Subject: Re: [PATCH v4 bpf 2/2] selftests/bpf: extend map-in-map selftest to detect memory leaks

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 29 Jul 2020 22:29:10 +0200

On Wed, Jul 29, 2020 at 07:48 PM CEST, Andrii Nakryiko wrote:
> On Wed, Jul 29, 2020 at 7:29 AM Jakub Sitnicki <[email-masked]> wrote:
>>
>> On Wed, Jul 29, 2020 at 06:09 AM CEST, Andrii Nakryiko wrote:

[...]

>> > +/*
>> > + * Trigger synchronize_cpu() in kernel.
>>
>> Nit: synchronize_*r*cu().
>
> welp, yeah
>
>>
>> > + *
>> > + * ARRAY_OF_MAPS/HASH_OF_MAPS lookup/update operations trigger
>> > + * synchronize_rcu(), if looking up/updating non-NULL element. Use this fact
>> > + * to trigger synchronize_cpu(): create map-in-map, create a trivial ARRAY
>> > + * map, update map-in-map with ARRAY inner map. Then cleanup. At the end, at
>> > + * least one synchronize_rcu() would be called.
>> > + */
>>
>> That's a cool trick. I'm a bit confused by "looking up/updating non-NULL
>> element". It looks like you're updating an element that is NULL/unset in
>> the code below. What am I missing?
>
> I was basically trying to say that it has to be a successful lookup or
> update. For lookup that means looking up non-NULL (existing) entry.
> For update -- setting valid inner map FD.
>
> Not sure fixing this and typo above is worth it to post v5.

I just wanted to understand that the helper is working as intended. It
seems handy. I agree that it's not worth respinning the patches just for
this.

>
>>
>> > +static int kern_sync_rcu(void)
>> > +{
>> > +     int inner_map_fd, outer_map_fd, err, zero = 0;
>> > +
>> > +     inner_map_fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, 4, 4, 1, 0);
>> > +     if (CHECK(inner_map_fd < 0, "inner_map_create", "failed %d\n", -errno))
>> > +             return -1;
>> > +
>> > +     outer_map_fd = bpf_create_map_in_map(BPF_MAP_TYPE_ARRAY_OF_MAPS, NULL,
>> > +                                          sizeof(int), inner_map_fd, 1, 0);
>> > +     if (CHECK(outer_map_fd < 0, "outer_map_create", "failed %d\n", -errno)) {
>> > +             close(inner_map_fd);
>> > +             return -1;
>> > +     }
>> > +
>> > +     err = bpf_map_update_elem(outer_map_fd, &zero, &inner_map_fd, 0);
>> > +     if (err)
>> > +             err = -errno;
>> > +     CHECK(err, "outer_map_update", "failed %d\n", err);
>> > +     close(inner_map_fd);
>> > +     close(outer_map_fd);
>> > +     return err;
>> > +}
>> > +
>
> [...]
>
> trimming's good ;)

You caught me. Just being lazy. No excuses :-)