一、devfs、udev和sysfs是什麼關係?
linux2.6之前使用devfs設備文件系統,它存在與內核空間;
linux2.6之後使用udev設備文件系統,它存在與用戶空間、但嚴重依賴與sysfs文件系統。
二、Android(使用linux2.6以後的設備節點創建策略)設備節點的創建
在Android中,沒有獨立的類似於udev或者mdev的用戶程序,這個功能集成到了init中做了。代碼見:system/core/init/init.c文件,如下:
int ueventd_main(int argc, char **argv)
{
struct pollfd ufd;
int nr;
char tmp[32];
open_devnull_stdio();
log_init();
INFO("starting ueventd\n");
get_hardware_name(hardware, &revision);
/*
/ueventd.rc中以行爲單位,除最後sysfs properties外,每一行由四部分組成:
如:/dev/diag 0660 radio radio
目錄 權限 用戶ID(uid) 組ID(gid)
# sysfs properties 多一個屬性
/sys/devices/virtual/input/input* enable 0660 root input
目錄 屬性 權限 用戶ID(uid) 組ID(gid)
*/
ueventd_parse_config_file("/ueventd.rc");
snprintf(tmp, sizeof(tmp), "/ueventd.%s.rc", hardware);
ueventd_parse_config_file(tmp);
//初始化uevent,建立socket,執行coldboot,用於檢查當前service啓動前操作系統已經處理的事件,add這些事件到應用層
device_init();
ufd.events = POLLIN;
ufd.fd = get_device_fd();
//在死循環中處理觸發事件
while(1) {
ufd.revents = 0;
nr = poll(&ufd, 1, -1);
if (nr <= 0)
continue;
if (ufd.revents == POLLIN)
handle_device_fd();
}
}
int ueventd_parse_config_file(const char *fn)
{
char *data;
data = read_file(fn, 0); //讀取文件內容返回給data;
if (!data) return -1;
parse_config(fn, data); //解析整個rc文件內容
DUMP(); //空函數什麼都不做
return 0;
}
以上步驟和Init進程解析init.rc文件的步驟相同,不過這裏調用的parse_config函數不同,該函數是專門用於解析ueventd.rc文件的,具體解析過程如下:
static void parse_config(const char *fn, char *s)
{
struct parse_state state;
char *args[UEVENTD_PARSER_MAXARGS]; //最多五個參數
int nargs;
nargs = 0;
state.filename = fn; //設置解析文件的路徑
state.line = 1;
state.ptr = s; //文件內容
state.nexttoken = 0;
state.parse_line = parse_line_device; //設置每行解析回調函數
for (;;) {
int token = next_token(&state); //用於獲得配置文件中特殊標記,如文件結尾(T_EOF),換行符(T_TEXT),文本(T_NEWLINE) ,從文件內容中查找token,與init.rc文件類似;
switch (token) {
case T_EOF: //文件結束
state.parse_line(&state, 0, 0); //state.parse_line 調用函數爲parse_line_device;
return;
case T_NEWLINE: //新的一行
if (nargs) {
state.parse_line(&state, nargs, args); //調用行解析函數解析每一行
nargs = 0;
}
break;
case T_TEXT:
if (nargs < UEVENTD_PARSER_MAXARGS) {
args[nargs++] = state.text;
}
break;
}
}
}
函數首先查找指定的token,然後對不同的token做不同的處理,對於發現新行時,調用parse_line_device函數對每一行進行詳細解析,該函數實現如下:
int next_token(struct parse_state *state)
{
char *x = state->ptr; //讀數據指針
char *s;
/*
#define T_EOF 0
#define T_TEXT 1
#define T_NEWLINE 2
非T_EOF時,直接返回下一個標記
*/
if (state->nexttoken) {
int t = state->nexttoken;
state->nexttoken = 0;
return t;
}
for (;;) {
switch (*x) {
case 0:
state->ptr = x;
return T_EOF;
case '\n':
x++;
state->ptr = x;
return T_NEWLINE; //換行符
case ' ':
case '\t':
case '\r':
x++;
continue; //跳過轉義字符 :空格 tab 回車
case '#':
while (*x && (*x != '\n')) x++; //單行註釋
if (*x == '\n') {
state->ptr = x+1;
return T_NEWLINE;
} else {
state->ptr = x;
return T_EOF;
}
default:
goto text;
}
}
textdone:
state->ptr = x;
*s = 0;
return T_TEXT;
text:
state->text = s = x;
textresume:
for (;;) {
switch (*x) {
case 0:
goto textdone;
case ' ':
case '\t':
case '\r':
x++;
goto textdone;
case '\n':
state->nexttoken = T_NEWLINE;
x++;
goto textdone;
case '"':
x++;
for (;;) {
switch (*x) {
case 0:
/* unterminated quoted thing */
state->ptr = x;
return T_EOF;
case '"':
x++;
goto textresume;
default:
*s++ = *x++;
}
}
break;
case '\\':
x++;
switch (*x) {
case 0:
goto textdone;
case 'n':
*s++ = '\n';
break;
case 'r':
*s++ = '\r';
break;
case 't':
*s++ = '\t';
break;
case '\\':
*s++ = '\\';
break;
case '\r':
/* \ <cr> <lf> -> line continuation */
if (x[1] != '\n') {
x++;
continue;
}
case '\n':
/* \ <lf> -> line continuation */
state->line++;
x++;
/* eat any extra whitespace */
while((*x == ' ') || (*x == '\t')) x++;
continue;
default:
/* unknown escape -- just copy */
*s++ = *x++;
}
continue;
default:
*s++ = *x++;
}
}
return T_EOF;
}
static void parse_line_device(struct parse_state* state, int nargs, char **args)
{
set_device_permission(nargs, args); //nargs參數個數 args參數
}
函數直接調用set_device_permission來實現;
非sysfs 設備文件:
|name| |permission| |user| |group|
/dev/cam 0660 root ca
sysfs 設備文件屬性:
/sys/devices/virtual/input/input* enable 0660 root input
void set_device_permission(int nargs, char **args)
{
char *name;
char *attr = 0;
mode_t perm;
uid_t uid;
gid_t gid;
int prefix = 0;
char *endptr;
int ret;
char *tmp = 0;
if (nargs == 0)
return;
if (args[0][0] == '#')
return;
/* |name| |permission| |user| |group| */
name = args[0];
if (!strncmp(name,"/sys/", 5) && (nargs == 5)) {
INFO("/sys/ rule %s %s\n",args[0],args[1]);
attr = args[1];
args++;
nargs--;
}
//參數檢查
if (nargs != 4) {
ERROR("invalid line ueventd.rc line for '%s'\n", args[0]);
return;
}
/* If path starts with mtd@ lookup the mount number. */
if (!strncmp(name, "mtd@", 4)) {
int n = mtd_name_to_number(name + 4);
if (n >= 0)
asprintf(&tmp, "/dev/mtd/mtd%d", n);
name = tmp;
} else {
int len = strlen(name);
if (name[len - 1] == '*') {
prefix = 1;
name[len - 1] = '\0';
}
}
//權限檢查
perm = strtol(args[1], &endptr, 8);
if (!endptr || *endptr != '\0') {
ERROR("invalid mode '%s'\n", args[1]);
free(tmp);
return;
}
//從android_ids數組中查找uid
ret = get_android_id(args[2]);
if (ret < 0) {
ERROR("invalid uid '%s'\n", args[2]);
free(tmp);
return;
}
uid = ret;
//從android_ids數組中查找gid
ret = get_android_id(args[3]);
if (ret < 0) {
ERROR("invalid gid '%s'\n", args[3]);
free(tmp);
return;
}
gid = ret;
//爲設備文件添加權限
add_dev_perms(name, attr, perm, uid, gid, prefix);
free(tmp);
}
首先檢查參數的合法性,並根據參數查找uid、gid,對不同的用戶和組的uid、gid已經事先配置在數組android_ids中了,如下:
- 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, },
- { "dhcp", AID_DHCP, },
- { "adb", AID_ADB, },
- { "install", AID_INSTALL, },
- { "media", AID_MEDIA, },
- { "drm", AID_DRM, },
- { "mdnsr", AID_MDNSR, },
- { "nfc", AID_NFC, },
- { "drmrpc", AID_DRMRPC, },
- { "shell", AID_SHELL, },
- { "cache", AID_CACHE, },
- { "diag", AID_DIAG, },
- { "net_bt_admin", AID_NET_BT_ADMIN, },
- { "net_bt", AID_NET_BT, },
- { "sdcard_r", AID_SDCARD_R, },
- { "sdcard_rw", AID_SDCARD_RW, },
- { "media_rw", AID_MEDIA_RW, },
- { "vpn", AID_VPN, },
- { "keystore", AID_KEYSTORE, },
- { "usb", AID_USB, },
- { "mtp", AID_MTP, },
- { "gps", AID_GPS, },
- { "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, },
- { "misc", AID_MISC, },
- { "nobody", AID_NOBODY, },
- };
這些uid、gid都是以宏的形式被定義:
- #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 */
通過調用get_android_id函數在數組android_ids中查找對應的uid、gid
- static int get_android_id(const char *id)
- {
- unsigned int i;
- for (i = 0; i < ARRAY_SIZE(android_ids); i++)
- if (!strcmp(id, android_ids[i].name))
- return android_ids[i].aid;
- return 0;
- }
函數實現比較簡單,通過遍歷數組,並匹配數組元素的name屬性來查找指定name的uid或gid。
最後通過add_dev_perms函數來設置設備文件的操作權限,該函數定義在system\core\init\devices.c文件中,在該文件中聲明瞭三個鏈表:
- static list_declare(sys_perms);
- static list_declare(dev_perms);
- static list_declare(platform_names);
add_dev_perms函數就是將解析得到的設備及設備屬性,添加到指定的鏈表中,
使用解析得到的內容來創建一個perm_node變量,並根據條件添加到sys_perms或dev_perms鏈表中。
- int add_dev_perms(const char *name, const char *attr,
- mode_t perm, unsigned int uid, unsigned int gid,
- unsigned short prefix) {
- //創建perm_node
- struct perm_node *node = calloc(1, sizeof(*node));
- if (!node)
- return -ENOMEM;
- node->dp.name = strdup(name);
- if (!node->dp.name)
- return -ENOMEM;
- if (attr) {
- node->dp.attr = strdup(attr);
- if (!node->dp.attr)
- return -ENOMEM;
- }
- //設置perm_node的成員屬性
- node->dp.perm = perm;
- node->dp.uid = uid;
- node->dp.gid = gid;
- node->dp.prefix = prefix;
- //根據attr 來選擇添加到sys_perms或dev_perms鏈表中
- if (attr)
- list_add_tail(&sys_perms, &node->plist);
- else
- list_add_tail(&dev_perms, &node->plist);
- return 0;
- }
至此ueventd.rc文件的解析工作完成了,uevent進程接下來將調用device_init()函數來初始化設備文件
- void device_init(void)
- {
- suseconds_t t0, t1;
- struct stat info;
- int fd;
- #ifdef HAVE_SELINUX
- struct selinux_opt seopts[] = {
- { SELABEL_OPT_PATH, "/file_contexts" }
- };
- if (is_selinux_enabled() > 0)
- sehandle = selabel_open(SELABEL_CTX_FILE, seopts, 1);
- #endif
- /* is 64K enough? udev uses 16MB! */
- //創建NETLINK socket,用於監聽內核發送過來的uevent消息
- device_fd = uevent_open_socket(64*1024, true);
- if(device_fd < 0)
- return;
- //設置socket相關屬性
- fcntl(device_fd, F_SETFD, FD_CLOEXEC);
- fcntl(device_fd, F_SETFL, O_NONBLOCK);
- //查看"/dev/.coldboot_done" 文件信息
- if (stat(coldboot_done, &info) < 0) {
- t0 = get_usecs();
- coldboot("/sys/class");
- coldboot("/sys/block");
- coldboot("/sys/devices");
- t1 = get_usecs();
- fd = open(coldboot_done, O_WRONLY|O_CREAT, 0000);
- close(fd);
- log_event_print("coldboot %ld uS\n", ((long) (t1 - t0)));
- } else {
- log_event_print("skipping coldboot, already done\n");
- }
- }
函數首先調用uevent_open_socket 來創建PF_NETLINK socket 並綁定到指定地址上:
- int uevent_open_socket(int buf_sz, bool passcred)
- {
- struct sockaddr_nl addr;
- int on = passcred;
- int s;
- memset(&addr, 0, sizeof(addr));
- addr.nl_family = AF_NETLINK;
- addr.nl_pid = getpid();
- addr.nl_groups = 0xffffffff;
- //創建socket
- s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_KOBJECT_UEVENT);
- if(s < 0)
- return -1;
- //設置該socket屬性
- setsockopt(s, SOL_SOCKET, SO_RCVBUFFORCE, &buf_sz, sizeof(buf_sz));
- setsockopt(s, SOL_SOCKET, SO_PASSCRED, &on, sizeof(on));
- //綁定該socket
- if(bind(s, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
- close(s);
- return -1;
- }
- return s;
- }
ueventd進程接下來將通過系統調用poll函數來監控該socket,如下所示:
- ufd.events = POLLIN;
- ufd.fd = get_device_fd();
- while(1) {
- ufd.revents = 0;
- nr = poll(&ufd, 1, -1);
- if (nr <= 0)
- continue;
- if (ufd.revents == POLLIN)
- handle_device_fd();
- }
函數get_device_fd()返回創建的socket句柄值,並設置到ufd中,最後ueventd進程進入閉環監控模式,使用poll函數監控ufd,同時將第三個參數設置爲-1,表示只有在監控的socket上有事件發生時,該函數才能返回。當熱插入某一設備時,Linux內核將通過NETLINKsocket 發送uevent事件,此時poll函數得以返回,並調用handle_device_fd()函數來出來設備變化事件:
- void handle_device_fd()
- {
- char msg[UEVENT_MSG_LEN+2];
- int n;
- //從socket中讀取消息內容
- while ((n = uevent_kernel_multicast_recv(device_fd, msg, UEVENT_MSG_LEN)) > 0) {
- //如果讀取的內容長度大於1024,繼續讀取
- if(n >= UEVENT_MSG_LEN) /* overflow -- discard */
- continue;
- msg[n] = '\0';
- msg[n+1] = '\0';
- //將uevent消息解析成uevent類型的事件
- struct uevent uevent;
- parse_event(msg, &uevent);
- //處理uevent事件
- handle_device_event(&uevent);
- handle_firmware_event(&uevent);
- }
- }
當有設備事件發生時,poll函數返回,並從socket中讀取內核發送過來的消息內容,並將該消息解析成uevent事件,同時調用handle_device_event函數和handle_firmware_event函數來分別處理設備事件或firmware事件
- static void handle_device_event(struct uevent *uevent)
- {
- //如果是設備添加事件
- if (!strcmp(uevent->action,"add"))
- fixup_sys_perms(uevent->path);
- //塊設備事件
- if (!strncmp(uevent->subsystem, "block", 5)) {
- handle_block_device_event(uevent);
- //平臺設備事件
- } else if (!strncmp(uevent->subsystem, "platform", 8)) {
- handle_platform_device_event(uevent);
- //通用設備事件
- } else {
- handle_generic_device_event(uevent);
- }
- }
- static void handle_firmware_event(struct uevent *uevent)
- {
- pid_t pid;
- int ret;
- if(strcmp(uevent->subsystem, "firmware"))
- return;
- if(strcmp(uevent->action, "add"))
- return;
- //創建一個線程來專門執行firmware事件
- /* we fork, to avoid making large memory allocations in init proper */
- pid = fork();
- if (!pid) {
- process_firmware_event(uevent);
- exit(EXIT_SUCCESS);
- }
- }
具體的處理過程這裏不在詳細分析,讀者有興趣請自行分析!至此就介紹完了整個ueventd進程的工作,