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

Song Liu
2025-03-10T17:23:28Z

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.

> +
> + /* 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.

Thanks,
Song

> + else
> + return -EINVAL;
> +}
> --
> 2.48.1
>
>