本地库覆盖glibc的getifaddrs

基本原理：（原理类似libSegFault.so这个库）

使用linux的环境变量“LD_PRELOAD”来提前载入一个库，然后这个库里面的同名API就可以覆盖后续的共享库的API了(glibc里面的若链接的API是可以被覆盖的)。如我们在自己的共享库中重写一个API

int getifaddrs (struct ifaddrs **ifap)；

主要修改代码点：

1. 在一个新的共享库里重写/覆盖getifaddrs(), 生产一个共享库libovfuns.so，把它拷贝到/opt/lib里面。主要修改点事去掉原有glibc里__netlink_assert_response(...)的abort处理，同时是在系统的文件里面记录一条异常log：（/opt/log/overwirte_fun.log）

   static void __netlink_assert_response (int fd, ssize_t result)
   {if (result < 0){/* Check if the error is unexpected.  */bool terminate = false;int error_code = errno;int family = get_address_family (fd);if (family != AF_NETLINK)/* If the address family does not match (or getsocknamefailed), report the original error.  */terminate = true;else if (error_code == EBADF|| error_code == ENOTCONN|| error_code == ENOTSOCK|| error_code == ECONNREFUSED)/* These errors indicate that the descriptor is not aconnected socket.  */terminate = true;else if (error_code == EAGAIN || error_code == EWOULDBLOCK){/* The kernel might return EAGAIN for other reasons than anon-blocking socket.  But if the socket is not blocking,it is not ours, so report the error.  */int mode = fcntl (fd, F_GETFL, 0);if (mode < 0 || (mode & O_NONBLOCK) != 0)terminate = true;}printf("[%s %d]\n", __func__, __LINE__);if (terminate){char message[200];if (family < 0)snprintf (message, sizeof (message),"Inovance overwrite： Unexpected error %d on netlink descriptor %d",error_code, fd);elsesnprintf (message, sizeof (message),"Inovance overwrite： Unexpected error %d on netlink descriptor %d"" (address family %d)",error_code, fd, family);//just use the log message replace the "__libc_fatal"//    __libc_fatal (message);overwrite_fun_log("getifaddrs", message);}else/* Restore orignal errno value.  */set_errno (error_code);}else if (result < sizeof (struct nlmsghdr)){char message[200];int family = get_address_family (fd);if (family < 0)snprintf (message, sizeof (message),"Inovance overwrite： Unexpected netlink response of size %zd"" on descriptor %d",result, fd);elsesnprintf (message, sizeof (message),"Inovance overwrite： Unexpected netlink response of size %zd"" on descriptor %d (address family %d)",result, fd, family);//just use the log message replace the "__libc_fatal"//   __libc_fatal (message);overwrite_fun_log("getifaddrs", message);}
   }
   

2. 修改test启动的服务脚本：在原有服务脚本里面增加如下行，用于设置环境变量Environment="LD_PRELOAD=/opt/runtime/libovfuns.so"

3. root@xxx:# cat /lib/systemd/system/test.service
   [Unit]
   Description=Inovance CoDeSys Control Progres
   Wants=inovance-daemon.service
   After=inovance-daemon.service[Service]
   Type=simple
   Environment="LD_PRELOAD=/opt/lib/libovfuns.so"
   PIDFile=/run/test.pid
   ExecStartPre=/bin/sleep 5
   ExecStart=/opt/xxx/test
   StandardOutput=null
   StandardError=null
   #Restart=always
   #RestartSec=1
   #StartLimitInterval=0[Install]
   WantedBy=multi-user.target

4. 下面是测试代码，编译时使用gcc -L/opt/runtime/ -I./lib -lovfuns -o test test.c

5. 生产test测试程序

    #define _GNU_SOURCE     /* To get defns of NI_MAXSERV and NI_MAXHOST */#include #include #include #include "my_ifaddrs.h"#include #include #include #include int main(int argc, char *argv[]){struct ifaddrs *ifaddr, *ifa;int family, s, n;char host[NI_MAXHOST];if (getifaddrs(&ifaddr) == -1) {perror("getifaddrs");exit(EXIT_FAILURE);}/* Walk through linked list, maintaining head pointer so wecan free list later */for (ifa = ifaddr, n = 0; ifa != NULL; ifa = ifa->ifa_next, n++) {if (ifa->ifa_addr == NULL)continue;family = ifa->ifa_addr->sa_family;/* Display interface name and family (including symbolicform of the latter for the common families) */printf("%-8s %s (%d)\n",ifa->ifa_name,(family == AF_PACKET) ? "AF_PACKET" :(family == AF_INET) ? "AF_INET" :(family == AF_INET6) ? "AF_INET6" : "???",family);/* For an AF_INET* interface address, display the address */if (family == AF_INET || family == AF_INET6) {s = getnameinfo(ifa->ifa_addr,(family == AF_INET) ? sizeof(struct sockaddr_in) :sizeof(struct sockaddr_in6),host, NI_MAXHOST,NULL, 0, NI_NUMERICHOST);if (s != 0) {printf("getnameinfo() failed: %s\n", gai_strerror(s));exit(EXIT_FAILURE);}printf("\t\taddress: <%s>\n", host);} else if (family == AF_PACKET && ifa->ifa_data != NULL) {struct rtnl_link_stats *stats = ifa->ifa_data;printf("\t\ttx_packets = %10u; rx_packets = %10u\n""\t\ttx_bytes   = %10u; rx_bytes   = %10u\n",stats->tx_packets, stats->rx_packets,stats->tx_bytes, stats->rx_bytes);}}freeifaddrs(ifaddr);exit(EXIT_SUCCESS);}
   

   测试添加和不添加环境量，程序使用的API是不同的，具体测试log如下所示：

   不添加环境变量，程序使用原始API

   root@xxx:# ./test lo AF_PACKET (17) tx_packets = 65; rx_packets = 65 tx_bytes = 3180; rx_bytes = 3180 enp1s0 AF_PACKET (17) tx_packets = 686; rx_packets = 3008 tx_bytes = 50762; rx_bytes = 192738 enp2s0 AF_PACKET (17) tx_packets = 118; rx_packets = 204 tx_bytes = 19366; rx_bytes = 22391 enp3s0 AF_PACKET (17) tx_packets = 0; rx_packets = 0 tx_bytes = 0; rx_bytes = 0 enp4s0 AF_PACKET (17) tx_packets = 0; rx_packets = 16 tx_bytes = 0; rx_bytes = 960 tunl0 AF_PACKET (17) tx_packets = 0; rx_packets = 0 tx_bytes = 0; rx_bytes = 0 tap0 AF_PACKET (17) tx_packets = 0; rx_packets = 0 tx_bytes = 0; rx_bytes = 0 tap1 AF_PACKET (17) tx_packets = 0; rx_packets = 0 tx_bytes = 0; rx_bytes = 0 lo AF_INET (2) address: <127.0.0.1> enp1s0 AF_INET (2) address: <10.61.64.75> enp2s0 AF_INET (2) address: <192.168.2.88>

   添加环境变量，程序使用新的API

   root@xxx:# env LD_PRELOAD=/opt/runtime/libovfuns.so ./test [__netlink_request 601] <------这个log是新的API里面的 [__netlink_request 604] [__netlink_request 601] [__netlink_request 604] [__netlink_request 601] [__netlink_request 604] [__netlink_request 601] [__netlink_request 604] [__netlink_request 601] [__netlink_request 604] lo AF_PACKET (17) tx_packets = 107; rx_packets = 107 tx_bytes = 5196; rx_bytes = 5196 enp1s0 AF_PACKET (17) tx_packets = 1138; rx_packets = 4968 tx_bytes = 83894; rx_bytes = 316171 enp2s0 AF_PACKET (17) tx_packets = 160; rx_packets = 251 tx_bytes = 23366; rx_bytes = 26263 enp3s0 AF_PACKET (17) tx_packets = 0; rx_packets = 0 tx_bytes = 0; rx_bytes = 0 enp4s0 AF_PACKET (17) tx_packets = 0; rx_packets = 24 tx_bytes = 0; rx_bytes = 1440 tunl0 AF_PACKET (17) tx_packets = 0; rx_packets = 0 tx_bytes = 0; rx_bytes = 0 tap0 AF_PACKET (17) tx_packets = 0; rx_packets = 0 tx_bytes = 0; rx_bytes = 0 tap1 AF_PACKET (17) tx_packets = 0; rx_packets = 0 tx_bytes = 0; rx_bytes = 0 lo AF_INET (2) address: <127.0.0.1> enp1s0 AF_INET (2) address: <10.61.64.75> enp2s0 AF_INET (2) address: <192.168.2.88>