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Message-ID: <aS6qfN4cXoEO82bE@google.com>
Date: Tue, 2 Dec 2025 08:59:40 +0000
From: Matt Bobrowski <mattbobrowski@...gle.com>
To: Shuran Liu <electronlsr@...il.com>
Cc: song@...nel.org, bpf@...r.kernel.org, ast@...nel.org,
daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev,
eddyz87@...il.com, yonghong.song@...ux.dev,
john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
haoluo@...gle.com, jolsa@...nel.org, rostedt@...dmis.org,
mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
Zesen Liu <ftyg@...e.com>, Peili Gao <gplhust955@...il.com>,
Haoran Ni <haoran.ni.cs@...il.com>
Subject: Re: [PATCH bpf v2 2/2] selftests/bpf: fix and consolidate d_path LSM
regression test
On Tue, Dec 02, 2025 at 03:54:41PM +0800, Shuran Liu wrote:
> Add a regression test for bpf_d_path() when invoked from an LSM program.
> The test attaches to the bprm_check_security hook, calls bpf_d_path() on
> the binary being executed, and verifies that a simple prefix comparison on
> the returned pathname behaves correctly after the fix in patch 1.
>
> To avoid nondeterminism, the LSM program now filters based on the
> expected PID, which is populated from userspace before the test binary is
> executed. This prevents unrelated processes that also trigger the
> bprm_check_security LSM hook from overwriting test results. Parent and
> child processes are synchronized through a pipe to ensure the PID is set
> before the child execs the test binary.
>
> Per review feedback, the new test is merged into the existing d_path
> selftest rather than adding new prog_tests/ or progs/ files.
>
> Co-developed-by: Zesen Liu <ftyg@...e.com>
> Signed-off-by: Zesen Liu <ftyg@...e.com>
> Co-developed-by: Peili Gao <gplhust955@...il.com>
> Signed-off-by: Peili Gao <gplhust955@...il.com>
> Co-developed-by: Haoran Ni <haoran.ni.cs@...il.com>
> Signed-off-by: Haoran Ni <haoran.ni.cs@...il.com>
> Signed-off-by: Shuran Liu <electronlsr@...il.com>
Feel free to add:
Reviewed-by: Matt Bobrowski <mattbobrowski@...gle.com>
> ---
> .../testing/selftests/bpf/prog_tests/d_path.c | 64 +++++++++++++++++++
> .../testing/selftests/bpf/progs/test_d_path.c | 33 ++++++++++
> 2 files changed, 97 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/d_path.c b/tools/testing/selftests/bpf/prog_tests/d_path.c
> index ccc768592e66..2909ca3bae0f 100644
> --- a/tools/testing/selftests/bpf/prog_tests/d_path.c
> +++ b/tools/testing/selftests/bpf/prog_tests/d_path.c
> @@ -195,6 +195,67 @@ static void test_d_path_check_types(void)
> test_d_path_check_types__destroy(skel);
> }
>
> +static void test_d_path_lsm(void)
> +{
> + struct test_d_path *skel;
> + int err;
> + int pipefd[2];
> + pid_t pid;
> +
> + skel = test_d_path__open_and_load();
> + if (!ASSERT_OK_PTR(skel, "d_path skeleton failed"))
> + return;
> +
> + err = test_d_path__attach(skel);
> + if (!ASSERT_OK(err, "attach failed"))
> + goto cleanup;
> +
> + /* Prepare the test binary */
> + system("cp /bin/true /tmp/bpf_d_path_test 2>/dev/null || :");
I'd much prefer if we also cleaned up after ourselves, but it's not
that much of an issue I guess.
> + if (!ASSERT_OK(pipe(pipefd), "pipe failed"))
> + goto cleanup;
> +
> + pid = fork();
> + if (!ASSERT_GE(pid, 0, "fork failed")) {
> + close(pipefd[0]);
> + close(pipefd[1]);
> + goto cleanup;
> + }
> +
> + if (pid == 0) {
> + /* Child */
> + char buf;
> +
> + close(pipefd[1]);
> + /* Wait for parent to set PID in BPF map */
> + if (read(pipefd[0], &buf, 1) != 1)
> + exit(1);
> + close(pipefd[0]);
> + execl("/tmp/bpf_d_path_test", "/tmp/bpf_d_path_test", NULL);
> + exit(1);
> + }
> +
> + /* Parent */
> + close(pipefd[0]);
> +
> + /* Update BPF map with child PID */
> + skel->bss->my_pid = pid;
> +
> + /* Signal child to proceed */
> + write(pipefd[1], "G", 1);
> + close(pipefd[1]);
> +
> + /* Wait for child */
> + waitpid(pid, NULL, 0);
> +
> + ASSERT_EQ(skel->bss->called_lsm, 1, "lsm hook called");
> + ASSERT_EQ(skel->bss->lsm_match, 1, "lsm match");
> +
> +cleanup:
> + test_d_path__destroy(skel);
> +}
> +
> void test_d_path(void)
> {
> if (test__start_subtest("basic"))
> @@ -205,4 +266,7 @@ void test_d_path(void)
>
> if (test__start_subtest("check_alloc_mem"))
> test_d_path_check_types();
> +
> + if (test__start_subtest("lsm"))
> + test_d_path_lsm();
> }
> diff --git a/tools/testing/selftests/bpf/progs/test_d_path.c b/tools/testing/selftests/bpf/progs/test_d_path.c
> index 84e1f883f97b..7f65c282069a 100644
> --- a/tools/testing/selftests/bpf/progs/test_d_path.c
> +++ b/tools/testing/selftests/bpf/progs/test_d_path.c
> @@ -17,6 +17,8 @@ int rets_close[MAX_FILES] = {};
>
> int called_stat = 0;
> int called_close = 0;
> +int called_lsm = 0;
> +int lsm_match = 0;
>
> SEC("fentry/security_inode_getattr")
> int BPF_PROG(prog_stat, struct path *path, struct kstat *stat,
> @@ -62,4 +64,35 @@ int BPF_PROG(prog_close, struct file *file, void *id)
> return 0;
> }
>
> +SEC("lsm/bprm_check_security")
> +int BPF_PROG(prog_lsm, struct linux_binprm *bprm)
> +{
> + pid_t pid = bpf_get_current_pid_tgid() >> 32;
> + char path[MAX_PATH_LEN] = {};
> + int ret;
> +
> + if (pid != my_pid)
> + return 0;
> +
> + called_lsm = 1;
> + ret = bpf_d_path(&bprm->file->f_path, path, MAX_PATH_LEN);
> + if (ret < 0)
> + return 0;
> +
> + {
> + static const char target_dir[] = "/tmp/";
> +
> +#pragma unroll
> + for (int i = 0; i < sizeof(target_dir) - 1; i++) {
> + if (path[i] != target_dir[i]) {
> + lsm_match = -1; /* mismatch */
> + return 0;
> + }
> + }
> + }
> +
> + lsm_match = 1; /* prefix match */
> + return 0;
> +}
> +
> char _license[] SEC("license") = "GPL";
> --
> 2.52.0
>
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