bpf: Allow access to const void pointer arguments in tracing programs

Adding support to access arguments with const void pointer arguments
in tracing programs.

Currently we allow tracing programs to access void pointers. If we try to
access argument which is pointer to const void like 2nd argument in kfree,
verifier will fail to load the program with;

0: R1=ctx() R10=fp0
; asm volatile ("r2 = *(u64 *)(r1 + 8); ");
0: (79) r2 = *(u64 *)(r1 +8)
func 'kfree' arg1 type UNKNOWN is not a struct

Changing the is_int_ptr to void and generic integer check and renaming
it to is_void_or_int_ptr.

Signed-off-by: KaFai Wan <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/bpf/btf.c

···
 		return prog->aux->attach_btf;
 }
 
-static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
+static bool is_void_or_int_ptr(struct btf *btf, const struct btf_type *t)
 {
 	/* skip modifiers */
 	t = btf_type_skip_modifiers(btf, t->type, NULL);
-
-	return btf_type_is_int(t);
+	return btf_type_is_void(t) || btf_type_is_int(t);
 }
 
 static u32 get_ctx_arg_idx(struct btf *btf, const struct btf_type *func_proto,
···
 		}
 	}
 
-	if (t->type == 0)
-		/* This is a pointer to void.
-		 * It is the same as scalar from the verifier safety pov.
-		 * No further pointer walking is allowed.
-		 */
-		return true;
-
-	if (is_int_ptr(btf, t))
+	/*
+	 * If it's a pointer to void, it's the same as scalar from the verifier
+	 * safety POV. Either way, no futher pointer walking is allowed.
+	 */
+	if (is_void_or_int_ptr(btf, t))
 		return true;
 
 	/* this is a pointer to another type */