[PATCH] bpf: Add support of skipping the current object for bpf_iter progs

[GONG, Ruiqi]

bpf_seq_read() can accept three different types of seq_ops->show()'s
return value:

  err > 0: skip the obj and reuse seq_num
  err < 0: abort the whole iter process
  err == 0 (implicitly): continue

but bpf_iter_run_prog() is limited to the last two cases. Extend the
legal return value of bpf_iter progs so that they can skip certain
objects and then proceed to the followings.

Signed-off-by: GONG, Ruiqi <gongruiqi@xxxxxxxxxxxxxxx>
---
 kernel/bpf/bpf_iter.c | 9 +++++----
 kernel/bpf/verifier.c | 1 +
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
index 96856f130cbf..1c1d67ec466c 100644
--- a/kernel/bpf/bpf_iter.c
+++ b/kernel/bpf/bpf_iter.c
@@ -716,13 +716,14 @@ int bpf_iter_run_prog(struct bpf_prog *prog, void *ctx)
 		rcu_read_unlock();
 	}
 
-	/* bpf program can only return 0 or 1:
-	 *  0 : okay
-	 *  1 : retry the same object
+	/* bpf program can return:
+	 *  0 : has shown the object, go next
+	 *  1 : has skipped the object, go next
+	 * -1 : encountered error and should terminate
 	 * The bpf_iter_run_prog() return value
 	 * will be seq_ops->show() return value.
 	 */
-	return ret == 0 ? 0 : -EAGAIN;
+	return ret == 0 ? 0 : (ret == 1 ? 1 : -EAGAIN);
 }
 
 BPF_CALL_4(bpf_for_each_map_elem, struct bpf_map *, map, void *, callback_fn,
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 930b5555cfd3..cebd3a0b3172 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14333,6 +14333,7 @@ static int check_return_code(struct bpf_verifier_env *env)
 		case BPF_MODIFY_RETURN:
 			return 0;
 		case BPF_TRACE_ITER:
+			range = tnum_range(-1, 1);
 			break;
 		default:
 			return -ENOTSUPP;
-- 
2.25.1