FAILED unresolved symbol vfs_truncate

[nathanchance]

Initially reported by @veruu: https://groups.google.com/g/clang-built-linux/c/de_mNh23FOc/m/E7cu5BwbBAAJ

Isolated set of configs:

$ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- LLVM=1 O=build/aarch64 defconfig

$ scripts/config --file build/aarch64/.config -e BPF_SYSCALL -e DEBUG_INFO_BTF -e FTRACE -e FUNCTION_TRACER

$ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- LLVM=1 O=build/aarch64 olddefconfig all
...
FAILED unresolved symbol vfs_truncate
...

I have reported this to the BPF folks upstream: https://lore.kernel.org/r/20210209034416.GA1669105@ubuntu-m3-large-x86/

[dileks]

I am not a BPF/BTF expert but I inspected my vmlinux on x86-64 where I have all above Kconfigs set:

$ /opt/llvm-toolchain/bin/llvm-objdump -Dr vmlinux

ffffffff8252ecfc <__BTF_ID__func__vfs_truncate__622>:
ffffffff8252ecfc: e0 76                 loopne  0xffffffff8252ed74 <__BTF_ID__struct__udp6_sock__1270>
ffffffff8252ecfe: 00 00                 addb    %al, (%rax)

ffffffff8252ed10 <bpf_d_path_btf_ids>:
ffffffff8252ed10: ca 00 00              lretl   $0
ffffffff8252ed13: 00 03                 addb    %al, (%rbx)

So, this seems to be arm64 specific.

[dileks]

Playing with bpftool and ripgrep (rg):

$ /usr/sbin/bpftool btf dump file vmlinux | rg 'vfs_truncate|bpf_d_path'
[22284] TYPEDEF 'btf_bpf_d_path' type_id=22285
[29999] FUNC 'vfs_truncate' type_id=29998 linkage=static

[nathanchance]

TL;DR of the upstream thread: This appears to be a pahole bug, exposed by arm64's BTI (which is why Sedat does not see anything). Jiri Olsa has posted a tentative pahole fix, which I have verified against Veronika's original config and the reduced one that I came up with.

[dileks]

@nathanchance
Cool!

My selfmade LLVM toolchain v12.0.0-rc1 supports AArch64 arch:

$ /opt/llvm-toolchain/bin/llc --version
LLVM (http://llvm.org/):
  LLVM version 12.0.0
  Optimized build.
  Default target: x86_64-unknown-linux-gnu
  Host CPU: sandybridge

  Registered Targets:
    aarch64    - AArch64 (little endian)
    aarch64_32 - AArch64 (little endian ILP32)
    aarch64_be - AArch64 (big endian)
    arm64      - ARM64 (little endian)
    arm64_32   - ARM64 (little endian ILP32)
    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

Can I cross-build for Linux/arm64?
If yes, how?

Just curious - I wanted to throw out Aarch64 target in my next ThinLTO+PGO optimized LLVM toolchain build (version 12.0.0-rc2) as the Linux/arm64 defconfig build takes here 02:30 [hh:mm] (vs. 00:45 for Linux/x86_64 defconfig).

[veruu]

Can I cross-build for Linux/arm64?
If yes, how?

We do

export ARCH=arm64
export CROSS_COMPILE=aarch64-linux-gnu-
make LLVM=1 -j30 INSTALL_MOD_STRIP=1 targz-pkg

[dileks]

@veruu

Is that your first comment in the CBL BTS :-)?

I normally build my LLVM toolchain with tc-build.

Kept the build-log - so I see:

$ grep aarch64-linux-gnu log_tc-build.txt | grep make
+ make -skj4 LLVM=1 O=out ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- KCONFIG_ALLCONFIG=/dev/fd/63 distclean defconfig Image.gz modules

That was my first Linux/arm64 cross-build :-).

Dunno, if I have time with this.
This Friday is milestone #2 of Debian/bullseye aka Debian-11 - that is of importance for Debianists.

[1] https://github.com/ClangBuiltLinux/tc-build

[veruu]

Is that your first comment in the CBL BTS :-)?

Hah possibly. I usually just annoy people on the mailing list and let them handle the rest (like with reporting this bug) 🙈

I'm a bit confused by your comment now as based on the question and issue I assumed you're asking about cross compiling the kernel to reproduce the problem/test the fix, but then linked LLVM building scripts. If you were asking about cross building LLVM and not the kernel then I'm sorry for misunderstanding!

[dileks]

@veruu

Indeed, 1st I like to confuse people and 2nd I wanted to reproduce this issue for Linux/arm64 but was not sure if I have all toolchain requirement to do so.

[veruu]

Given that your target list contains both ARM and BPF targets you should be good to go :)

[dileks]

/me gets red (hat).

[nathanchance]

pahole patch posted: https://lore.kernel.org/r/[email protected]/

[nickdesaulniers]

looks like pahole has this as 8e1f8c904e303d5d142d24e820f01b47a35bb72a which was in v1.21.