bpf: Enforce returning 0 for fentry/fexit progs

Currently, tracing/fentry and tracing/fexit prog
return values are not enforced. In trampoline codes,
the fentry/fexit prog return values are ignored.
Let us enforce it to be 0 to avoid confusion and
allows potential future extension.

This patch also explicitly added return value
checking for tracing/raw_tp, tracing/fmod_ret,
and freplace programs such that these program
return values can be anything. The purpose are
two folds:
 1. to make it explicit about return value expectations
    for these programs in verifier.
 2. for tracing prog_type, if a future attach type
    is added, the default is -ENOTSUPP which will
    enforce to specify return value ranges explicitly.

Fixes: fec56f5890d9 ("bpf: Introduce BPF trampoline")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200514053206.1298415-1-yhs@fb.com

kernel/bpf/verifier.c

@@ -7059,6 +7059,23 @@ static int check_return_code(struct bpf_verifier_env *env)
 			return 0;
 		range = tnum_const(0);
 		break;
+	case BPF_PROG_TYPE_TRACING:
+		switch (env->prog->expected_attach_type) {
+		case BPF_TRACE_FENTRY:
+		case BPF_TRACE_FEXIT:
+			range = tnum_const(0);
+			break;
+		case BPF_TRACE_RAW_TP:
+		case BPF_MODIFY_RETURN:
+			return 0;
+		default:
+			return -ENOTSUPP;
+		}
+		break;
+	case BPF_PROG_TYPE_EXT:
+		/* freplace program can return anything as its return value
+		 * depends on the to-be-replaced kernel func or bpf program.
+		 */
 	default:
 		return 0;
 	}