selftests/bpf: Add selftests for set_mempolicy with a lsm prog

In the straightforward LSM prog, it denies the use of mbind(2) with the
mode MPOL_BIND and permits other modes.

Consequently:

- Absent the LSM prog, mbind(2) should invariably succeed regardless of
  the mode
    #263/1   set_mempolicy/MPOL_BIND_without_lsm:OK
    #263/2   set_mempolicy/MPOL_DEFAULT_without_lsm:OK

- With the LSM prog
  - mbind(2) with the mode MPOL_BIND should result in failure
    #263/3   set_mempolicy/MPOL_BIND_with_lsm:OK

  - mbind(2) with the mode MPOL_DEFAULT should succeed
    #263/4   set_mempolicy/MPOL_DEFAULT_with_lsm:OK

- Summary
  #263     set_mempolicy:OK
  Summary: 1/4 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>

Author: laoar

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

@@ -0,0 +1,84 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2023 Yafang Shao <laoar.shao@gmail.com> */
+
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/mman.h>
+#include <linux/mempolicy.h>
+#include <test_progs.h>
+#include "test_set_mempolicy.skel.h"
+
+#define SIZE 4096
+
+static void mempolicy_bind(bool success)
+{
+	unsigned long mask = 1;
+	char *addr;
+	int err;
+
+	addr = mmap(NULL, SIZE, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+	if (!ASSERT_OK_PTR(addr, "mmap"))
+		return;
+
+	/* -lnuma is required by mbind(2), so use __NR_mbind to avoid the dependency. */
+	err = syscall(__NR_mbind, addr, SIZE, MPOL_BIND, &mask, sizeof(mask), 0);
+	if (success)
+		ASSERT_OK(err, "mbind_success");
+	else
+		ASSERT_ERR(err, "mbind_fail");
+
+	munmap(addr, SIZE);
+}
+
+static void mempolicy_default(void)
+{
+	char *addr;
+	int err;
+
+	addr = mmap(NULL, SIZE, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+	if (!ASSERT_OK_PTR(addr, "mmap"))
+		return;
+
+	err = syscall(__NR_mbind, addr, SIZE, MPOL_DEFAULT, NULL, 0, 0);
+	ASSERT_OK(err, "mbind_success");
+
+	munmap(addr, SIZE);
+}
+
+void test_set_mempolicy(void)
+{
+	struct test_set_mempolicy *skel;
+	int err;
+
+	skel = test_set_mempolicy__open();
+	if (!ASSERT_OK_PTR(skel, "open"))
+		return;
+
+	skel->bss->target_pid = getpid();
+
+	err = test_set_mempolicy__load(skel);
+	if (!ASSERT_OK(err, "load"))
+		goto destroy;
+
+	/* Without LSM, mbind(2) should succeed regardless of the mode. */
+	if (test__start_subtest("MPOL_BIND_without_lsm"))
+		mempolicy_bind(true);
+	if (test__start_subtest("MPOL_DEFAULT_without_lsm"))
+		mempolicy_default();
+
+	/* Attach LSM prog, in which it will deny MPOL_BIND */
+	err = test_set_mempolicy__attach(skel);
+	if (!ASSERT_OK(err, "attach"))
+		goto destroy;
+
+	/* MPOL_BIND should fail. */
+	if (test__start_subtest("MPOL_BIND_with_lsm"))
+		mempolicy_bind(false);
+
+	/* MPOL_DEFAULT should succeed. */
+	if (test__start_subtest("MPOL_DEFAULT_with_lsm"))
+		mempolicy_default();
+
+destroy:
+	test_set_mempolicy__destroy(skel);
+}

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

@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2023 Yafang Shao <laoar.shao@gmail.com> */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+int target_pid;
+
+static int mem_policy_adjustment(u64 mode)
+{
+	struct task_struct *task = bpf_get_current_task_btf();
+
+	if (task->pid != target_pid)
+		return 0;
+
+	if (mode != MPOL_BIND)
+		return 0;
+	return -1;
+}
+
+SEC("lsm/set_mempolicy")
+int BPF_PROG(setmempolicy, u64 mode, u16 mode_flags, nodemask_t *nmask, u32 flags)
+{
+	return mem_policy_adjustment(mode);
+}
+
+char _license[] SEC("license") = "GPL";