selftests/bpf: add regression test for bpf_d_path()

Add a simple LSM BPF program and a corresponding test_progs test case
to exercise bpf_d_path() and ensure that prefix comparisons on the
returned path keep working.

The LSM program hooks bprm_check_security, calls bpf_d_path() on the
binary being executed, and compares the returned path against the
"/tmp/" prefix. The result is recorded in an array map.

The user space test runs /tmp/bpf_d_path_test (copied from /bin/true)
and checks that the BPF program records a successful prefix match.

Without the preceding fix to bpf_d_path()'s helper prototype, the
test can fail due to the verifier incorrectly assuming that the
buffer contents are unchanged across the helper call and misoptimizing
the program. With the fix applied, the test passes.

Co-developed-by: Zesen Liu <[email protected]>
Signed-off-by: Zesen Liu <[email protected]>
Co-developed-by: Peili Gao <[email protected]>
Signed-off-by: Peili Gao <[email protected]>
Co-developed-by: Haoran Ni <[email protected]>
Signed-off-by: Haoran Ni <[email protected]>
Signed-off-by: Shuran Liu <[email protected]>

Author: electronlsr

tools/testing/selftests/bpf/prog_tests/d_path_lsm.c

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <test_progs.h>
+#include "d_path_lsm.skel.h"
+
+void test_d_path_lsm(void)
+{
+	struct d_path_lsm *skel = NULL;
+	int err, map_fd, key = 0, val = 0;
+
+	skel = d_path_lsm__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	err = d_path_lsm__attach(skel);
+	if (!ASSERT_OK(err, "attach"))
+		goto out;
+
+	system("cp /bin/true /tmp/bpf_d_path_test 2>/dev/null || :");
+	system("/tmp/bpf_d_path_test >/dev/null 2>&1");
+
+	map_fd = bpf_map__fd(skel->maps.result);
+	err = bpf_map_lookup_elem(map_fd, &key, &val);
+	ASSERT_OK(err, "lookup_result");
+	ASSERT_EQ(val, 1, "prefix_match");
+out:
+	d_path_lsm__destroy(skel);
+}

tools/testing/selftests/bpf/progs/d_path_lsm.bpf.c

@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char LICENSE[] SEC("license") = "GPL";
+
+#define FILENAME_MAX_SIZE 256
+#define TARGET_DIR "/tmp/"
+#define TARGET_DIR_LEN 5
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, int);
+} result SEC(".maps");
+
+SEC("lsm/bprm_check_security")
+int BPF_PROG(d_path_lsm_prog, struct linux_binprm *bprm)
+{
+	char path[FILENAME_MAX_SIZE] = {};
+	long len;
+	int key = 0;
+	int val = 0;
+
+	len = bpf_d_path(&bprm->file->f_path, path, sizeof(path));
+	if (len < 0)
+		return 0;
+
+#pragma unroll
+	for (int i = 0; i < TARGET_DIR_LEN; i++) {
+		if ((u8)path[i] != (u8)TARGET_DIR[i]) {
+			val = -1; /* mismatch */
+			bpf_map_update_elem(&result, &key, &val, BPF_ANY);
+			return 0;
+		}
+	}
+
+	val = 1; /* prefix match */
+	bpf_map_update_elem(&result, &key, &val, BPF_ANY);
+	return 0;
+}