浅析dev目录下设备文件mknod节点gid,uid和mode的如何方便设置

init

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handle_device_fd 收到uevent信息

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handle_device_event

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static void handle_device_event(struct uevent *uevent)
{
...
/* are we block or char? where should we live? */
if(!strncmp(uevent->path, "/block", 6)) {
block = 1;
base = "/dev/block/";//根据uevent路径改变该节点路径
mkdir(base, 0755);
} else {
block = 0;
/* this should probably be configurable somehow */
if(!strncmp(uevent->path, "/class/graphics/", 16)) {
base = "/dev/graphics/";//根据uevent路径改变该uevent需要创建节点的路径
mkdir(base, 0755);
} else if (!strncmp(uevent->path, "/class/oncrpc/", 14)) {
base = "/dev/oncrpc/";
mkdir(base, 0755);
} else if (!strncmp(uevent->path, "/class/adsp/", 12)) {
base = "/dev/adsp/";
mkdir(base, 0755);
} else if(!strncmp(uevent->path, "/class/input/", 13)) {
base = "/dev/input/";//根据uevent路径改变该uevent需要创建节点的路径
mkdir(base, 0755);
} else if(!strncmp(uevent->path, "/class/sensors/", 15)) {
base = "/dev/sensors/";
mkdir(base, 0755);
} else if(!strncmp(uevent->path, "/class/mtd/", 11)) {
base = "/dev/mtd/";//根据uevent路径改变该uevent需要创建节点的路径
mkdir(base, 0755);
} else if(!strncmp(uevent->path, "/class/misc/", 12) &&
!strncmp(name, "log_", 4)) {
base = "/dev/log/";//根据uevent路径改变该uevent需要创建节点的路径
mkdir(base, 0755);
name += 4;
} else if(!strncmp(uevent->path, "/class/sound/", 13)) {
base = "/dev/snd/";
mkdir(base, 0755);
} else
base = "/dev/";
}

snprintf(devpath, sizeof(devpath), "%s%s", base, name);
if(!strcmp(uevent->action, "add")) {
make_device(devpath, block, uevent->major, uevent->minor);//创建节点文件文件devpath
return;
}
...
}
static void make_device(const char *path, int block, int major, int minor)
{
unsigned uid;
unsigned gid;
mode_t mode;
dev_t dev;
if(major > 255 || minor > 255)
return;
mode = get_device_perm(path, &uid, &gid) | (block ? S_IFBLK : S_IFCHR);//获取将要创建的节点是否需要重设它的mode数值
dev = (major << 8) | minor;
mknod(path, mode, dev);
chown(path, uid, gid);
}

get_device_perm函数会比较path路径是否和下面的devperms[
]
数组中的inode路径相一致,
如果一致,
那么返回devperms[
]
数组中指定的uid,
gid和mode数值,
这样make_device就会向/
dev这个tmpfs'内存虚拟物理盘'
创建inode节点,
同时改变该inode的uid和gid.

于是在devperms[
]
数组中我们就可以方便的控制生成的/
dev文件的uid,
gid和mode了.

static struct perms_ devperms[] = {
{ "/dev/null", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/zero", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/full", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/ptmx", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/tty", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/random", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/urandom", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/ashmem", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/binder", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/m2d", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/bmm", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/innodev", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/innodev0", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/innodev1", 0666, AID_ROOT, AID_ROOT, 0 },
{ "/dev/innodev2", 0666, AID_ROOT, AID_ROOT, 0 },
/* logger should be world writable (for logging) but not readable */
{ "/dev/log/", 0662, AID_ROOT, AID_LOG, 1 },
/* these should not be world writable */
{ "/dev/android_adb", 0660, AID_ADB, AID_ADB, 0 },
{ "/dev/android_adb_enable", 0660, AID_ADB, AID_ADB, 0 },
{ "/dev/ttyMSM0", 0660, AID_BLUETOOTH, AID_BLUETOOTH, 0 },
{ "/dev/alarm", 0664, AID_SYSTEM, AID_RADIO, 0 },
{ "/dev/tty0", 0666, AID_ROOT, AID_SYSTEM, 0 },
{ "/dev/graphics/", 0666, AID_ROOT, AID_GRAPHICS, 1 },
{ "/dev/hw3d", 0660, AID_SYSTEM, AID_GRAPHICS, 0 },
{ "/dev/input/", 0660, AID_ROOT, AID_INPUT, 1 },
{ "/dev/eac", 0660, AID_ROOT, AID_AUDIO, 0 },
{ "/dev/cam", 0660, AID_ROOT, AID_CAMERA, 0 },
{ "/dev/video0", 0660, AID_ROOT, AID_CAMERA, 0 },
{ "/dev/mved", 0660, AID_SYSTEM, AID_CAMERA, 0 },
{ "/dev/pmem", 0660, AID_SYSTEM, AID_GRAPHICS, 0 },
{ "/dev/pmem_gpu", 0660, AID_SYSTEM, AID_GRAPHICS, 1 },
{ "/dev/pmem_adsp", 0660, AID_SYSTEM, AID_AUDIO, 1 },
{ "/dev/pmem_camera", 0660, AID_SYSTEM, AID_CAMERA, 1 },
{ "/dev/oncrpc/", 0660, AID_ROOT, AID_SYSTEM, 1 },
{ "/dev/adsp/", 0660, AID_SYSTEM, AID_AUDIO, 1 },
{ "/dev/sensors/", 0666, AID_SYSTEM, AID_SYSTEM, 1 },
{ "/dev/mt9t013", 0660, AID_SYSTEM, AID_SYSTEM, 0 },
{ "/dev/akm8976_daemon",0640, AID_COMPASS, AID_SYSTEM, 0 },
{ "/dev/akm8976_aot", 0640, AID_COMPASS, AID_SYSTEM, 0 },
{ "/dev/akm8976_pffd", 0640, AID_COMPASS, AID_SYSTEM, 0 },
{ "/dev/msm_pcm_out", 0660, AID_SYSTEM, AID_AUDIO, 1 },
{ "/dev/msm_pcm_in", 0660, AID_SYSTEM, AID_AUDIO, 1 },
{ "/dev/msm_pcm_ctl", 0660, AID_SYSTEM, AID_AUDIO, 1 },
{ "/dev/msm_mp3", 0660, AID_SYSTEM, AID_AUDIO, 1 },
{ "/dev/smd0", 0640, AID_RADIO, AID_RADIO, 0 },
{ "/dev/qmi", 0640, AID_RADIO, AID_RADIO, 0 },
{ "/dev/qmi0", 0640, AID_RADIO, AID_RADIO, 0 },
{ "/dev/qmi1", 0640, AID_RADIO, AID_RADIO, 0 },
{ "/dev/qmi2", 0640, AID_RADIO, AID_RADIO, 0 },
{ "/dev/htc-acoustic", 0640, AID_RADIO, AID_RADIO, 0 },
{ "/dev/snd/", 0666, AID_ROOT, AID_AUDIO, 1 },
{ NULL, 0, 0, 0, 0 },
};