I'm trying to send a string to the kernel module from the user space and the kernel returns back the string reversed. I followed a simple example code and added strrev() function in the kernel code which is defined in the string.h header. But that gives me an error "fatal error: string.h: No such file or directory" and no other error. The user code is same as the sample code from the link. Can anybody explain why this is happening ?
User Code
#include <linux/netlink.h>#include <stdio.h>#include <stdlib.h>#include <string.h>#include <sys/socket.h>#include <unistd.h>#define NETLINK_USER 31#define MAX_PAYLOAD 1024 /* maximum payload size*/struct sockaddr_nl src_addr, dest_addr;struct nlmsghdr *nlh = NULL;struct iovec iov;int sock_fd;struct msghdr msg;int main(){ char str[20]; sock_fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_USER); if (sock_fd < 0) return -1; memset(&src_addr, 0, sizeof(src_addr)); src_addr.nl_family = AF_NETLINK; src_addr.nl_pid = getpid(); /* self pid */ bind(sock_fd, (struct sockaddr *)&src_addr, sizeof(src_addr)); memset(&dest_addr, 0, sizeof(dest_addr)); dest_addr.nl_family = AF_NETLINK; dest_addr.nl_pid = 0; /* For Linux Kernel */ dest_addr.nl_groups = 0; /* unicast */ nlh = (struct nlmsghdr *)malloc(NLMSG_SPACE(MAX_PAYLOAD)); memset(nlh, 0, NLMSG_SPACE(MAX_PAYLOAD)); nlh->nlmsg_len = NLMSG_SPACE(MAX_PAYLOAD); nlh->nlmsg_pid = getpid(); nlh->nlmsg_flags = 0; printf("Enter a string for reversal:- "); scanf("%s",str); strcpy(NLMSG_DATA(nlh),str); iov.iov_base = (void *)nlh; iov.iov_len = nlh->nlmsg_len; msg.msg_name = (void *)&dest_addr; msg.msg_namelen = sizeof(dest_addr); msg.msg_iov = &iov; msg.msg_iovlen = 1; printf("\nSending message to kernel\n"); sendmsg(sock_fd, &msg, 0); printf("Waiting for message from kernel\n"); /* Read message from kernel */ recvmsg(sock_fd, &msg, 0); printf("Received message payload: %s\n", NLMSG_DATA(nlh)); close(sock_fd); return 0;}Kernel Code
#include <linux/module.h>#include <net/sock.h> #include <linux/netlink.h>#include <linux/skbuff.h> #include<string.h>#define NETLINK_USER 31struct sock *nl_sk = NULL;static void hello_nl_recv_msg(struct sk_buff *skb){ struct nlmsghdr *nlh; int pid; struct sk_buff *skb_out; int msg_size; char msg[20]; int res; printk(KERN_INFO "Entering: %s\n", __FUNCTION__); nlh = (struct nlmsghdr *)skb->data; printk(KERN_INFO "Netlink received msg payload:%s\n", (char *)nlmsg_data(nlh)); pid = nlh->nlmsg_pid; /*pid of sending process */ strcpy(msg,strrev((char *)nlmsg_data(nlh))); msg_size = strlen(msg); skb_out = nlmsg_new(msg_size, 0); if (!skb_out) { printk(KERN_ERR "Failed to allocate new skb\n"); return; } nlh = nlmsg_put(skb_out, 0, 0, NLMSG_DONE, msg_size, 0); NETLINK_CB(skb_out).dst_group = 0; /* not in mcast group */ strncpy(nlmsg_data(nlh), msg, msg_size); res = nlmsg_unicast(nl_sk, skb_out, pid); if (res < 0) printk(KERN_INFO "Error while sending bak to user\n");}static int __init hello_init(void){ printk("Entering: %s\n", __FUNCTION__); //nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, 0, hello_nl_recv_msg, NULL, THIS_MODULE); struct netlink_kernel_cfg cfg = { .input = hello_nl_recv_msg, }; nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, &cfg); if (!nl_sk) { printk(KERN_ALERT "Error creating socket.\n"); return -10; } return 0;}static void __exit hello_exit(void){ printk(KERN_INFO "exiting hello module\n"); netlink_kernel_release(nl_sk);}module_init(hello_init); module_exit(hello_exit);MODULE_LICENSE("GPL");Terminal window
make -C /lib/modules/4.15.0-1096-oem/build/ M=/home/Username/Documents/Task_KtoUcom modulesmake[1]: Entering directory '/usr/src/linux-headers-4.15.0-1096-oem' CC [M] /home/Username/Documents/Task_KtoUcom/kernl.o/home/Username/Documents/Task_KtoUcom/kernl.c:5:9: fatal error: string.h: No such file or directory #include<string.h> ^~~~~~~~~~compilation terminated.scripts/Makefile.build:337: recipe for target '/home/Username/Documents/Task_KtoUcom/kernl.o' failedmake[2]: *** [/home/Username/Documents/Task_KtoUcom/kernl.o] Error 1Makefile:1585: recipe for target '_module_/home/Username/Documents/Task_KtoUcom' failedmake[1]: *** [_module_/home/Username/Documents/Task_KtoUcom] Error 2make[1]: Leaving directory '/usr/src/linux-headers-4.15.0-1096-oem'Makefile:4: recipe for target 'all' failedmake: *** [all] Error 2