Android环境下使用call_usermodehelper()以及调试

有时候设备驱动需要做一些与其他的设备通信的操作，但是驱动本身又不可以去实作，那这个时候就可以通过调用用户态的软件，通过这个软件和其他的设备进行通信。

那在内核态如何去调用用户态的程序呢？call_usermodehelper() 可以做到。这个linux kernel提供的一个接口，并且这个程序具体root 权限。

这个函数的调用的方法是非常简单的。如下：

    char cmd_path[] = "/vendor/bin/XX";
    char* cmd_argv_low[] = {cmd_path,"wlan0","write","xxxx",NULL};
    char* cmd_envp[] = {"PATH=/sbin:/system/bin", NULL};
    int touch_int = 0;
    touch_int = call_usermodehelper(cmd_path, cmd_argv_low, cmd_envp, UMH_WAIT_EXEC);
        printk(KERN_DEBUG "call_usermodehelper is %d 
", touch_int);

如果顺利执行，上面的代码相当于在shell中执行了 /vendor/bin/XX wlan0 write xxxx 这么一条指令。下面简单解释一下各个参数的意思：

cmd_path 就是这个用户态的程序存放的位置。
cmd_argv_low 是传给用户态程序的参数列表，相当于argv[0],argv[1],最后要以NULL 结尾
cmd_envp 是传给程序的环境变量，同样也要以NULL 结尾
最后如果执行成功，那么返回0

但是当我执行这段程序的时候，并没有返回0，而是返回了-13，搜索了一下linux 内核关于错误码的定义，发现是Permission denied ，程序是root权限为什么还是会是权限不通过呢？原来是Android 有selinux 机制挡掉了这次调用。

暂时关闭selinux机制就可以使用了，当然要彻底解决的话，就要通过添加te 规则来通过Android的安全检查。

这里顺便也列一下kernel 的常见的错误码：

#define EPERM   1 /* Operation not permitted */
#define ENOENT   2 /* No such file or directory */
#define ESRCH   3 /* No such process */
#define EINTR   4 /* Interrupted system call */
#define EIO   5 /* I/O error */
#define ENXIO   6 /* No such device or address */
#define E2BIG   7 /* Argument list too long */
#define ENOEXEC   8 /* Exec format error */
#define EBADF   9 /* Bad file number */
#define ECHILD  10 /* No child processes */
#define EAGAIN  11 /* Try again */
#define ENOMEM  12 /* Out of memory */
#define EACCES  13 /* Permission denied */
#define EFAULT  14 /* Bad address */
#define ENOTBLK  15 /* Block device required */
#define EBUSY  16 /* Device or resource busy */
#define EEXIST  17 /* File exists */
#define EXDEV  18 /* Cross-device link */
#define ENODEV  19 /* No such device */
#define ENOTDIR  20 /* Not a directory */
#define EISDIR  21 /* Is a directory */
#define EINVAL  22 /* Invalid argument */
#define ENFILE  23 /* File table overflow */
#define EMFILE  24 /* Too many open files */
#define ENOTTY  25 /* Not a typewriter */
#define ETXTBSY  26 /* Text file busy */
#define EFBIG  27 /* File too large */
#define ENOSPC  28 /* No space left on device */
#define ESPIPE  29 /* Illegal seek */
#define EROFS  30 /* Read-only file system */
#define EMLINK  31 /* Too many links */
#define EPIPE  32 /* Broken pipe */
#define EDOM  33 /* Math argument out of domain of func */
#define ERANGE  34 /* Math result not representable */
#define EDEADLK  35 /* Resource deadlock would occur */
#define ENAMETOOLONG 36 /* File name too long */
#define ENOLCK  37 /* No record locks available */
#define ENOSYS  38 /* Function not implemented */
#define ENOTEMPTY 39 /* Directory not empty */
#define ELOOP  40 /* Too many symbolic links encountered */
#define EWOULDBLOCK EAGAIN /* Operation would block */
#define ENOMSG  42 /* No message of desired type */
#define EIDRM  43 /* Identifier removed */
#define ECHRNG  44 /* Channel number out of range */
#define EL2NSYNC 45 /* Level 2 not synchronized */
#define EL3HLT  46 /* Level 3 halted */
#define EL3RST  47 /* Level 3 reset */
#define ELNRNG  48 /* Link number out of range */
#define EUNATCH  49 /* Protocol driver not attached */
#define ENOCSI  50 /* No CSI structure available */
#define EL2HLT  51 /* Level 2 halted */
#define EBADE  52 /* Invalid exchange */
#define EBADR  53 /* Invalid request descriptor */
#define EXFULL  54 /* Exchange full */
#define ENOANO  55 /* No anode */
#define EBADRQC  56 /* Invalid request code */
#define EBADSLT  57 /* Invalid slot */

#define EDEADLOCK EDEADLK

#define EBFONT  59 /* Bad font file format */
#define ENOSTR  60 /* Device not a stream */
#define ENODATA  61 /* No data available */
#define ETIME  62 /* Timer expired */
#define ENOSR  63 /* Out of streams resources */
#define ENONET  64 /* Machine is not on the network */
#define ENOPKG  65 /* Package not installed */
#define EREMOTE  66 /* Object is remote */
#define ENOLINK  67 /* Link has been severed */
#define EADV  68 /* Advertise error */
#define ESRMNT  69 /* Srmount error */
#define ECOMM  70 /* Communication error on send */
#define EPROTO  71 /* Protocol error */
#define EMULTIHOP 72 /* Multihop attempted */
#define EDOTDOT  73 /* RFS specific error */
#define EBADMSG  74 /* Not a data message */
#define EOVERFLOW 75 /* Value too large for defined data type */
#define ENOTUNIQ 76 /* Name not unique on network */
#define EBADFD  77 /* File descriptor in bad state */
#define EREMCHG  78 /* Remote address changed */
#define ELIBACC  79 /* Can not access a needed shared library */
#define ELIBBAD  80 /* Accessing a corrupted shared library */
#define ELIBSCN  81 /* .lib section in a.out corrupted */
#define ELIBMAX  82 /* Attempting to link in too many shared libraries */
#define ELIBEXEC 83 /* Cannot exec a shared library directly */
#define EILSEQ  84 /* Illegal byte sequence */
#define ERESTART 85 /* Interrupted system call should be restarted */
#define ESTRPIPE 86 /* Streams pipe error */
#define EUSERS  87 /* Too many users */
#define ENOTSOCK 88 /* Socket operation on non-socket */
#define EDESTADDRREQ 89 /* Destination address required */
#define EMSGSIZE 90 /* Message too long */
#define EPROTOTYPE 91 /* Protocol wrong type for socket */
#define ENOPROTOOPT 92 /* Protocol not available */
#define EPROTONOSUPPORT 93 /* Protocol not supported */
#define ESOCKTNOSUPPORT 94 /* Socket type not supported */
#define EOPNOTSUPP 95 /* Operation not supported on transport endpoint */
#define EPFNOSUPPORT 96 /* Protocol family not supported */
#define EAFNOSUPPORT 97 /* Address family not supported by protocol */
#define EADDRINUSE 98 /* Address already in use */
#define EADDRNOTAVAIL 99 /* Cannot assign requested address */
#define ENETDOWN 100 /* Network is down */
#define ENETUNREACH 101 /* Network is unreachable */
#define ENETRESET 102 /* Network dropped connection because of reset */
#define ECONNABORTED 103 /* Software caused connection abort */
#define ECONNRESET 104 /* Connection reset by peer */
#define ENOBUFS  105 /* No buffer space available */
#define EISCONN  106 /* Transport endpoint is already connected */
#define ENOTCONN 107 /* Transport endpoint is not connected */
#define ESHUTDOWN 108 /* Cannot send after transport endpoint shutdown */
#define ETOOMANYREFS 109 /* Too many references: cannot splice */
#define ETIMEDOUT 110 /* Connection timed out */
#define ECONNREFUSED 111 /* Connection refused */
#define EHOSTDOWN 112 /* Host is down */
#define EHOSTUNREACH 113 /* No route to host */
#define EALREADY 114 /* Operation already in progress */
#define EINPROGRESS 115 /* Operation now in progress */
#define ESTALE  116 /* Stale NFS file handle */
#define EUCLEAN  117 /* Structure needs cleaning */
#define ENOTNAM  118 /* Not a XENIX named type file */
#define ENAVAIL  119 /* No XENIX semaphores available */
#define EISNAM  120 /* Is a named type file */
#define EREMOTEIO 121 /* Remote I/O error */
#define EDQUOT  122 /* Quota exceeded */

#define ENOMEDIUM 123 /* No medium found */
#define EMEDIUMTYPE 124 /* Wrong medium type */