android- 启动文件权限

在system.img和data.img，我们发现目录和文件有自己的UID/GID及相应的访问权限
但在Android的编译环境中，这些目录和文件目录并没有对此有相应的配置。
事实上，这些修改是有mkyaffs2image工具完成的。分析mkyaffs2image.c源代码，其过程如下：1. 预制条件. 当我们使用 mkyaffs2image 工具生成yaffs2文件系统时，这里有两个预制条件：
a. 使用'-f'标志
/external/yaffs2/yaffs2/utils/mkyaffs2image.c.
如果'-f'，代码中fixstats标志会被设置，一些特殊操作将会根据标志来操作
        if (strcmp(argv[1], "-f") == 0) {
            fixstats = 1;
            argc--;
            argv++;
        }b. 输入的文件夹的名字必须是"data"或者"system"
    if (fixstats) {
        int len = strlen(argv[1]);
       
        if((len >= 4) && (!strcmp(argv[1] + len - 4, "data"))) {
            source_path_len = len - 4;
        } else if((len >= 7) && (!strcmp(argv[1] + len - 6, "system"))) {
            source_path_len = len - 6;
        } else {           
            fprintf(stderr,"Fixstats (-f) option requested but filesystem is not data or android!\n");
            exit(1);
        }
        fix_stat(argv[1], &stats);
    }2. 正常配置：访问权限、UID/GID.
mkyaffs2image 给所有的目录和文件uid:gid/ROOT:ROOT，使用在编译环境中得到原始的访问权限3. Android特殊的配置如果fixstats标志被设置了，mkyaffs2image 将会做一些特殊的配置用于Android操作系统 /external/yaffs2/yaffs2/utils/mkyaffs2image.c.
main() -> process_directory() -> fix_stat() -> fs_config() -> fs_config() 定义在 sysem/core/include/private/android_filesystem_config.h.
该函数根据android_dirs和android_files数据结构做Android相关的配置4. android_dirs和android_files数据结构定义在system/core/include/private/android_filesystem_config.h文件中：static struct fs_path_config android_dirs[] = {
    { 00770, AID_SYSTEM, AID_CACHE,  "cache" },
    { 00771, AID_SYSTEM, AID_SYSTEM, "data/app" },
    { 00771, AID_SYSTEM, AID_SYSTEM, "data/app-private" },
    { 00771, AID_SYSTEM, AID_SYSTEM, "data/dalvik-cache" },
    { 00771, AID_SYSTEM, AID_SYSTEM, "data/data" },
    { 00771, AID_SHELL,  AID_SHELL,  "data/local/tmp" },
    { 00771, AID_SHELL,  AID_SHELL,  "data/local" },
    { 01771, AID_SYSTEM, AID_MISC,   "data/misc" },
    { 00770, AID_DHCP,   AID_DHCP,   "data/misc/dhcp" },
    { 00771, AID_SYSTEM, AID_SYSTEM, "data" },
    { 00750, AID_ROOT,   AID_SHELL,  "sbin" },
    { 00755, AID_ROOT,   AID_SHELL,  "system/bin" },
    { 00755, AID_ROOT,   AID_SHELL,  "system/xbin" },
    { 00755, AID_ROOT,   AID_ROOT,   "system/etc/ppp" },
    { 00777, AID_ROOT,   AID_ROOT,   "sdcard" },
    { 00755, AID_SYSTEM, AID_SYSTEM, "system/midletbox" },
    { 00777, AID_SYSTEM, AID_SYSTEM, "system/bin/midletvm" },
    { 00755, AID_ROOT,   AID_ROOT,   0 },
};
static struct fs_path_config android_files[] = {
    { 00440, AID_ROOT,      AID_SHELL,     "system/etc/init.goldfish.rc" },
    { 00550, AID_ROOT,      AID_SHELL,     "system/etc/init.goldfish.sh" },
    { 00440, AID_ROOT,      AID_SHELL,     "system/etc/init.trout.rc" },
    { 00550, AID_ROOT,      AID_SHELL,     "system/etc/init.ril" },
    { 00550, AID_ROOT,      AID_SHELL,     "system/etc/init.testmenu" },
    { 00550, AID_DHCP,      AID_SHELL,     "system/etc/dhcpcd/dhcpcd-run-hooks" },
    { 00440, AID_BLUETOOTH, AID_BLUETOOTH, "system/etc/dbus.conf" },
    { 00440, AID_BLUETOOTH, AID_BLUETOOTH, "system/etc/bluez/main.conf" },
    { 00440, AID_BLUETOOTH, AID_BLUETOOTH, "system/etc/bluez/input.conf" },
    { 00440, AID_BLUETOOTH, AID_BLUETOOTH, "system/etc/bluez/audio.conf" },
    { 00444, AID_RADIO,     AID_AUDIO,     "system/etc/AudioPara4.csv" },
    { 00555, AID_ROOT,      AID_ROOT,      "system/etc/ppp
    { 02755, AID_ROOT,      AID_NET_RAW,   "system/bin/ping" },
    { 02755, AID_ROOT,      AID_INET,      "system/bin/netcfg" },
        
    { 06755, AID_ROOT,      AID_ROOT,      "system/xbin/su" },
    { 06755, AID_ROOT,      AID_ROOT,      "system/xbin/librank" },
    { 06755, AID_ROOT,      AID_ROOT,      "system/xbin/procrank" },
    { 06755, AID_ROOT,      AID_ROOT,      "system/xbin/procmem" },
    { 06755, AID_ROOT,      AID_ROOT,      "system/xbin/tcpdump" },
    { 04755, AID_ROOT,      AID_ROOT,      "system/bin/fota" },
    { 00755, AID_ROOT,      AID_SHELL,     "system/bin/*" },
    { 00755, AID_ROOT,      AID_SHELL,     "system/xbin/*" },
    { 00750, AID_ROOT,      AID_SHELL,     "sbin/*" },
    { 00755, AID_ROOT,      AID_ROOT,      "bin/*" },
    { 00750, AID_ROOT,      AID_SHELL,     "init*" },
    { 00644, AID_ROOT,      AID_ROOT,       0 },
};