bpf: Fix modifier skipping logic

Fix the way modifiers are skipped while walking pointers. Otherwise second
level dereferences of 'const struct foo *' will be rejected by the verifier.

Fixes: 9e15db66 ("bpf: Implement accurate raw_tp context access via BTF")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200201000314.261392-1-ast@kernel.org

kernel/bpf/btf.c

@@ -3931,6 +3931,7 @@ int btf_struct_access(struct bpf_verifier_log *log,
 
 		if (btf_type_is_ptr(mtype)) {
 			const struct btf_type *stype;
+			u32 id;
 
 			if (msize != size || off != moff) {
 				bpf_log(log,
@@ -3939,12 +3940,9 @@ int btf_struct_access(struct bpf_verifier_log *log,
 				return -EACCES;
 			}
 
-			stype = btf_type_by_id(btf_vmlinux, mtype->type);
-			/* skip modifiers */
-			while (btf_type_is_modifier(stype))
-				stype = btf_type_by_id(btf_vmlinux, stype->type);
+			stype = btf_type_skip_modifiers(btf_vmlinux, mtype->type, &id);
 			if (btf_type_is_struct(stype)) {
-				*next_btf_id = mtype->type;
+				*next_btf_id = id;
 				return PTR_TO_BTF_ID;
 			}
 		}