Re: [PATCH v6 bpf-next 2/2] selftests/bpf: Add a kernel flag test for LSM bpf hook

[Blaise Boscaccy <bboscaccy@linux.microsoft.com>]

Song Liu <song@kernel.org> writes:

> On Fri, Mar 7, 2025 at 5:33 PM Blaise Boscaccy
> <bboscaccy@linux.microsoft.com> wrote:
>>
>> This test exercises the kernel flag added to security_bpf by
>> effectively blocking light-skeletons from loading while allowing
>> normal skeletons to function as-is. Since this should work with any
>> arbitrary BPF program, an existing program from LSKELS_EXTRA was
>> used as a test payload.
>>
>> Signed-off-by: Blaise Boscaccy <bboscaccy@linux.microsoft.com>
>> ---
>>  .../selftests/bpf/prog_tests/kernel_flag.c    | 43 +++++++++++++++++++
>>  .../selftests/bpf/progs/test_kernel_flag.c    | 28 ++++++++++++
>>  2 files changed, 71 insertions(+)
>>  create mode 100644 tools/testing/selftests/bpf/prog_tests/kernel_flag.c
>>  create mode 100644 tools/testing/selftests/bpf/progs/test_kernel_flag.c
>>
>> diff --git a/tools/testing/selftests/bpf/prog_tests/kernel_flag.c b/tools/testing/selftests/bpf/prog_tests/kernel_flag.c
>> new file mode 100644
>> index 0000000000000..479ad5de3737e
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/prog_tests/kernel_flag.c
>> @@ -0,0 +1,43 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/* Copyright (c) 2025 Microsoft */
>> +#include <test_progs.h>
>> +#include "kfunc_call_test.skel.h"
>> +#include "kfunc_call_test.lskel.h"
>> +#include "test_kernel_flag.skel.h"
>> +
>> +void test_kernel_flag(void)
>> +{
>> +       struct test_kernel_flag *lsm_skel;
>> +       struct kfunc_call_test *skel = NULL;
>> +       struct kfunc_call_test_lskel *lskel = NULL;
>> +       int ret;
>> +
>> +       lsm_skel = test_kernel_flag__open_and_load();
>> +       if (!ASSERT_OK_PTR(lsm_skel, "lsm_skel"))
>> +               return;
>> +
>> +       ret = test_kernel_flag__attach(lsm_skel);
>> +       if (!ASSERT_OK(ret, "test_kernel_flag__attach"))
>> +               goto close_prog;
>> +
>> +       lsm_skel->bss->monitored_pid = getpid();
>
> We usually set monitored_pid before attaching the program.
>

Okay, copy that. 

>> +
>> +       /* Test with skel. This should pass the gatekeeper */
>> +       skel = kfunc_call_test__open_and_load();
>> +       if (!ASSERT_OK_PTR(skel, "skel"))
>> +               goto close_prog;
>> +
>> +       /* Test with lskel. This should fail due to blocking kernel-based bpf() invocations */
>> +       lskel = kfunc_call_test_lskel__open_and_load();
>> +       if (!ASSERT_ERR_PTR(lskel, "lskel"))
>> +               goto close_prog;
>> +
>> +close_prog:
>> +       if (skel)
>> +               kfunc_call_test__destroy(skel);
>> +       if (lskel)
>> +               kfunc_call_test_lskel__destroy(lskel);
>> +
>> +       lsm_skel->bss->monitored_pid = 0;
>> +       test_kernel_flag__destroy(lsm_skel);
>> +}
>> diff --git a/tools/testing/selftests/bpf/progs/test_kernel_flag.c b/tools/testing/selftests/bpf/progs/test_kernel_flag.c
>> new file mode 100644
>> index 0000000000000..9ca01aadb6656
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/progs/test_kernel_flag.c
>> @@ -0,0 +1,28 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +/*
>> + * Copyright (C) 2025 Microsoft Corporation
>> + *
>> + * Author: Blaise Boscaccy <bboscaccy@linux.microsoft.com>
>> + */
>> +
>> +#include "vmlinux.h"
>> +#include <errno.h>
>> +#include <bpf/bpf_helpers.h>
>> +#include <bpf/bpf_tracing.h>
>> +
>> +char _license[] SEC("license") = "GPL";
>> +
>> +__u32 monitored_pid;
>> +
>> +SEC("lsm.s/bpf")
>> +int BPF_PROG(bpf, int cmd, union bpf_attr *attr, unsigned int size, bool kernel)
>> +{
>> +       __u32 pid;
>> +
>> +       pid = bpf_get_current_pid_tgid() >> 32;
>> +       if (!kernel || pid != monitored_pid)
>> +               return 0;
>
> We are blocking lskel load for the pid. This could make
> parallel testing (test_progs -j) flaky. We should probably
> change the logic to filtering on monitored_tiid.
>

Curious on this for my own edification. The

pid = bpf_get_current_pid_tgid() >> 32;

is used extensively in the current test suite in a bunch of other
tests. Why does that not cause an issue with the other tests during
parallel testing? 

> Thanks,
> Song
>
>> +       else
>> +               return -EINVAL;
>> +}
>> --
>> 2.48.1
>>
>>