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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Mar 25, 2020 at 3:16 PM Matt Cover <werekraken@gmail.com> 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.

BTF is, but that BTF doesn't have information about FUNCs (only
FUNC_PROTOs). You need pahole 1.16 for fentry/fexit.

>
>
> [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`
> 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
> [vagrant@localhost bpf]$ ~/bpftool btf dump file
> /sys/kernel/btf/vmlinux | grep -i fexit
>     'BPF_TRAMP_FEXIT' val=1
>     'BPF_TRACE_FEXIT' val=25
>
>
> The fexit_test.o file also has BTF information.
>
>
> [vagrant@localhost bpf]$ ~/bpftool btf dump file fexit_test.o | grep FUNC_PROTO
> [4] FUNC_PROTO '(anon)' ret_type_id=5 vlen=1
> [7] FUNC_PROTO '(anon)' ret_type_id=5 vlen=1
> [9] FUNC_PROTO '(anon)' ret_type_id=5 vlen=1
> [11] FUNC_PROTO '(anon)' ret_type_id=5 vlen=1
> [13] FUNC_PROTO '(anon)' ret_type_id=5 vlen=1
> [15] FUNC_PROTO '(anon)' ret_type_id=5 vlen=1
>
>
> However, I get libbpf/BTF load errors when trying to run any
> fentry/fexit tests.
>
>
> [vagrant@localhost bpf]$ sudo ./test_progs -t fexit_test | grep '^libbpf\|FAIL'
> libbpf: Error loading BTF: Invalid argument(22)
> libbpf: magic: 0xeb9f
> libbpf: Error loading .BTF into kernel: -22.
> libbpf: Error loading BTF: Invalid argument(22)
> libbpf: magic: 0xeb9f
> libbpf: Error loading .BTF into kernel: -22.
> 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
>
>
> I saw in a similar thread that -vvv output was requested. Figured the
> same applies here.

Yeah, for tricky issues that good. In this case it was pretty obvious,
but generally it's a good idea for sure, thanks!
>
>
> [vagrant@localhost bpf]$ sudo ./test_progs -vvv -t fexit_test | grep
> '^libbpf\|FAIL'

[...]

>
>
> Any hints on the issue?
>
> -Matt C.