Commit 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking") started distinguishing read vs write accesses performed by helpers.

The second argument of bpf_d_path() is a pointer to a buffer that the helper fills with the resulting path. However, its prototype currently uses ARG_PTR_TO_MEM without MEM_WRITE.

Before 37cce22dbd51, helper accesses were conservatively treated as potential writes, so this mismatch did not cause issues. Since that commit, the verifier may incorrectly assume that the buffer contents are unchanged across the helper call and base its optimizations on this wrong assumption. This can lead to misbehaviour in BPF programs that read back the buffer, such as prefix comparisons on the returned path.

Fix this by marking the second argument of bpf_d_path() as ARG_PTR_TO_MEM | MEM_WRITE so that the verifier correctly models the write to the caller-provided buffer.

Fixes: 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking") Co-developed-by: Zesen Liu ftyg@live.com Signed-off-by: Zesen Liu ftyg@live.com Co-developed-by: Peili Gao gplhust955@gmail.com Signed-off-by: Peili Gao gplhust955@gmail.com Co-developed-by: Haoran Ni haoran.ni.cs@gmail.com Signed-off-by: Haoran Ni haoran.ni.cs@gmail.com Signed-off-by: Shuran Liu electronlsr@gmail.com Reviewed-by: Matt Bobrowski mattbobrowski@google.com --- kernel/trace/bpf_trace.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c index 4f87c16d915a..49e0bdaa7a1b 100644 --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c @@ -965,7 +965,7 @@ static const struct bpf_func_proto bpf_d_path_proto = { .ret_type = RET_INTEGER, .arg1_type = ARG_PTR_TO_BTF_ID, .arg1_btf_id = &bpf_d_path_btf_ids[0], - .arg2_type = ARG_PTR_TO_MEM, + .arg2_type = ARG_PTR_TO_MEM | MEM_WRITE, .arg3_type = ARG_CONST_SIZE_OR_ZERO, .allowed = bpf_d_path_allowed, };

On Tue, Dec 2, 2025 at 6:20 AM Shuran Liu electronlsr@gmail.com 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.

I don't get why we add this selftest here. It doesn't appear to be related to patch 1/2.

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.

The paragraph above is not really necessary. Just curious, did some AI write it?

Per review feedback, the new LSM coverage is merged into the existing d_path selftest rather than adding new prog_tests/ or progs/ files. The loop that checks the pathname prefix now uses bpf_for(), which is a verifier-friendly way to express a small, fixed-iteration loop, and the temporary /tmp/bpf_d_path_test binary is removed in the test cleanup path.

Co-developed-by: Zesen Liu ftyg@live.com Signed-off-by: Zesen Liu ftyg@live.com Co-developed-by: Peili Gao gplhust955@gmail.com Signed-off-by: Peili Gao gplhust955@gmail.com Co-developed-by: Haoran Ni haoran.ni.cs@gmail.com Signed-off-by: Haoran Ni haoran.ni.cs@gmail.com Signed-off-by: Shuran Liu electronlsr@gmail.com Reviewed-by: Matt Bobrowski mattbobrowski@google.com

---

.../testing/selftests/bpf/prog_tests/d_path.c | 65 +++++++++++++++++++ .../testing/selftests/bpf/progs/test_d_path.c | 33 ++++++++++ 2 files changed, 98 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..202b44e6f482 100644 --- a/tools/testing/selftests/bpf/prog_tests/d_path.c +++ b/tools/testing/selftests/bpf/prog_tests/d_path.c @@ -195,6 +195,68 @@ 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 || :");
  

•   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:

•   unlink("/tmp/bpf_d_path_test");
  

•   test_d_path__destroy(skel);
  

+}

void test_d_path(void) { if (test__start_subtest("basic")) @@ -205,4 +267,7 @@ 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..9ae36eabcd07 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;
  

•   {
  

This {} block is not necessary.

•           static const char target_dir[] = "/tmp/";
  

•           int i;
  

•           bpf_for(i, 0, sizeof(target_dir) - 1) {
  

•                   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