bpf: Mark OBJ_RELEASE argument as MEM_RCU when possible

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>
Link: https://lore.kernel.org/r/20230827152816.2000760-1-yonghong.song@linux.devSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -9660,6 +9660,26 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 				return -EFAULT;
 			}
 			err = unmark_stack_slots_dynptr(env, &regs[meta.release_regno]);
+		} else if (func_id == BPF_FUNC_kptr_xchg && meta.ref_obj_id) {
+			u32 ref_obj_id = meta.ref_obj_id;
+			bool in_rcu = in_rcu_cs(env);
+			struct bpf_func_state *state;
+			struct bpf_reg_state *reg;
+
+			err = release_reference_state(cur_func(env), ref_obj_id);
+			if (!err) {
+				bpf_for_each_reg_in_vstate(env->cur_state, state, reg, ({
+					if (reg->ref_obj_id == ref_obj_id) {
+						if (in_rcu && (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);
+						}
+					}
+				}));
+			}
 		} else if (meta.ref_obj_id) {
 			err = release_reference(env, meta.ref_obj_id);
 		} else if (register_is_null(&regs[meta.release_regno])) {