android探索之UID u0_axxx的由来
通过PackageManager的学习，我们知道，android的UID和linux的UID根本是两回事，Linux的UID是用于针对多用户操作系统中用于区分用户的。而Android中的UID是用于系统进行权限管理的，相信大家在shell环境中进行ps命令查询的时候，屏幕都会输出如下的信息，那么，对于输出信息中的USER一栏中具体的system，radio，u0_a1（CanlendarProvider）,u0_a27(deskClock)等等这就是我们今天的主人公UID，奇怪的就是之前我们理解的uid都是有正整数来表示，为毛这里是字串的形式，uid是整数这点毋庸置疑，会出现字串的情况应该是有一个一一对应的关系。
为了探索这个对应关系，今天我们就以此为例，探索下这些UID的具体由来。
USER PID PPID VSIZE RSS WCHAN PC NAME
u0_a42 1971 597 1443692 35452 SyS_epoll_ 7f7dca9ba4 S com.android.printspooler
u0_a24 2003 597 1440292 33136 SyS_epoll_ 7f7dca9ba4 S com.qualcomm.qti.accesscache
u0_a50 2110 597 1440316 32912 SyS_epoll_ 7f7dca9ba4 S com.android.smspush
system 2127 597 1470876 37476 SyS_epoll_ 7f7dca9ba4 S com.android.settings
radio 2140 597 1442476 43824 SyS_epoll_ 7f7dca9ba4 S com.qualcomm.qcrilmsgtunnel
system 2212 597 1444100 35500 SyS_epoll_ 7f7dca9ba4 S com.qualcomm.telephony
system 2286 597 1442632 34892 SyS_epoll_ 7f7dca9ba4 S com.qti.diagservices
system 2301 597 1440236 34528 SyS_epoll_ 7f7dca9ba4 S com.qualcomm.qti.qs
u0_a1 2318 597 1443108 39640 SyS_epoll_ 7f7dca9ba4 S com.android.providers.calendar
u0_a8 2335 597 1442188 34216 SyS_epoll_ 7f7dca9ba4 S com.android.managedprovisioning
u0_a9 2351 597 1440216 33492 SyS_epoll_ 7f7dca9ba4 S com.android.onetimeinitializer
u0_a20 2368 597 1440176 32664 SyS_epoll_ 7f7dca9ba4 S com.qualcomm.qti.calendarlocalaccount
u0_a26 2381 597 1440680 33512 SyS_epoll_ 7f7dca9ba4 S com.qti.csk
u0_a27 2394 597 1448988 40792 SyS_epoll_ 7f7dca9ba4 S com.android.deskclock首先，通过对于android系统的了解，由于android进程是由zygote进程孵化而来，因此通过zygote的fork流程，我们可以断定，这个字符串并非在这里生成的。如下简要列出zygote孵化的重要部分：
最终在fork的子进程中通过系统调用setresgid和setresuid对当前的进程uid和gid进行了设置，但是这里仍然是数字。无法确认整数和字符的对应关系。所以并非是zygote对其进行了映射转换。
接着我们看第二个方向，由于这些信息终是由ps命令而输出，而这里的ps命令又与原生linux中的ps命令不同，android中源码中，具体的实现代码在/system/core/toolbox/ps.c中，其对应的函数为ps_line方法。
static int ps_line(int pid, int tid, char *namefilter)
{
// 略去无关代码
pw = getpwuid(stats.st_uid);
if(pw == 0 || (display_flags & SHOW_NUMERIC_UID)) {
sprintf(user,"%d",(int)stats.st_uid);
} else {
strcpy(user,pw->pw_name);
}
// 略去无关代码
}这个方法体很长，我们只关注重点部分，getpwuid即是我们要获取UID的方法了。这里需要注意，这个方法是在libc(android实现的轻量级c库boinic)中,路径bionic/libc/bionic/stubs.c
passwd* getpwuid(uid_t uid) { // NOLINT: implementing bad function.
passwd_state_t* state = g_passwd_tls_buffer.get();
if (state == NULL) {
return NULL;
}
passwd* pw = android_id_to_passwd(state, uid);
if (pw != NULL) {
return pw;
}
return app_id_to_passwd(uid, state);
}这里首先通过传入的uid通过android_id_to_passwd方法去查找，如果有查到，便直接返回了。这里我插一下，这个uid啊，是由userid+appid得来的， 而userid即为我们android的用户id，appid即PKMS在apk安装的时候给每个应用分配的，想要了解PKMS是怎样给安装的应用程序分配uid的同学请戳如下文章深入了解：
Android M PackageManagerService 启动过程分析
Android M PackageManager对于应用程序apk的安装流程分析
我们跟进android_id_to_passwd方法。
static passwd* android_id_to_passwd(passwd_state_t* state, unsigned id) {
for (size_t n = 0; n < android_id_count; ++n) {
if (android_ids[n].aid == id) {
return android_iinfo_to_passwd(state, android_ids + n);
}
}
return NULL;
}android_id_passwd的情况是通过查询一个android_ids的数组，其中定义了各个特殊uid和字符串的对应关系，如system root shell等系统级别的，定义在文件/system/core/include/private/android_filesystem_config.h当中的，看一下，是不是似曾相识？
#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_USB 1018 /* USB devices */
#define AID_DRM 1019 /* DRM server */
#define AID_MDNSR 1020 /* MulticastDNSResponder (service discovery) */
#define AID_GPS 1021 /* GPS daemon */
#define AID_UNUSED1 1022 /* deprecated, DO NOT USE */
#define AID_MEDIA_RW 1023 /* internal media storage write access */
#define AID_MTP 1024 /* MTP USB driver access */
#define AID_UNUSED2 1025 /* deprecated, DO NOT USE */
#define AID_DRMRPC 1026 /* group for drm rpc */
#define AID_NFC 1027 /* nfc subsystem */
#define AID_SDCARD_R 1028 /* external storage read access */
#define AID_CLAT 1029 /* clat part of nat464 */
#define AID_LOOP_RADIO 1030 /* loop radio devices */
#define AID_MEDIA_DRM 1031 /* MediaDrm plugins */
#define AID_PACKAGE_INFO 1032 /* access to installed package details */
#define AID_SDCARD_PICS 1033 /* external storage photos access */
#define AID_SDCARD_AV 1034 /* external storage audio/video access */
#define AID_SDCARD_ALL 1035 /* access all users external storage */
#define AID_LOGD 1036 /* log daemon */
#define AID_SHARED_RELRO 1037 /* creator of shared GNU RELRO files */
#define AID_SHELL 2000 /* adb and debug shell user */
#define AID_CACHE 2001 /* cache access */
#define AID_DIAG 2002 /* access to diagnostic resources */
/* The range 2900-2999 is reserved for OEM, and must never be
* used here */
#define AID_OEM_RESERVED_START 2900
#define AID_OEM_RESERVED_END 2999
/* 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_NET_BW_STATS 3006 /* read bandwidth statistics */
#define AID_NET_BW_ACCT 3007 /* change bandwidth statistics accounting */
#define AID_NET_BT_STACK 3008 /* bluetooth: access config files */
#define AID_QCOM_DIAG 3009 /* can read/write /dev/diag */
#define AID_SENSORS 3011 /* access to /dev/socket/sensor_ctl_socket & QCCI/QCSI */
#define AID_RFS 3012 /* Remote Filesystem for peripheral processors */
#define AID_RFS_SHARED 3013 /* Shared files for Remote Filesystem for peripheral processors */
#define AID_EVERYBODY 9997 /* shared between all apps in the same profile */
#define AID_MISC 9998 /* access to misc storage */
#define AID_NOBODY 9999
#define AID_APP 10000 /* first app user */
#define AID_ISOLATED_START 99000 /* start of uids for fully isolated sandboxed processes */
#define AID_ISOLATED_END 99999 /* end of uids for fully isolated sandboxed processes */
#define AID_USER 100000 /* offset for uid ranges for each user */
#define AID_SHARED_GID_START 50000 /* start of gids for apps in each user to share */
#define AID_SHARED_GID_END 59999 /* start of gids for apps in each user to share */
static const struct android_id_info android_ids[] = {
{ "root", AID_ROOT, },
{ "system", AID_SYSTEM, },
{ "radio", AID_RADIO, },
{ "bluetooth", AID_BLUETOOTH, },
{ "graphics", AID_GRAPHICS, },
{ "input", AID_INPUT, },
{ "audio", AID_AUDIO, },
{ "camera", AID_CAMERA, },
{ "log", AID_LOG, },
{ "compass", AID_COMPASS, },
{ "mount", AID_MOUNT, },
{ "wifi", AID_WIFI, },
{ "adb", AID_ADB, },
{ "install", AID_INSTALL, },
{ "media", AID_MEDIA, },
{ "dhcp", AID_DHCP, },
{ "sdcard_rw", AID_SDCARD_RW, },
{ "vpn", AID_VPN, },
{ "keystore", AID_KEYSTORE, },
{ "usb", AID_USB, },
{ "drm", AID_DRM, },
{ "mdnsr", AID_MDNSR, },
{ "gps", AID_GPS, },
// AID_UNUSED1
{ "media_rw", AID_MEDIA_RW, },
{ "mtp", AID_MTP, },
// AID_UNUSED2
{ "drmrpc", AID_DRMRPC, },
{ "nfc", AID_NFC, },
{ "sdcard_r", AID_SDCARD_R, },
{ "clat", AID_CLAT, },
{ "loop_radio", AID_LOOP_RADIO, },
{ "mediadrm", AID_MEDIA_DRM, },
{ "package_info", AID_PACKAGE_INFO, },
{ "shared_relro", AID_SHARED_RELRO, },
{ "shell", AID_SHELL, },
{ "cache", AID_CACHE, },
{ "diag", AID_DIAG, },
{ "qcom_diag", AID_QCOM_DIAG, },
{ "net_bt_admin", AID_NET_BT_ADMIN, },
{ "net_bt", AID_NET_BT, },
{ "inet", AID_INET, },
{ "net_raw", AID_NET_RAW, },
{ "net_admin", AID_NET_ADMIN, },
{ "net_bw_stats", AID_NET_BW_STATS, },
{ "net_bw_acct", AID_NET_BW_ACCT, },
{ "net_bt_stack", AID_NET_BT_STACK, },
{ "sensors", AID_SENSORS, },
{ "rfs", AID_RFS, },
{ "rfs_shared", AID_RFS_SHARED, },
{ "everybody", AID_EVERYBODY, },
{ "misc", AID_MISC, },
{ "nobody", AID_NOBODY, },
};这里就很清楚了，那些系统级别的uid对应的字符串都是在这里定义的，比如我们开篇提到的system，radio等等，而普通的应用呢则不是在这里匹配得到，方法自然会返回null，那么就会进入后面的流程 app_id_to_passwd。
#define AID_USER 100000 /* offset for uid ranges for each user */
#define AID_ISOLATED_START 99000 /* start of uids for fully isolated sandboxed processes */
#define AID_APP 10000
// Translate a uid into the corresponding name.
// 0 to AID_APP-1 -> "system", "radio", etc.
// AID_APP to AID_ISOLATED_START-1 -> u0_a1234
// AID_ISOLATED_START to AID_USER-1 -> u0_i1234
// AID_USER+ -> u1_radio, u1_a1234, u2_i1234, etc.
// returns a passwd structure (sets errno to ENOENT on failure).
static passwd* app_id_to_passwd(uid_t uid, passwd_state_t* state) {
if (uid < AID_APP) {
errno = ENOENT;
return NULL;
}
print_app_name_from_uid(uid, state->name_buffer_, sizeof(state->name_buffer_));
const uid_t appid = uid % AID_USER;
if (appid < AID_APP) {
snprintf(state->dir_buffer_, sizeof(state->dir_buffer_), "/");
} else {
snprintf(state->dir_buffer_, sizeof(state->dir_buffer_), "/data");
}
snprintf(state->sh_buffer_, sizeof(state->sh_buffer_), "/system/bin/sh");
passwd* pw = &state->passwd_;
pw->pw_name = state->name_buffer_;
pw->pw_dir = state->dir_buffer_;
pw->pw_shell = state->sh_buffer_;
pw->pw_uid = uid;
pw->pw_gid = uid;
return pw;
}
static void print_app_name_from_uid(const uid_t uid, char* buffer, const int bufferlen) {
const uid_t appid = uid % AID_USER;
const uid_t userid = uid / AID_USER;
if (appid >= AID_ISOLATED_START) {
snprintf(buffer, bufferlen, "u%u_i%u", userid, appid - AID_ISOLATED_START);
} else if (appid < AID_APP) {
for (size_t n = 0; n < android_id_count; n++) {
if (android_ids[n].aid == appid) {
snprintf(buffer, bufferlen, "u%u_%s", userid, android_ids[n].name);
return;
}
}
} else {
snprintf(buffer, bufferlen, "u%u_a%u", userid, appid - AID_APP);
}
}看到这个方法后，我们不禁恍然大悟啊，通过上面的宏定义以及计算方法，我们不难得出：
- app的uid/100000的结果为userid，填到ux的x处。
- app的uid减去10000为appid，填到axx的xx处。
- 例如某个app的uid是10022，经过计算，userid为10022/100000=0，appid为10022-10000=22，则那么最终通过ps打印得到uid字串就是u0_a22
那么这个useid在系统中哪里可以查到呢？当然是在pkms中啦 ，比如在packages.list中就会有哦。
com.gd.mobicore.pa 10047 0 /data/data/com.gd.mobicore.pa platform 3003
com.qualcomm.qti.auth.sampleextauthservice 10049 0 /data/data/com.qualcomm.qti.auth.sampleextauthservice platform none
com.cootek.smartinputv5.language.indonesian 10062 0 /data/data/com.cootek.smartinputv5.language.indonesian default none
com.qrd.omadownload 10036 0 /data/data/com.qrd.omadownload platform 3003
com.cootek.smartinputv5.language.vietnam 10072 0 /data/data/com.cootek.smartinputv5.language.vietnam default none
com.android.providers.telephony 1001 0 /data/data/com.android.providers.telephony platform 2001,1005,3002,1023,1015,3003,3001,3009,3006
com.cootek.smartinputv5.language.cangjie 10059 0 /data/data/com.cootek.smartinputv5.language.cangjie default none
com.android.providers.calendar 10001 0 /data/data/com.android.providers.calendar default 3003
com.android.providers.media 10006 0 /data/data/com.android.providers.media default 2001,1023,1015,3003,1024,3007
com.qti.service.colorservice 1000 0 /data/data/com.qti.service.colorservice platform 2001,3002,1023,1015,3003,3001,1021,3004,3005,1000,2002,3009,1010
com.qualcomm.shutdownlistner 10053 0 /data/data/com.qualcomm.shutdownlistner platform none
com.android.wallpapercropper 10016 0 /data/data/com.android.wallpapercropper platform none
com.quicinc.cne.CNEService 1000 0 /data/data/com.quicinc.cne.CNEService platform 2001,3002,1023,1015,3003,3001,1021,3004,3005,1000,2002,3009,1010
com.android.protips 10043 0 /data/data/com.android.protips default none
com.qualcomm.qti.phonefeature 1001 0 /data/data/com.qualcomm.qti.phonefeature platform 2001,1005,3002,1023,1015,3003,3001,3009,3006
com.cootek.smartinputv5.language.marathi 10063 0 /data/data/com.cootek.smartinputv5.language.marathi default none
org.simalliance.openmobileapi.service 1000 0 /data/data/org.simalliance.openmobileapi.service platform 2001,3002,1023,1015,3003,3001,1021,3004,3005,1000,2002,3009,1010
com.android.documentsui 10028 0 /data/data/com.android.documentsui platform none
com.android.externalstorage 10007 0 /data/data/com.android.externalstorage platform 1023,1015
com.cootek.smartinputv5.language.spanishlatin 10066 0 /data/data/com.cootek.smartinputv5.language.spanishlatin default none
com.qualcomm.uimremoteclient 1001 0 /data/data/com.qualcomm.uimremoteclient platform 2001,1005,3002,1023,1015,3003,3001,3009,3006
com.android.htmlviewer 10030 0 /data/data/com.android.htmlviewer default none
com.qualcomm.ftm 10029 0 /data/data/com.qualcomm.ftm platform none
com.qualcomm.sta 1000 0 /data/data/com.qualcomm.sta platform 2001,3002,1023,1015,3003,3001,1021,3004,3005,1000,2002,3009,1010
com.qualcomm.svi 1000 0 /data/data/com.qualcomm.svi platform 2001,3002,1023,1015,3003,3001,1021,3004,3005,1000,2002,3009,1010
com.android.deskclock 10027 0 /data/data/com.android.deskclock platform none上面的第二列即是每个app对应的uid了，我们可以看到，canlendarprovider的uid为10001，而deskclock的是10027，那么经过计算，我们得到的USER分别为u0_a1, u0_a27。
可能有些人会问：博主，看你打印都是u0_axx，那有些u10_axx中是u10是怎么回事，我看到过的哦。
/**
* @hide Range of uids allocated for a user.
*/
public static final int PER_USER_RANGE = 100000;
public static final int getUid(int userId, int appId) {
if (MU_ENABLED) {
return userId * PER_USER_RANGE + (appId % PER_USER_RANGE);
} else {
return appId;
}
}很简单，u10_axx就是多用户下的进程了，我们打开多用户开关，切换到访客模式或者通过新建user的方式，完成后进行ps一下，输出如下：
// ps输出 访客模式
u0_a34 13056 597 1440200 32780 SyS_epoll_ 7f7dca9ba4 S com.qualcomm.qti.lunarinfo
system 13069 597 1440288 33880 SyS_epoll_ 7f7dca9ba4 S com.qualcomm.timeservice
u10_a42 32319 597 1443772 35444 SyS_epoll_ 7f7dca9ba4 S com.android.printspooler
u10_a8 32341 597 1442188 33064 SyS_epoll_ 7f7dca9ba4 S com.android.managedprovisioning
u10_system 32355 597 1470876 36316 SyS_epoll_ 7f7dca9ba4 S com.android.settings
u10_radio 32388 597 1440252 32800 SyS_epoll_ 7f7dca9ba4 S com.qualcomm.qcrilmsgtunnel
u10_a44 32406 597 1440248 35280 SyS_epoll_ 7f7dca9ba4 S com.android.provision
u10_a14 32419 597 1453712 44656 SyS_epoll_ 7f7dca9ba4 S com.android.systemui
u10_system 32434 597 1448136 37004 SyS_epoll_ 7f7dca9ba4 S com.qualcomm.location.XT
system 32492 1 14332 3832 hrtimer_na 7f964c9594 S /system/bin/ATFWD-daemon
// 新建user 11
u11_system 3022 591 1440164 33184 SyS_epoll_ 7f96147ba4 S com.qualcomm.qti.notificationservice
u11_a37 3035 591 1447512 35676 SyS_epoll_ 7f96147ba4 S com.oma.drm
u11_radio 3050 591 1451772 39164 SyS_epoll_ 7f96147ba4 S com.android.phone
u11_a45 3063 594 878888 36016 SyS_epoll_ 00f7427c64 S com.qti.smq.qualcommFeedback
u11_system 3077 591 1441432 34276 SyS_epoll_ 7f96147ba4 S com.qualcomm.qti.GBAHttpAuthentication.auth
u11_a46 3096 591 1440756 33672 SyS_epoll_ 7f96147ba4 S com.qualcomm.qti.telephonyservice
u11_system 3114 591 1441200 34632 SyS_epoll_ 7f96147ba4 S com.qualcomm.qualcommsettings
u11_radio 3128 591 1442344 34000 SyS_epoll_ 7f96147ba4 S com.qualcomm.qti.rcsbootstraputil
u11_system 3143 591 1441420 34084 SyS_epoll_ 7f96147ba4 S com.qualcomm.telephony
是不是很神奇？
