selftests/bpf: Implement BPF programs for kernel socket operations

This patch lays out a set of SYSCALL programs that can be used to invoke
the socket operation kfuncs in bpf_testmod, allowing a test program to
manipulate kernel socket operations from userspace.
Signed-off-by: Jordan Rife <jrife@google.com>
Link: https://lore.kernel.org/r/20240429214529.2644801-4-jrife@google.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Google LLC */
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "../bpf_testmod/bpf_testmod_kfunc.h"
+
+SEC("syscall")
+int init_sock(struct init_sock_args *args)
+{
+	bpf_kfunc_init_sock(args);
+
+	return 0;
+}
+
+SEC("syscall")
+int close_sock(void *ctx)
+{
+	bpf_kfunc_close_sock();
+
+	return 0;
+}
+
+SEC("syscall")
+int kernel_connect(struct addr_args *args)
+{
+	return bpf_kfunc_call_kernel_connect(args);
+}
+
+SEC("syscall")
+int kernel_bind(struct addr_args *args)
+{
+	return bpf_kfunc_call_kernel_bind(args);
+}
+
+SEC("syscall")
+int kernel_listen(struct addr_args *args)
+{
+	return bpf_kfunc_call_kernel_listen();
+}
+
+SEC("syscall")
+int kernel_sendmsg(struct sendmsg_args *args)
+{
+	return bpf_kfunc_call_kernel_sendmsg(args);
+}
+
+SEC("syscall")
+int sock_sendmsg(struct sendmsg_args *args)
+{
+	return bpf_kfunc_call_sock_sendmsg(args);
+}
+
+SEC("syscall")
+int kernel_getsockname(struct addr_args *args)
+{
+	return bpf_kfunc_call_kernel_getsockname(args);
+}
+
+SEC("syscall")
+int kernel_getpeername(struct addr_args *args)
+{
+	return bpf_kfunc_call_kernel_getpeername(args);
+}
+
+char _license[] SEC("license") = "GPL";