add userspace library skeleton

Signed-off-by: Mohammad Shehar Yaar Tausif <[email protected]>

README.md

@@ -1203,6 +1203,18 @@ netfilter hooks to Lua.
 * `"LOCAL_OUT"`: `NF_INET_LOCAL_OUT`. The packet is generated by the local system.
 * `"POST_ROUTING"`: `NF_INET_POST_ROUTING`. The packet is about to be sent out.
 
+### xtable userspace library (libxtable)
+
+The `libxtable` [userspace library](usr/lib/xtable) provides support for generating userspace code for [xtable extensions](https://inai.de/projects/xtables-addons/). The user can modify the generated lua code to implement the userspace handlers for the corresponding xtable extension.
+
+To generate the library, the following steps are required:
+
+1. Go to `usr/lib/xtable` and create a `libxt_<ext_name>.lua` file.
+2. Register your callbacks for the xtable extension by importing the library (`libxtable`) in the created file.
+3. Run `LUAXTABLE_MODULE=<ext_name> make` to build the extension and `LUAXTABLE_MODULE=<ext_name> make install` (as root) to install the userspace plugin to the system.
+
+Now load the extension normally using `iptables`.
+
 # Examples
 
 ### spyglass

usr/lib/xtable/Makefile

@@ -0,0 +1,27 @@
+# SPDX-FileCopyrightText: (c) 2024 Mohammad Shehar Yaar Tausif <[email protected]>
+# SPDX-License-Identifier: MIT OR GPL-2.0-only
+
+LUAXTABLE_MODULE ?= test
+
+CFLAGS = -DLUAXTABLE_MODULE=\"${LUAXTABLE_MODULE}\" -O2 -Wall -I../../../lib
+XTABLES_SO_DIR = $(shell pkg-config xtables --variable xtlibdir)
+LUA_FLAGS = -llua -lm
+
+all:
+	make libxt_${LUAXTABLE_MODULE}.so
+
+install:
+	sudo cp libxt_${LUAXTABLE_MODULE}.so  ${XTABLES_SO_DIR}
+
+uninstall:
+	sudo rm -f ${XTABLES_SO_DIR}/libxt_${LUAXTABLE_MODULE}.so
+
+clean:
+	rm -f libxt_*.so libxt_*.o
+
+lib%.so: lib%.o
+	gcc -shared -fPIC -o libxt_${LUAXTABLE_MODULE}.so libxt_${LUAXTABLE_MODULE}.o ${LUA_FLAGS};
+
+lib%.o: libxtable.c
+	gcc ${CFLAGS} ${LUA_FLAGS} -D_INIT=lib$*_init -fPIC -c -o libxt_${LUAXTABLE_MODULE}.o $<;
+

usr/lib/xtable/libxtable.c

@@ -0,0 +1,183 @@
+/*
+* SPDX-FileCopyrightText: (c) 2024 Mohammad Shehar Yaar Tausif <[email protected]>
+* SPDX-License-Identifier: MIT OR GPL-2.0-only
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <lauxlib.h>
+#include <lua.h>
+#include <lualib.h>
+
+#include <xtables.h>
+
+#include "luaxtable.h"
+
+#ifndef LUAXTABLE_MODULE
+#error "LUAXTABLE_MODULE is not defined"
+#endif
+
+static lua_State *L = NULL;
+const char *libxtable_match_key = "libxtable_match";
+const char *libxtable_target_key = "libxtable_target";
+
+int luaopen_libxtable(lua_State *L);
+
+static inline int libxtable_run(lua_State *L, const char *func_name, const char *key, int nargs, int nresults)
+{
+	int ret = 0;
+	int base = lua_gettop(L) - nargs;
+	lua_rawgetp(L, LUA_REGISTRYINDEX, key);
+
+	if (lua_getfield(L, -1, func_name) != LUA_TFUNCTION) {
+		fprintf(stderr, "Function %s not found\n", func_name);
+		goto restore;
+	}
+
+	lua_insert(L, base + 1); /* func */
+	lua_pop(L, 1); /* table */
+
+	if (lua_pcall(L, nargs, nresults, 0) != LUA_OK) {
+		fprintf(stderr, "Failed to call Lua function %s: %s\n", func_name,
+				lua_tostring(L, -1));
+		goto restore;
+	}
+
+	if (nresults == 1)
+		ret = lua_toboolean(L, -1);
+
+restore:
+	lua_settop(L, base);
+	return ret;
+}
+
+// stubs for xtables functions
+#define LIBXTABLE_HELPER_CB(hook)			   	\
+static void libxtable_##hook##_help(void)		   	\
+{							   	\
+	libxtable_run(L, "help", libxtable_##hook##_key, 0, 0);	\
+}
+
+#define LIBXTABLE_INITER_CB(hook)				   	\
+static void libxtable_##hook##_init(struct xt_entry_##hook *hook)   	\
+{								   	\
+	libxtable_run(L, "init", libxtable_##hook##_key, 1, 0);		\
+}
+
+#define LIBXTABLE_PARSER_CB(hook)			   		\
+static int libxtable_##hook##_parse(int c, char **argv, int invert, unsigned int *flags,	\
+					const void *entry, struct xt_entry_##hook **hook)	\
+{							   		\
+	return libxtable_run(L, "parse", libxtable_##hook##_key, 0, 1);	\
+}
+
+#define LIBXTABLE__FINALCHECKER_CB(hook)				\
+static void libxtable_##hook##_finalcheck(unsigned int flags)		\
+{							  		\
+	libxtable_run(L, "final_check", libxtable_##hook##_key, 0, 0);	\
+}
+
+#define LIBXTABLE_PRINTER_CB(hook)  					\
+static void libxtable_##hook##_print(const void *entry, const struct xt_entry_##hook *hook, int numeric)	\
+{							   		\
+	libxtable_run(L, "print", libxtable_##hook##_key, 0, 0);	\
+}
+
+#define LIBXTABLE_SAVER_CB(hook)				\
+static void libxtable_##hook##_save(const void *entry, const struct xt_entry_##hook *hook)  \
+{								\
+	libxtable_run(L, "save", libxtable_##hook##_key, 0, 0);	\
+}
+
+#define LIBXTABLE_NEWCB(hook)  		\
+	LIBXTABLE_HELPER_CB(hook)   	\
+	LIBXTABLE_INITER_CB(hook)   	\
+	LIBXTABLE_PARSER_CB(hook)   	\
+	LIBXTABLE__FINALCHECKER_CB(hook)	\
+	LIBXTABLE_PRINTER_CB(hook)  	\
+	LIBXTABLE_SAVER_CB(hook)
+
+LIBXTABLE_NEWCB(match);
+LIBXTABLE_NEWCB(target);
+
+
+#define LIBXTABLE_NEWREG(hook)					\
+static struct xtables_##hook libxtable_##hook##_reg = {		\
+	.version = XTABLES_VERSION,				\
+	.name = LUAXTABLE_MODULE,					\
+	.size = XT_ALIGN(sizeof(luaxtable_info_t)),		\
+	.userspacesize = 0,					\
+	.help = libxtable_##hook##_help,			\
+	.init = libxtable_##hook##_init,			\
+	.parse = libxtable_##hook##_parse,		 	\
+	.final_check = libxtable_##hook##_finalcheck,	  	\
+	.print = libxtable_##hook##_print,		  	\
+	.save = libxtable_##hook##_save			 	\
+}
+
+LIBXTABLE_NEWREG(match);
+LIBXTABLE_NEWREG(target);
+
+static inline int libxtable_checkint(lua_State *L, int idx, const char *key)
+{
+	if (lua_getfield(L, idx, key) != LUA_TNUMBER) {
+		fprintf(stderr, "invalid \'%s\' in match\n", key);
+		return 0;
+	}
+	int ret = lua_tointeger(L, -1);
+	lua_pop(L, 1);
+	return ret;
+}
+
+#define LIBXTABLE_LIB_CB(hook)				\
+static int libxtable_##hook##_lib(lua_State *L)		\
+{							\
+	luaL_checktype(L, 1, LUA_TTABLE);		\
+							\
+	libxtable_##hook##_reg.revision = libxtable_checkint(L, 1, "revision");		\
+	libxtable_##hook##_reg.family = libxtable_checkint(L, 1, "family");		\
+	xtables_register_##hook(&libxtable_##hook##_reg);		\
+	lua_pushvalue(L, 1);						\
+	lua_rawsetp(L, LUA_REGISTRYINDEX, libxtable_##hook##_key);	\
+	return 0;							\
+}
+
+LIBXTABLE_LIB_CB(match);
+LIBXTABLE_LIB_CB(target);
+
+static const luaL_Reg libxtable_lib[] = {
+	{"match", libxtable_match_lib},
+	{"target", libxtable_target_lib},
+	{NULL, NULL}
+};
+
+int luaopen_libxtable(lua_State *L)
+{
+	luaL_newlib(L, libxtable_lib);
+	return 1;
+}
+
+static int __attribute__((constructor)) _init(void)
+{
+	if ((L = luaL_newstate()) == NULL) {
+		fprintf(stderr, "Failed to create Lua state\n");
+		return -1;
+	}
+	luaL_openlibs(L);
+	luaL_requiref(L, "libxtable", luaopen_libxtable, 1);
+
+	if (luaL_dofile(L, "libxt_"LUAXTABLE_MODULE".lua") != LUA_OK) {
+		fprintf(stderr, "Failed to load Lua script: %s\n", lua_tostring(L, -1));
+		return -1;
+	}
+
+	return 0;
+}
+
+static void __attribute__((destructor)) _fini(void)
+{
+	if (L)
+		lua_close(L);
+}