Ideal way to read FUNC_PROTO in raw tp?

[Yan Zhai <yan@cloudflare.com>]

Hi,

 I am building a tracing program around workqueue. But I encountered
following problem when I try to record a function pointer value from
trace_workqueue_execute_end on net-next kernel:

...
libbpf: prog 'workqueue_end': BPF program load failed: Permission
denied
libbpf: prog 'workqueue_end': -- BEGIN PROG LOAD LOG --
reg type unsupported for arg#0 function workqueue_end#5
0: R1=ctx(off=0,imm=0) R10=fp0
; int BPF_PROG(workqueue_end, struct work_struct *w, work_func_t f)
0: (79) r3 = *(u64 *)(r1 +8)
func 'workqueue_execute_end' arg1 type FUNC_PROTO is not a struct
invalid bpf_context access off=8 size=8
processed 1 insns (limit 1000000) max_states_per_insn 0 total_states 0
peak_states 0 mark_read 0
-- END PROG LOAD LOG --
libbpf: prog 'workqueue_end': failed to load: -13
libbpf: failed to load object 'configs/test.bpf.o'
Error: failed to load object file
Warning: bpftool is now running in libbpf strict mode and has more
stringent requirements about BPF programs.
If it used to work for this object file but now doesn't, see --legacy
option for more details.
...

A simple reproducer for me is like:
#include "vmlinux.h"
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>

SEC("tp_btf/workqueue_execute_end")
int BPF_PROG(workqueue_end, struct work_struct *w, work_func_t f)
{
        u64 addr = (u64) f;
        bpf_printk("f is %lu\n", addr);

        return 0;
}

char LICENSE[] SEC("license") = "GPL";

I would like to use the function address to decode the kernel symbol
and track execution of these functions. Replacing raw tp to regular tp
solves the problem, but I am wondering if there is any go-to approach
to read the pointer value in a raw tp? Doesn't seem to find one in
selftests/samples. If not, does it make sense if we allow it in
the verifier for tracing programs like the attached patch?

Yan

---
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 821063660d9f..5f000ab4c8d0 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6308,6 +6308,11 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
                        __btf_name_by_offset(btf, t->name_off),
                        btf_type_str(t));
                return false;
+       } else if (prog->type == BPF_PROG_TYPE_TRACING || prog->type == BPF_PROG_TYPE_RAW_TRACEPOINT) {
+               /* allow reading function pointer value from a tracing program */
+               const struct btf_type *pointed = btf_type_by_id(btf, t->type);
+               if (btf_type_is_func_proto(pointed))
+                       return true;
        }

        /* check for PTR_TO_RDONLY_BUF_OR_NULL or PTR_TO_RDWR_BUF_OR_NULL */