[PATCH bpf v2 1/2] bpf: enforce returning 0 for fentry/fexit progs

[Yonghong Song <yhs@fb.com>]

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>
---
 kernel/bpf/verifier.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

 bpf-next Commit 15d83c4d7cef ("bpf: Allow loading of a bpf
 iter program") contains the following change:

  --- a/kernel/bpf/verifier.c
  +++ b/kernel/bpf/verifier.c
  @@ -7101,6 +7101,10 @@ static int check_return_code(struct bpf_verifier_env *env)
                        return 0;
                range = tnum_const(0);
                break;
  +       case BPF_PROG_TYPE_TRACING:
  +               if (env->prog->expected_attach_type != BPF_TRACE_ITER)
  +                       return 0;
  +               break;
        default:
                return 0;
        }

  If this patch is accepted, it will have a merge conflict when syncing the change
  back to net-next/bpf-next, To resolve it, we can change to something like below:
                case BPF_TRACE_RAW_TP:
                case BPF_MODIFY_RETURN:
                        return 0;
		case BPF_TRACE_ITER:
			break;
                default:
                        return -ENOTSUPP;

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index fa1d8245b925..2d80cce0a28a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7059,6 +7059,24 @@ 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;
 	}
-- 
2.24.1