Subject: Re: [PATCH bpf v4 2/2] selftests/bpf: Add a test for using a cpumap from an freplace-to-XDP program

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Wed, 14 Dec 2022 23:58:32 +0100

Andrii Nakryiko <[email-masked]> writes:

> On Tue, Dec 13, 2022 at 5:05 PM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>
>> This adds a simple test for inserting an XDP program into a cpumap that is
>> "owned" by an XDP program that was loaded as PROG_TYPE_EXT (as libxdp
>> does). Prior to the kernel fix this would fail because the map type
>> ownership would be set to PROG_TYPE_EXT instead of being resolved to
>> PROG_TYPE_XDP.
>>
>> v4:
>> - Use skeletons for selftest
>> v3:
>> - Update comment to better explain the cause
>> - Add Yonghong's ACK
>>
>> Acked-by: Yonghong Song <[email-masked]>
>> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
>> ---
>>  .../selftests/bpf/prog_tests/fexit_bpf2bpf.c  | 54 +++++++++++++++++++
>>  .../selftests/bpf/progs/freplace_progmap.c    | 24 +++++++++
>>  2 files changed, 78 insertions(+)
>>  create mode 100644 tools/testing/selftests/bpf/progs/freplace_progmap.c
>>
>> diff --git a/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c b/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
>> index d1e32e792536..efa1fc65840d 100644
>> --- a/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
>> +++ b/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
>> @@ -4,6 +4,8 @@
>>  #include <network_helpers.h>
>>  #include <bpf/btf.h>
>>  #include "bind4_prog.skel.h"
>> +#include "freplace_progmap.skel.h"
>> +#include "xdp_dummy.skel.h"
>>
>>  typedef int (*test_cb)(struct bpf_object *obj);
>>
>> @@ -500,6 +502,56 @@ static void test_fentry_to_cgroup_bpf(void)
>>         bind4_prog__destroy(skel);
>>  }
>>
>> +static void test_func_replace_progmap(void)
>> +{
>> +       struct bpf_cpumap_val value = { .qsize = 1 };
>> +       struct freplace_progmap *skel = NULL;
>> +       struct xdp_dummy *tgt_skel = NULL;
>> +       int err, tgt_fd;
>> +       __u32 key = 0;
>> +
>> +       skel = freplace_progmap__open();
>> +       if (!ASSERT_OK_PTR(skel, "prog_open"))
>> +               return;
>> +
>> +       tgt_skel = xdp_dummy__open_and_load();
>> +       if (!ASSERT_OK_PTR(tgt_skel, "tgt_prog_load"))
>> +               goto out;
>> +
>> +       tgt_fd = bpf_program__fd(tgt_skel->progs.xdp_dummy_prog);
>> +
>> +       /* Change the 'redirect' program type to be a PROG_TYPE_EXT
>> +        * with an XDP target
>> +        */
>> +       bpf_program__set_type(skel->progs.xdp_cpumap_prog, BPF_PROG_TYPE_EXT);
>> +       bpf_program__set_expected_attach_type(skel->progs.xdp_cpumap_prog, 0);
>
> you shouldn't need this manual override if you mark xdp_cpumap_prog as
> SEC("freplace"), or am I missing something?

No, you're right; I guess I was just too focused on recreating the flow
we use in libxdp. Will fix!

>> +       err = bpf_program__set_attach_target(skel->progs.xdp_cpumap_prog,
>> +                                            tgt_fd, "xdp_dummy_prog");
>> +       if (!ASSERT_OK(err, "set_attach_target"))
>> +               goto out;
>> +
>> +       err = freplace_progmap__load(skel);
>> +       if (!ASSERT_OK(err, "obj_load"))
>> +               goto out;
>> +
>> +       /* Prior to fixing the kernel, loading the PROG_TYPE_EXT 'redirect'
>> +        * program above will cause the map owner type of 'cpumap' to be set to
>> +        * PROG_TYPE_EXT. This in turn will cause the bpf_map_update_elem()
>> +        * below to fail, because the program we are inserting into the map is
>> +        * of PROG_TYPE_XDP. After fixing the kernel, the initial ownership will
>> +        * be correctly resolved to the *target* of the PROG_TYPE_EXT program
>> +        * (i.e., PROG_TYPE_XDP) and the map update will succeed.
>> +        */
>> +       value.bpf_prog.fd = bpf_program__fd(skel->progs.xdp_drop_prog);
>> +       err = bpf_map_update_elem(bpf_map__fd(skel->maps.cpu_map),
>> +                                 &key, &value, 0);
>> +       ASSERT_OK(err, "map_update");
>> +
>> +out:
>> +       xdp_dummy__destroy(tgt_skel);
>> +       freplace_progmap__destroy(skel);
>> +}
>> +
>>  /* NOTE: affect other tests, must run in serial mode */
>>  void serial_test_fexit_bpf2bpf(void)
>>  {
>> @@ -525,4 +577,6 @@ void serial_test_fexit_bpf2bpf(void)
>>                 test_func_replace_global_func();
>>         if (test__start_subtest("fentry_to_cgroup_bpf"))
>>                 test_fentry_to_cgroup_bpf();
>> +       if (test__start_subtest("func_replace_progmap"))
>> +               test_func_replace_progmap();
>>  }
>> diff --git a/tools/testing/selftests/bpf/progs/freplace_progmap.c b/tools/testing/selftests/bpf/progs/freplace_progmap.c
>> new file mode 100644
>> index 000000000000..68174c3d7b37
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/progs/freplace_progmap.c
>> @@ -0,0 +1,24 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +#include <linux/bpf.h>
>> +#include <bpf/bpf_helpers.h>
>> +
>> +struct {
>> +       __uint(type, BPF_MAP_TYPE_CPUMAP);
>> +       __uint(key_size, sizeof(__u32));
>> +       __uint(value_size, sizeof(struct bpf_cpumap_val));
>
> ok, another minor nit which you ignored, libbpf should be smart enough to accept
>
> __type(key, __u32);
> __type(value, struct bpf_cpumap_val);
>
> And if it's not it would be good to know that it's not (and trivially
> fix it).

Ah, actually saw that comment on the previous version, and then
completely forgot about it when I was fixing things up. Sorry about
that; will change!

-Toke