer
[off-list ref] wrote:  

quoted

On Wed, 19 Feb 2020 08:41:27 -0800
Alexei Starovoitov [off-list ref] wrote:
 

quoted

On Wed, Feb 19, 2020 at 4:30 AM Jesper Dangaard Brouer
[off-list ref] wrote:  

quoted

I'm willing to help out, such that we can do either version or feature
detection, to either skip compiling specific test programs or at least
give users a proper warning of they are using a too "old" LLVM version.  

...  

quoted

progs/test_core_reloc_bitfields_probed.c:47:13: error: use of unknown builtin '__builtin_preserve_field_info' [-Wimplicit-function-declaration]
        out->ub1 = BPF_CORE_READ_BITFIELD_PROBED(in, ub1);  

imo this is proper warning message already.  

This is an error, not a warning.  The build breaks as the make process stops.
 

Latest Clang was a requirement for building and running all selftests
for a long time now. There were few previous discussions on mailing
list about this and each time the conclusion was the same: latest
Clang is a requirement for BPF selftests.  

The latest Clang is 9.0.1, and it doesn't build with that.  

Latest as in "latest built from sources".

When I download a specific kernel release, how can I know what LLVM
git-hash or version I need (to use BPF-selftests)?

Do you think it is reasonable to require end-users to compile their own
bleeding edge version of LLVM, to use BPF-selftests?

I do hope that some end-users of BPF-selftests will be CI-systems.
That also implies that CI-system maintainers need to constantly do
"latest built from sources" of LLVM git-tree to keep up.  Is that a
reasonable requirement when buying a CI-system in the cloud?

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer