string.h：尝试使用kbuild Makefile构建时没有此类文件或目录

问题描述

我正在尝试从用户空间向内核模块发送一个字符串，内核将反向返回的字符串返回。我遵循一个简单的示例code，并在string.h标头中定义的内核代码中添加了strrev（）函数。但这给了我一个错误“严重错误：string.h：没有这样的文件或目录”，也没有其他错误。用户代码与链接中的示例代码相同。谁能解释为什么会这样？

用户代码

#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,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,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,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,0);
    printf("Received message payload: %s\n",NLMSG_DATA(nlh));
    close(sock_fd);
    return 0;
}

内核代码

#include <linux/module.h>
#include <net/sock.h> 
#include <linux/netlink.h>
#include <linux/skbuff.h> 
#include<string.h>
#define NETLINK_USER 31

struct 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,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,hello_nl_recv_msg,NULL,THIS_MODULE);
    struct netlink_kernel_cfg cfg = {
        .input = hello_nl_recv_msg,};

    nl_sk = netlink_kernel_create(&init_net,&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");

终端窗口

make -C /lib/modules/4.15.0-1096-oem/build/ M=/home/Username/Documents/Task_KtoUcom modules
make[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' Failed
make[2]: *** [/home/Username/Documents/Task_KtoUcom/kernl.o] Error 1
Makefile:1585: recipe for target '_module_/home/Username/Documents/Task_KtoUcom' Failed
make[1]: *** [_module_/home/Username/Documents/Task_KtoUcom] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-4.15.0-1096-oem'
Makefile:4: recipe for target 'all' Failed
make: *** [all] Error 2

解决方法

Linux内核在linux/string.h标头中提供了字符串函数：

#include <linux/string.h>

故意选择了不同的标头名称：Linux内核<linux/string.h>提供的一组功能，与标准<string.h>提供的一组功能有所不同

头文件<linux/string.h>提供了大多数标准的字符串函数，并替代了其他一些字符串函数。

例如函数strlen，strcpy和strncpy由该标头提供，并具有与C标准中相同的语义。