selftests/bpf: Test freplace from user namespace

Add selftests to verify that it is possible to load freplace program
from user namespace if BPF token is initialized by bpf_object__prepare
before calling bpf_program__set_attach_target.
Negative test is added as well.

Modified type of the priv_prog to xdp, as kprobe did not work on aarch64
and s390x.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20250317174039.161275-5-mykyta.yatsenko5@gmail.com

Author: mykyta5

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

@@ -19,6 +19,7 @@
 #include "priv_prog.skel.h"
 #include "dummy_st_ops_success.skel.h"
 #include "token_lsm.skel.h"
+#include "priv_freplace_prog.skel.h"
 
 static inline int sys_mount(const char *dev_name, const char *dir_name,
 			    const char *type, unsigned long flags,
@@ -788,6 +789,84 @@ static int userns_obj_priv_prog(int mnt_fd, struct token_lsm *lsm_skel)
 	return 0;
 }
 
+static int userns_obj_priv_freplace_setup(int mnt_fd, struct priv_freplace_prog **fr_skel,
+					  struct priv_prog **skel, int *tgt_fd)
+{
+	LIBBPF_OPTS(bpf_object_open_opts, opts);
+	int err;
+	char buf[256];
+
+	/* use bpf_token_path to provide BPF FS path */
+	snprintf(buf, sizeof(buf), "/proc/self/fd/%d", mnt_fd);
+	opts.bpf_token_path = buf;
+	*skel = priv_prog__open_opts(&opts);
+	if (!ASSERT_OK_PTR(*skel, "priv_prog__open_opts"))
+		return -EINVAL;
+	err = priv_prog__load(*skel);
+	if (!ASSERT_OK(err, "priv_prog__load"))
+		return -EINVAL;
+
+	*fr_skel = priv_freplace_prog__open_opts(&opts);
+	if (!ASSERT_OK_PTR(*skel, "priv_freplace_prog__open_opts"))
+		return -EINVAL;
+
+	*tgt_fd = bpf_program__fd((*skel)->progs.xdp_prog1);
+	return 0;
+}
+
+/* Verify that freplace works from user namespace, because bpf token is loaded
+ * in bpf_object__prepare
+ */
+static int userns_obj_priv_freplace_prog(int mnt_fd, struct token_lsm *lsm_skel)
+{
+	struct priv_freplace_prog *fr_skel = NULL;
+	struct priv_prog *skel = NULL;
+	int err, tgt_fd;
+
+	err = userns_obj_priv_freplace_setup(mnt_fd, &fr_skel, &skel, &tgt_fd);
+	if (!ASSERT_OK(err, "setup"))
+		goto out;
+
+	err = bpf_object__prepare(fr_skel->obj);
+	if (!ASSERT_OK(err, "freplace__prepare"))
+		goto out;
+
+	err = bpf_program__set_attach_target(fr_skel->progs.new_xdp_prog2, tgt_fd, "xdp_prog1");
+	if (!ASSERT_OK(err, "set_attach_target"))
+		goto out;
+
+	err = priv_freplace_prog__load(fr_skel);
+	ASSERT_OK(err, "priv_freplace_prog__load");
+
+out:
+	priv_freplace_prog__destroy(fr_skel);
+	priv_prog__destroy(skel);
+	return err;
+}
+
+/* Verify that replace fails to set attach target from user namespace without bpf token */
+static int userns_obj_priv_freplace_prog_fail(int mnt_fd, struct token_lsm *lsm_skel)
+{
+	struct priv_freplace_prog *fr_skel = NULL;
+	struct priv_prog *skel = NULL;
+	int err, tgt_fd;
+
+	err = userns_obj_priv_freplace_setup(mnt_fd, &fr_skel, &skel, &tgt_fd);
+	if (!ASSERT_OK(err, "setup"))
+		goto out;
+
+	err = bpf_program__set_attach_target(fr_skel->progs.new_xdp_prog2, tgt_fd, "xdp_prog1");
+	if (ASSERT_ERR(err, "attach fails"))
+		err = 0;
+	else
+		err = -EINVAL;
+
+out:
+	priv_freplace_prog__destroy(fr_skel);
+	priv_prog__destroy(skel);
+	return err;
+}
+
 /* this test is called with BPF FS that doesn't delegate BPF_BTF_LOAD command,
  * which should cause struct_ops application to fail, as BTF won't be uploaded
  * into the kernel, even if STRUCT_OPS programs themselves are allowed
@@ -1004,12 +1083,28 @@ void test_token(void)
 	if (test__start_subtest("obj_priv_prog")) {
 		struct bpffs_opts opts = {
 			.cmds = bit(BPF_PROG_LOAD),
-			.progs = bit(BPF_PROG_TYPE_KPROBE),
+			.progs = bit(BPF_PROG_TYPE_XDP),
 			.attachs = ~0ULL,
 		};
 
 		subtest_userns(&opts, userns_obj_priv_prog);
 	}
+	if (test__start_subtest("obj_priv_freplace_prog")) {
+		struct bpffs_opts opts = {
+			.cmds = bit(BPF_BTF_LOAD) | bit(BPF_PROG_LOAD) | bit(BPF_BTF_GET_FD_BY_ID),
+			.progs = bit(BPF_PROG_TYPE_EXT) | bit(BPF_PROG_TYPE_XDP),
+			.attachs = ~0ULL,
+		};
+		subtest_userns(&opts, userns_obj_priv_freplace_prog);
+	}
+	if (test__start_subtest("obj_priv_freplace_prog_fail")) {
+		struct bpffs_opts opts = {
+			.cmds = bit(BPF_BTF_LOAD) | bit(BPF_PROG_LOAD) | bit(BPF_BTF_GET_FD_BY_ID),
+			.progs = bit(BPF_PROG_TYPE_EXT) | bit(BPF_PROG_TYPE_XDP),
+			.attachs = ~0ULL,
+		};
+		subtest_userns(&opts, userns_obj_priv_freplace_prog_fail);
+	}
 	if (test__start_subtest("obj_priv_btf_fail")) {
 		struct bpffs_opts opts = {
 			/* disallow BTF loading */

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

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+SEC("freplace/xdp_prog1")
+int new_xdp_prog2(struct xdp_md *xd)
+{
+	return XDP_DROP;
+}

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

@@ -6,8 +6,8 @@
 
 char _license[] SEC("license") = "GPL";
 
-SEC("kprobe")
-int kprobe_prog(void *ctx)
+SEC("xdp")
+int xdp_prog1(struct xdp_md *xdp)
 {
-	return 1;
+	return XDP_DROP;
 }