Re: [PATCH bpf-next v6 2/2] selftests/bpf: Add test to verify freeing the special fields when update [lru_,]percpu_hash maps
From: Yonghong Song
Date: Tue Nov 11 2025 - 16:58:38 EST
On 11/11/25 5:52 AM, Leon Hwang wrote:
On 2025/11/11 21:38, Leon Hwang wrote:
They are the same in theory.
On 2025/11/7 10:00, Yonghong Song wrote:
Hi Yonghong,
On 11/5/25 7:14 AM, Leon Hwang wrote:
Add test to verify that updating [lru_,]percpu_hash maps decrementsLGTM with a few nits below.
refcount when BPF_KPTR_REF objects are involved.
The tests perform the following steps:
1. Call update_elem() to insert an initial value.
2. Use bpf_refcount_acquire() to increment the refcount.
3. Store the node pointer in the map value.
4. Add the node to a linked list.
5. Probe-read the refcount and verify it is *2*.
6. Call update_elem() again to trigger refcount decrement.
7. Probe-read the refcount and verify it is *1*.
Signed-off-by: Leon Hwang <leon.hwang@xxxxxxxxx>
Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
Thanks for your review and ack.
Nope — node_ref and node_new are different for the verifier.---Empty line here.
.../bpf/prog_tests/refcounted_kptr.c | 57 ++++++++++++++++++
.../selftests/bpf/progs/refcounted_kptr.c | 60 +++++++++++++++++++
2 files changed, 117 insertions(+)
diff --git a/tools/testing/selftests/bpf/prog_tests/refcounted_kptr.c
b/tools/testing/selftests/bpf/prog_tests/refcounted_kptr.c
index d6bd5e16e6372..086f679fa3f61 100644
--- a/tools/testing/selftests/bpf/prog_tests/refcounted_kptr.c
+++ b/tools/testing/selftests/bpf/prog_tests/refcounted_kptr.c
@@ -44,3 +44,60 @@ void test_refcounted_kptr_wrong_owner(void)
ASSERT_OK(opts.retval, "rbtree_wrong_owner_remove_fail_a2 retval");
refcounted_kptr__destroy(skel);
}
+
+void test_percpu_hash_refcounted_kptr_refcount_leak(void)
+{
+ struct refcounted_kptr *skel;
+ int cpu_nr, fd, err, key = 0;
+ struct bpf_map *map;
+ size_t values_sz;
+ u64 *values;
+ LIBBPF_OPTS(bpf_test_run_opts, opts,
+ .data_in = &pkt_v4,
+ .data_size_in = sizeof(pkt_v4),
+ .repeat = 1,
+ );
+
+ cpu_nr = libbpf_num_possible_cpus();
+ if (!ASSERT_GT(cpu_nr, 0, "libbpf_num_possible_cpus"))
+ return;
+
+ values = calloc(cpu_nr, sizeof(u64));
+ if (!ASSERT_OK_PTR(values, "calloc values"))
+ return;
+
+ skel = refcounted_kptr__open_and_load();
+ if (!ASSERT_OK_PTR(skel, "refcounted_kptr__open_and_load")) {
+ free(values);
+ return;
+ }
+
+ values_sz = cpu_nr * sizeof(u64);
+ memset(values, 0, values_sz);
+
+ map = skel->maps.percpu_hash;
+ err = bpf_map__update_elem(map, &key, sizeof(key), values,
values_sz, 0);
+ if (!ASSERT_OK(err, "bpf_map__update_elem"))
+ goto out;
+
+ fd = bpf_program__fd(skel->progs.percpu_hash_refcount_leak);
+ err = bpf_prog_test_run_opts(fd, &opts);
+ if (!ASSERT_OK(err, "bpf_prog_test_run_opts"))
+ goto out;
+ if (!ASSERT_EQ(opts.retval, 2, "opts.retval"))
+ goto out;
+
+ err = bpf_map__update_elem(map, &key, sizeof(key), values,
values_sz, 0);
+ if (!ASSERT_OK(err, "bpf_map__update_elem"))
+ goto out;
+
+ fd = bpf_program__fd(skel->progs.check_percpu_hash_refcount);
+ err = bpf_prog_test_run_opts(fd, &opts);
+ ASSERT_OK(err, "bpf_prog_test_run_opts");
+ ASSERT_EQ(opts.retval, 1, "opts.retval");
+
+out:
+ refcounted_kptr__destroy(skel);
+ free(values);
+}
+
diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr.c b/Change the above to node_old = bpf_kptr_xchg(node, node_node_ref); might
tools/testing/selftests/bpf/progs/refcounted_kptr.c
index 893a4fdb4b6e9..1aca85d86aebc 100644
--- a/tools/testing/selftests/bpf/progs/refcounted_kptr.c
+++ b/tools/testing/selftests/bpf/progs/refcounted_kptr.c
@@ -568,4 +568,64 @@ int
BPF_PROG(rbtree_sleepable_rcu_no_explicit_rcu_lock,
return 0;
}
+private(kptr_ref) u64 ref;
+
+static int probe_read_refcount(void)
+{
+ u32 refcount;
+
+ bpf_probe_read_kernel(&refcount, sizeof(refcount), (void *) ref);
+ return refcount;
+}
+
+static int __insert_in_list(struct bpf_list_head *head, struct
bpf_spin_lock *lock,
+ struct node_data __kptr **node)
+{
+ struct node_data *node_new, *node_ref, *node_old;
+
+ node_new = bpf_obj_new(typeof(*node_new));
+ if (!node_new)
+ return -1;
+
+ node_ref = bpf_refcount_acquire(node_new);
+ node_old = bpf_kptr_xchg(node, node_new);
be better for reasoning although node_ref/node_new are the same.
The verifier failure was likely caused by something else, but I'm not
sure of the exact reason.
I did some analysis and your code works as expected:
node_ref = bpf_refcount_acquire(node_new);
node_old = bpf_kptr_xchg(node, node_new);
if (node_old) {
bpf_obj_drop(node_old);
bpf_obj_drop(node_ref);
return -2;
}
bpf_spin_lock(lock);
bpf_list_push_front(head, &node_ref->l);
ref = (u64)(void *) &node_ref->ref;
bpf_spin_unlock(lock);
In the above, after the following insn:
node_old = bpf_kptr_xchg(node, node_new);
the second argument 'node_new' will become a scalar since it
may be changed by another bpf program accessing the same map.
So your code is okay as node_ref still valid ptr_node_data
and can be used in following codes.
My suggestion to replace
node_old = bpf_kptr_xchg(node, node_new);
with
node_old = bpf_kptr_xchg(node, node_ref);
will not work since node_ref will be a scalar
so subsequent bpf_obj_drop(node_ref) and bpf_list_push_front(...)
won't work.
In summary, your change look okay to me. Sorry for noise.
When trying node_old = bpf_kptr_xchg(node, node_ref), the verifier reported:
[verifier log snipped for brevity...]
; bpf_obj_drop(node_ref); @ refcounted_kptr.c:594
26: (bf) r1 = r6 ; R1=scalar(id=7) R6=scalar(id=7)
refs=3
27: (b7) r2 = 0 ; R2=0 refs=3
28: (85) call bpf_obj_drop_impl#54490
R1 must be referenced or trusted
processed 27 insns (limit 1000000) max_states_per_insn 0 total_states 2
peak_states 2 mark_read 0
So the verifier rejected it because R6 became scalar(id=7) from
ptr_node_data(ref_obj_id=4).
---
Hi Alexei, could you please drop the extra empty line when applying this
patch?
Then I don't need to send another revision.
Thanks,
Leon
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