bpf: Allow access to int pointer arguments in tracing programs

Adding support to access arguments with int pointer arguments
in tracing programs.

Currently we allow tracing programs to access only pointers to
string (char pointer), void pointers and pointers to structs.

If we try to access argument which is pointer to int, verifier
will fail to load the program with;

  R1 type=ctx expected=fp
  ; int BPF_PROG(fmod_ret_test, int _a, __u64 _b, int _ret)
  0: (bf) r6 = r1
  ; int BPF_PROG(fmod_ret_test, int _a, __u64 _b, int _ret)
  1: (79) r9 = *(u64 *)(r6 +8)
  func 'bpf_modify_return_test' arg1 type INT is not a struct

There is no harm for the program to access int pointer argument.
We are already doing that for string pointer, which is pointer
to int with 1 byte size.

Changing the is_string_ptr to generic integer check and renaming
it to btf_type_is_int.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211208193245.172141-2-jolsa@kernel.org

kernel/bpf/btf.c

@@ -4826,7 +4826,7 @@ struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
 		return prog->aux->attach_btf;
 }
 
-static bool is_string_ptr(struct btf *btf, const struct btf_type *t)
+static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
 {
 	/* t comes in already as a pointer */
 	t = btf_type_by_id(btf, t->type);
@@ -4835,8 +4835,7 @@ static bool is_string_ptr(struct btf *btf, const struct btf_type *t)
 	if (BTF_INFO_KIND(t->info) == BTF_KIND_CONST)
 		t = btf_type_by_id(btf, t->type);
 
-	/* char, signed char, unsigned char */
-	return btf_type_is_int(t) && t->size == 1;
+	return btf_type_is_int(t);
 }
 
 bool btf_ctx_access(int off, int size, enum bpf_access_type type,
@@ -4957,7 +4956,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		 */
 		return true;
 
-	if (is_string_ptr(btf, t))
+	if (is_int_ptr(btf, t))
 		return true;
 
 	/* this is a pointer to another type */