selftests/bpf: Add test for bpf_sock_read_xattr() kfunc

Add a selftest that loads the kfunc in sleepable and non-sleepable
lsm/socket_connect programs and checks that a value set via fsetxattr()
on a socket is read back.

Link: https://patch.msgid.link/20260617-work-bpf-sock-xattr-v1-2-a1276f7c9da3@kernel.org
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

tools/testing/selftests/bpf/bpf_experimental.h

@@ -364,6 +364,9 @@ extern void bpf_iter_dmabuf_destroy(struct bpf_iter_dmabuf *it) __weak __ksym;
 extern int bpf_cgroup_read_xattr(struct cgroup *cgroup, const char *name__str,
 				 struct bpf_dynptr *value_p) __weak __ksym;
 
+extern int bpf_sock_read_xattr(struct socket *sock, const char *name__str,
+			       struct bpf_dynptr *value_p) __weak __ksym;
+
 #define PREEMPT_BITS	8
 #define SOFTIRQ_BITS	8
 #define HARDIRQ_BITS	4

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

@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2026 Christian Brauner */
+
+#include <errno.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/xattr.h>
+#include <sys/socket.h>
+#include <netinet/in.h>
+#include <test_progs.h>
+
+#include "sock_read_xattr.skel.h"
+
+static const char xattr_value[] = "bpf_sock_value";
+static const char xattr_name[] = "user.bpf_test";
+
+static void test_read_sock_xattr(void)
+{
+	struct sockaddr_in addr = {};
+	struct sock_read_xattr *skel = NULL;
+	struct bpf_link *link = NULL;
+	int sock_fd = -1, err;
+
+	sock_fd = socket(AF_INET, SOCK_STREAM, 0);
+	if (!ASSERT_OK_FD(sock_fd, "socket"))
+		return;
+
+	err = fsetxattr(sock_fd, xattr_name, xattr_value, sizeof(xattr_value), 0);
+	if (!ASSERT_OK(err, "fsetxattr"))
+		goto out;
+
+	skel = sock_read_xattr__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "sock_read_xattr__open_and_load"))
+		goto out;
+
+	skel->bss->monitored_pid = sys_gettid();
+
+	/* Only attach the functional program; the verifier-only programs
+	 * above are not pid-gated and would clobber the shared globals.
+	 */
+	link = bpf_program__attach(skel->progs.read_sock_xattr);
+	if (!ASSERT_OK_PTR(link, "attach read_sock_xattr"))
+		goto out;
+
+	addr.sin_family = AF_INET;
+	addr.sin_port = htons(1234);
+	addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+	/* Only the lsm/socket_connect hook matters; the connect may fail. */
+	connect(sock_fd, (struct sockaddr *)&addr, sizeof(addr));
+
+	ASSERT_EQ(skel->data->read_ret, sizeof(xattr_value), "read_ret");
+	ASSERT_STREQ(skel->bss->value, xattr_value, "value");
+
+out:
+	bpf_link__destroy(link);
+	if (sock_fd >= 0)
+		close(sock_fd);
+	sock_read_xattr__destroy(skel);
+}
+
+void test_sock_xattr(void)
+{
+	RUN_TESTS(sock_read_xattr);
+
+	if (test__start_subtest("read_sock_xattr"))
+		test_read_sock_xattr();
+}

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

@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Christian Brauner */
+
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_core_read.h>
+#include "bpf_experimental.h"
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+char value[16];
+int read_ret = -1;
+__u32 monitored_pid = 0;
+
+static __always_inline void read_xattr(struct socket *sock)
+{
+	struct bpf_dynptr value_ptr;
+
+	bpf_dynptr_from_mem(value, sizeof(value), 0, &value_ptr);
+	bpf_sock_read_xattr(sock, "user.bpf_test", &value_ptr);
+}
+
+SEC("lsm.s/socket_connect")
+__success
+int BPF_PROG(trusted_sock_ptr_sleepable, struct socket *sock)
+{
+	read_xattr(sock);
+	return 0;
+}
+
+SEC("lsm/socket_connect")
+__success
+int BPF_PROG(trusted_sock_ptr_non_sleepable, struct socket *sock)
+{
+	read_xattr(sock);
+	return 0;
+}
+
+SEC("lsm.s/socket_connect")
+__success
+int BPF_PROG(read_sock_xattr, struct socket *sock)
+{
+	struct bpf_dynptr value_ptr;
+	__u32 pid = bpf_get_current_pid_tgid() >> 32;
+
+	if (pid != monitored_pid)
+		return 0;
+
+	bpf_dynptr_from_mem(value, sizeof(value), 0, &value_ptr);
+	read_ret = bpf_sock_read_xattr(sock, "user.bpf_test", &value_ptr);
+	return 0;
+}