bpf: allow pointers for ARG_ANYTHING parameters of global functions

The goal of the commit is to simplify writing global functions that
convert their operands to read-only probed memory using
bpf_rdonly_cast(). Depending on the context such parameters might be
either pointers or scalars. E.g. here is how scalar parameters might
occur:

	...
	// `task` is `PTR_TO_BTF_ID`
	task = bpf_get_current_task_btf();
	// .name is defined as `char *`
	name = task->mm->exe_file->f_path.dentry->d_name.name;
	global_fn(name);

Verifier considers `name` to be a scalar because of how
`btf_struct_access` and `btf_struct_walk` process paths that end up
with pointers to non-struct types: such pointers are considered to be
scalar values.

This commit allows declaring global_fn as `global_fn(u64)` and pass
both pointers and scalars as in the example above as parameters of the
`global_fn`.

Suggested-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Eduard Zingerman <[email protected]>

Author: eddyz87

kernel/bpf/verifier.c

@@ -10401,7 +10401,10 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog,
 		struct bpf_subprog_arg_info *arg = &sub->args[i];
 
 		if (arg->arg_type == ARG_ANYTHING) {
-			if (reg->type != SCALAR_VALUE) {
+			if (reg->type == NOT_INIT) {
+				bpf_log(log, "parameter R%d is not initialized\n", regno);
+				return -EINVAL;
+			} else if (!env->allow_ptr_leaks && reg->type != SCALAR_VALUE) {
 				bpf_log(log, "R%d is not a scalar\n", regno);
 				return -EINVAL;
 			}