Re: libbpf/BTF loading issue with fentry/fexit selftests

[KP Singh <kpsingh@chromium.org>]

On 25-Mär 15:16, Matt Cover wrote:
> I'm looking to explore the bpf trampoline Alexei introduced for
> tracing progs, but am encountering a libbpf/BTF issue with loading
> the selftests. Hoping you guys might have a pointer or two.
> 
> The kernel build used pahole 1.15. All llvm-project components used
> in compiling the selftests were 10.0.0-rc6.
> 
> I believe the following confirms that BTF is indeed present in this kernel.
> 
> 
> [vagrant@localhost bpf]$ uname -r
> 5.5.9-1.btf.el7.x86_64
> [vagrant@localhost bpf]$ grep CONFIG_DEBUG_INFO_BTF /boot/config-`uname -r`

It seems like you might have CONFIG_TEST_BPF disabled.

This is explained in Documentation/filter.txt:

Testing
-------

Next to the BPF toolchain, the kernel also ships a test module that
contains various test cases for classic and internal BPF that can be
executed against the BPF interpreter and JIT compiler. It can be found
in lib/test_bpf.c and enabled via Kconfig:

  CONFIG_TEST_BPF=m

After the module has been built and installed, the test suite can be
executed via insmod or modprobe against 'test_bpf' module. Results of
the test cases including timings in nsec can be found in the kernel
log (dmesg).

- KP

> CONFIG_DEBUG_INFO_BTF=y
> [vagrant@localhost bpf]$ ~/bpftool btf dump file ~/vmlinux-`uname -r`
> | grep -i fexit
>     'BPF_TRAMP_FEXIT' val=1
>     'BPF_TRACE_FEXIT' val=25

[...]

> libbpf: collecting relocating info for: 'fexit/bpf_fentry_test6'
> libbpf: relo for shdr 16, symb 32, value 40, type 1, bind 1, name 34
> ('test6_result'), insn 18
> libbpf: found data map 0 (fexit_te.bss, sec 16, off 0) for insn 18
> libbpf: loading kernel BTF '/sys/kernel/btf/vmlinux': 0
> libbpf: fexit/bpf_fentry_test1 is not found in vmlinux BTF
> test_fexit_test:FAIL:prog_load fail err -2 errno 22
> #10 fexit_test:FAIL
> Summary: 0/0 PASSED, 0 SKIPPED, 1 FAILED
> 
> 
> Any hints on the issue?
> 
> -Matt C.