Re: [PATCH bpf-next v6 2/2] selftests/bpf: Add test to verify freeing the special fields when update [lru_,]percpu_hash maps

[Leon Hwang]

On 2025/11/7 10:00, Yonghong Song wrote:
>
>
> On 11/5/25 7:14 AM, Leon Hwang wrote:
>> Add test to verify that updating [lru_,]percpu_hash maps decrements
>> 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>
>
> LGTM with a few nits below.
>
> Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
>

Hi Yonghong,

Thanks for your review and ack.

>> ---
>>   .../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);
>> +}
>> +
>
> Empty line here.
>
>> diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr.c b/
>> 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);
>
> Change the above to node_old = bpf_kptr_xchg(node, node_node_ref); might
> be better for reasoning although node_ref/node_new are the same.
>

Nope — node_ref and node_new are different for the verifier.

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

[...]