Option for BPF support when CONFIG_DEBUG_INFO_BTF=y

[dileks]

Debian-kernel team started to set CONFIG_DEBUG_INFO_BTF=y.

First, this needs pahole binary from dwarves package (minimum: v1.16).

$ pahole --version
v1.19

Second, your LLVM toolchain needs BPF support enabled.

My selfmade LLVM toolchain v11.0.1 does not have this:

$ which llc
/home/dileks/src/llvm-toolchain/install/bin/llc

$ llc --version
LLVM (http://llvm.org/):
 LLVM version 11.0.1
 Optimized build.
 Default target: x86_64-unknown-linux-gnu
 Host CPU: sandybridge

 Registered Targets:
   x86    - 32-bit X86: Pentium-Pro and above
   x86-64 - 64-bit X86: EM64T and AMD64

Debian's LLC has BPF support:

$ /usr/bin/llc-11 --version | grep -i bpf
   bpf        - BPF (host endian)
   bpfeb      - BPF (big endian)
   bpfel      - BPF (little endian)

Is it possible to have an option to enable BPF support?

Thanks.

[1] https://salsa.debian.org/kernel-team/linux/-/commit/929891281c61ce4403ddd869664c949692644a2f
[2] https://www.kernel.org/doc/html/latest/bpf/bpf_devel_QA.html?highlight=pahole#llvm
[3] https://www.kernel.org/doc/html/latest/bpf/btf.html?highlight=pahole#btf-generation

[msfjarvis]

Any issues you see with just having it always enabled instead?

[dileks]

It breaks here with CONFIG_DEBUG_INFO_BTF=y:

+ info BTFIDS vmlinux
+ [  != silent_ ]
+ printf   %-7s %s\n BTFIDS vmlinux
 BTFIDS  vmlinux
+ ./tools/bpf/resolve_btfids/resolve_btfids vmlinux
FAILED: load BTF from vmlinux: Invalid argument

[1] https://lists.debian.org/debian-kernel/2021/01/msg00110.html

[dileks]

Just FYI:
I started a thread on BPF and linux-kbuild mailing-ist:
Check pahole availibity and BPF support of toolchain before starting a Linux kernel build

[1] https://lore.kernel.org/bpf/CA+icZUVuk5PVY4_HoCoY2ymd27UjuDi6kcAmFb_3=dqkvOA_Qw@mail.gmail.com/T/#u
[2] https://marc.info/?l=linux-kbuild&m=161036940824132&w=2

[dileks]

@msfjarvis

Do you have a quick (and dirty) fix?

[msfjarvis]

@msfjarvis

Do you have a quick (and dirty) fix?

I do not.

[dileks]

[llvm-dev] BPF: How can I enable BPF support in my LLVM toolchain?

[1] https://lists.llvm.org/pipermail/llvm-dev/2021-January/147728.html

[dileks]

Add BPF to targets (see -t X86;BPF):

python3 ./build-llvm.py --no-update --build-type Release -p clang;lld -t X86;BPF --clang-vendor dileks -B /home/dileks/src/llvm-toolchain/build -I /home/dileks/src/llvm-toolchain/install --check-targets clang lld --build-stage1-only --install-stage1-only

Logs:

-- Targeting X86
-- Targeting BPF

[dileks]

It might be enough to add a hint to targets option in case of BPF?

[tstellar]

I ran into this same bug trying to build the Fedora kernel. The problem is that pahole segfaults at: scripts/link-vmlinux.sh:131. This looks to me like a bug in pahole.

[dileks]

@tstellar

Which version of pahole do you use?

[dileks]

BPF support looks good:

$ ./build/stage1/bin/llc --version
LLVM (http://llvm.org/):
  LLVM version 11.0.1
  Optimized build.
  Default target: x86_64-unknown-linux-gnu
  Host CPU: sandybridge

  Registered Targets:
    bpf    - BPF (host endian)
    bpfeb  - BPF (big endian)
    bpfel  - BPF (little endian)
    x86    - 32-bit X86: Pentium-Pro and above
    x86-64 - 64-bit X86: EM64T and AMD64

[tstellar]

@tstellar

Which version of pahole do you use?

@dileks 1.19. This build: https://koji.fedoraproject.org/koji/buildinfo?buildID=1644955

[dileks]

@tstellar

Upstream Git pahole has btf_encoder fixes - post v1.19.

[1] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/

[dileks]

Wow how fast, thanks!

[JiaHuann]

Why i meet this again in pahole 1.27.....

[ptr1337]

Why i meet this again in pahole 1.27.....

Pahole needs to be patched currently. Should be too fixed in -git now.

[dileks]

ClangBuiltLinux/linux#2032
ClangBuiltLinux/linux#2032 (comment)

[JiaHuann]

为什么我在 pahole 1.27 中再次遇到这种情况......

Pahole 目前需要修补。现在应该在 -git 中修复了。

ClangBuiltLinux/linux#2032 ClangBuiltLinux/linux#2032（评论）

Thanks. Looks good to me.