Re: [PATCH v3 bpf-next 5/5] selftests/bpf: Tweak cgroup kfunc test.

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Feb 28, 2023 at 9:07 AM David Vernet <void@manifault.com> wrote:
> libbpf: prog 'on_lookup': failed to load: -13
> libbpf: failed to load object 'cgrp_ls_recursion'
> libbpf: failed to load BPF skeleton 'cgrp_ls_recursion': -13
> test_recursion:FAIL:skel_open_and_load unexpected error: -13
> #43/3    cgrp_local_storage/recursion:FAIL
> #43      cgrp_local_storage:FAIL
>
> All error logs:
> test_cgrp_local_storage:PASS:join_cgroup /cgrp_local_storage 0 nsec
> libbpf: prog 'on_lookup': BPF program load failed: Permission denied
> libbpf: prog 'on_lookup': -- BEGIN PROG LOAD LOG --
> reg type unsupported for arg#0 function on_lookup#16
> 0: R1=ctx(off=0,imm=0) R10=fp0
> ; struct task_struct *task = bpf_get_current_task_btf();
> 0: (85) call bpf_get_current_task_btf#158     ; R0_w=trusted_ptr_task_struct(off=0,imm=0)
> 1: (bf) r6 = r0                       ; R0_w=trusted_ptr_task_struct(off=0,imm=0) R6_w=trusted_ptr_task_struct(off=0,imm=0)
> ; bpf_cgrp_storage_delete(&map_a, task->cgroups->dfl_cgrp);
> 2: (79) r1 = *(u64 *)(r6 +2296)       ; R1_w=rcu_ptr_or_null_css_set(off=0,imm=0) R6_w=trusted_ptr_task_struct(off=0,imm=0)
> ; bpf_cgrp_storage_delete(&map_a, task->cgroups->dfl_cgrp);
> 3: (79) r2 = *(u64 *)(r1 +120)
> R1 invalid mem access 'rcu_ptr_or_null_'

This one was tricky :)
Turned out btf_nested_type_is_trusted() was able to find
'cgroups' field in gcc compiled kernel and was failing on clang
compiled kernel because patch 2 did:
BTF_TYPE_SAFE_NESTED(struct task_struct) {
        const cpumask_t *cpus_ptr;
        struct css_set *cgroups;
};
instead of
BTF_TYPE_SAFE_NESTED(struct task_struct) {
        const cpumask_t *cpus_ptr;
        struct css_set __rcu *cgroups;
};
The missing tag was causing a miscompare.
Something to keep in mind.
This ugliness will go away once GCC supports btf tag.