Android Permission

1. 文件(夾)讀寫權限

 

init.rc 中建立test1 test2 test3 文件夾

mkdir /data/misc/test1 0770 root root   

mkdir /data/misc/test2 0770 wifi wifi

mkdir /data/misc/test3 0770 system misc

 

其中

test1 目錄的owner是root, group 也是root

test2 目錄的owner是wifi , group 也是wifi

test3 目錄的owner是system , group 是misc （任何用戶都屬於group misc）

 

service xxxx /system/bin/xxxx
    user root
    disabled
    oneshot

 

service yyyy /system/bin/yyyy 
    user system
    disabled
    oneshot

 

service zzzz /system/bin/zzzz
    user wifi
    disabled
    oneshot

 

結果:

xxxx 服務可以訪問 test1, test2, test3

yyyy 服務可以訪問 test3

zzzz 服務可以訪問 test2, test3

 

 Android 中mkdir 的定義

 

int do_mkdir(int nargs, char **args)
{
    mode_t mode = 0755;

    /* mkdir <path> [mode] [owner] [group] */

    if (nargs >= 3) {
        mode = strtoul(args[2], 0, 8);
    }

    if (mkdir(args[1], mode)) {
        return -errno;
    }

    if (nargs >= 4) {
        uid_t uid = decode_uid(args[3]);
        gid_t gid = -1;

        if (nargs == 5) {
            gid = decode_uid(args[4]);
        }

        if (chown(args[1], uid, gid)) {
            return -errno;
        }
    }

    return 0;
}

 

 

2. Property 權限

 Android Property 也是有權限的。

2.1 以前綴 ctl. 開頭的控制屬性， 設置前，Android 代碼會調用函數check_control_perms()檢查調用者的 user id 和 group id 

struct {
    const char *service;
    unsigned int uid;
    unsigned int gid;
} control_perms[] = {
    { "dumpstate",AID_SHELL, AID_LOG },
     {NULL, 0, 0 }
};

/*
 * Checks permissions for setting system properties.
 * Returns 1 if uid allowed, 0 otherwise.
 */
static int check_control_perms(const char *name, int uid, int gid) {
    int i;
    if (uid == AID_SYSTEM || uid == AID_ROOT)
        return 1;

    /* Search the ACL */
    for (i = 0; control_perms[i].service; i++) {
        if (strcmp(control_perms[i].service, name) == 0) {
            if ((uid && control_perms[i].uid == uid) ||
                (gid && control_perms[i].gid == gid)) {
                return 1;
            }
        }
    }
    return 0;
}

 

 2.2 其它屬性， 設置前，Android 代碼會調用函數check_perms()檢查調用者的 user id 和 group id 

check_perms(msg.name, cr.uid, cr.gid)

 

 

struct {
    const char *prefix;
    unsigned int uid;
    unsigned int gid;
} property_perms[] = {
    { "net.rmnet0.",      AID_RADIO,    0 },
    { "net.gprs.",        AID_RADIO,    0 },
    { "net.ppp",          AID_RADIO,    0 },
    { "ril.",             AID_RADIO,    0 },
    { "gsm.",             AID_RADIO,    0 },
    { "persist.radio",    AID_RADIO,    0 },
    { "net.dns",          AID_RADIO,    0 },
    { "net.",             AID_SYSTEM,   0 },
    { "dev.",             AID_SYSTEM,   0 },
    { "runtime.",         AID_SYSTEM,   0 },
    { "hw.",              AID_SYSTEM,   0 },
    { "sys.",             AID_SYSTEM,   0 },
    { "service.",         AID_SYSTEM,   0 },
    { "wlan.",            AID_SYSTEM,   0 },
    { "dhcp.",            AID_SYSTEM,   0 },
    { "dhcp.",            AID_DHCP,     0 },
    { "vpn.",             AID_SYSTEM,   0 },
    { "vpn.",             AID_VPN,      0 },
    { "debug.",           AID_SHELL,    0 },
    { "log.",             AID_SHELL,    0 },
    { "service.adb.root", AID_SHELL,    0 },
    { "persist.sys.",     AID_SYSTEM,   0 },
    { "persist.service.", AID_SYSTEM,   0 },
    { NULL, 0, 0 }
};
  
/*
 * Checks permissions for starting/stoping system services.
 * AID_SYSTEM and AID_ROOT are always allowed.
 *
 * Returns 1 if uid allowed, 0 otherwise.
 */
static int check_perms(const char *name, unsigned int uid, int gid)
{
    int i;
    if (uid == 0)
        return 1;

    if(!strncmp(name, "ro.", 3))
        name +=3;

    for (i = 0; property_perms[i].prefix; i++) {
        int tmp;
        if (strncmp(property_perms[i].prefix, name,
                    strlen(property_perms[i].prefix)) == 0) {
            if ((uid && property_perms[i].uid == uid) ||
                (gid && property_perms[i].gid == gid)) {
                return 1;
            }
        }
    }

    return 0;
}

從代碼中可以看到， 任何不以property_perms[]  中定義的前綴開頭的property 是

無法被除root以外的用戶訪問的，包括system用戶。

 

3. 最後補充Android 的uid gid 定義

#define AID_ROOT             0  /* traditional unix root user */

#define AID_SYSTEM        1000  /* system server */

#define AID_RADIO         1001  /* telephony subsystem, RIL */
#define AID_BLUETOOTH     1002  /* bluetooth subsystem */
#define AID_GRAPHICS      1003  /* graphics devices */
#define AID_INPUT         1004  /* input devices */
#define AID_AUDIO         1005  /* audio devices */
#define AID_CAMERA        1006  /* camera devices */
#define AID_LOG           1007  /* log devices */
#define AID_COMPASS       1008  /* compass device */
#define AID_MOUNT         1009  /* mountd socket */
#define AID_WIFI          1010  /* wifi subsystem */
#define AID_ADB           1011  /* android debug bridge (adbd) */
#define AID_INSTALL       1012  /* group for installing packages */
#define AID_MEDIA         1013  /* mediaserver process */
#define AID_DHCP          1014  /* dhcp client */
#define AID_SDCARD_RW     1015  /* external storage write access */
#define AID_VPN           1016  /* vpn system */
#define AID_KEYSTORE      1017  /* keystore subsystem */

#define AID_SHELL         2000  /* adb and debug shell user */
#define AID_CACHE         2001  /* cache access */
#define AID_DIAG          2002  /* access to diagnostic resources */

/* The 3000 series are intended for use as supplemental group id's only.
 * They indicate special Android capabilities that the kernel is aware of. */
#define AID_NET_BT_ADMIN  3001  /* bluetooth: create any socket */
#define AID_NET_BT        3002  /* bluetooth: create sco, rfcomm or l2cap sockets */
#define AID_INET          3003  /* can create AF_INET and AF_INET6 sockets */
#define AID_NET_RAW       3004  /* can create raw INET sockets */
#define AID_NET_ADMIN     3005  /* can configure interfaces and routing tables. */

#define AID_MISC          9998  /* access to misc storage */
#define AID_NOBODY        9999

#define AID_APP          10000 /* first app user */

 

可見root (AID_ROOT = 0) 的權限最高， app (AID_APP = 10000) 權限最低， misc (AID_MISC = 9998) 權限倒數第三低。

所以#1 中描述的目錄test3的group 屬性設置成了 misc, 則除了 app/nobody 這兩個用戶，

android系統中其它所有用戶都有該目錄的group權限！

轉自：http://blog.csdn.net/stevenliyong/article/details/5343085