[PATCH bpf-next 09/15] bpf: Mark OBJ_RELEASE argument as MEM_RCU when possible

[Yonghong Song <yonghong.song@linux.dev>]

In previous selftests/bpf patch, we have
  p = bpf_percpu_obj_new(struct val_t);
  if (!p)
          goto out;

  p1 = bpf_kptr_xchg(&e->pc, p);
  if (p1) {
          /* race condition */
          bpf_percpu_obj_drop(p1);
  }

  p = e->pc;
  if (!p)
          goto out;

After bpf_kptr_xchg(), we need to re-read e->pc into 'p'.
This is due to that the second argument of bpf_kptr_xchg() is marked
OBJ_RELEASE and it will be marked as invalid after the call.
So after bpf_kptr_xchg(), 'p' is an unknown scalar,
and the bpf program needs to reread from the map value.

This patch checks if the 'p' has type MEM_ALLOC and MEM_PERCPU,
and if 'p' is RCU protected. If this is the case, 'p' can be marked
as MEM_RCU. MEM_ALLOC needs to be removed since 'p' is not
an owning reference any more. Such a change makes re-read
from the map value unnecessary.

Note that re-reading 'e->pc' after bpf_kptr_xchg() might get
a different value from 'p' if immediately before 'p = e->pc',
another cpu may do another bpf_kptr_xchg() and swap in another value
into 'e->pc'. If this is the case, then 'p = e->pc' may
get either 'p' or another value, and race condition already exists.
So removing direct re-reading seems fine too.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 kernel/bpf/verifier.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6fc200cb68b6..6fa458e13bfc 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8854,8 +8854,15 @@ static int release_reference(struct bpf_verifier_env *env,
 		return err;
 
 	bpf_for_each_reg_in_vstate(env->cur_state, state, reg, ({
-		if (reg->ref_obj_id == ref_obj_id)
-			mark_reg_invalid(env, reg);
+		if (reg->ref_obj_id == ref_obj_id) {
+			if (in_rcu_cs(env) && (reg->type & MEM_ALLOC) && (reg->type & MEM_PERCPU)) {
+				reg->ref_obj_id = 0;
+				reg->type &= ~MEM_ALLOC;
+				reg->type |= MEM_RCU;
+			} else {
+				mark_reg_invalid(env, reg);
+			}
+		}
 	}));
 
 	return 0;
-- 
2.34.1