我們?cè)诓僮饕粋€(gè)調(diào)和時(shí)都 是通過(guò)open系統(tǒng)調(diào)用先去打開這個(gè)設(shè)備，不管是設(shè)備還是文件，我們要訪問(wèn)它都要稱通過(guò)open函數(shù)來(lái)先打開， 這樣才能調(diào)用其它的函數(shù)如read、write來(lái)操作它,即通知內(nèi)核新建一個(gè)代表該文件的結(jié)構(gòu)，并且返回該文件的描述符(一個(gè)整數(shù))，該描述符在進(jìn)程內(nèi)唯一。

在Linux系統(tǒng)進(jìn)程中，分為內(nèi)核空間和用戶空間，當(dāng)一個(gè)任務(wù)（進(jìn)程）執(zhí)行系統(tǒng)調(diào)用而陷入內(nèi)核代碼中執(zhí)行時(shí)，我們就稱進(jìn)程處于內(nèi)核運(yùn)行態(tài)（內(nèi)核態(tài)）。在內(nèi)核態(tài)下，CPU可執(zhí)行任何指令。當(dāng)進(jìn)程在執(zhí)行用戶自己的代碼時(shí)，則稱其處于用戶運(yùn)行態(tài)（用戶態(tài)）。用戶態(tài)不能訪問(wèn)內(nèi)核空間，包括代碼和數(shù)據(jù)。所有進(jìn)程的內(nèi)核空間（3G－4G）都是共享的。當(dāng)我們?cè)谟脩艨臻g調(diào)用open之后，會(huì)產(chǎn)生一個(gè)軟中斷，然后通過(guò)系統(tǒng)調(diào)用進(jìn)入內(nèi)核空間。通過(guò)系統(tǒng)調(diào)用號(hào)，我們就可以跳轉(zhuǎn)到該中斷例程的入口地址。

這里分析一下open的調(diào)用流程.

open()的格式如下：

int open(const char * pathname,int oflag, mode_t mode )
pathname:代表需要打開的文件的文件名；
oflag：表示打開的標(biāo)識(shí)，具體的內(nèi)核支持如下標(biāo)記位(includeasm-genericfcntl.h)：

[cpp]?view plain?copy

#define?O_ACCMODE???00000003??

#define?O_RDONLY????00000000??//只讀打開??

#define?O_WRONLY????00000001??//只寫打開??

#define?O_RDWR??????00000002??//讀寫打開??

#ifndef?O_CREAT??

#define?O_CREAT?????00000100????//文件不存在則創(chuàng)建，需要mode_t??

#endif??

#ifndef?O_EXCL??

#define?O_EXCL??????00000200????//如果同時(shí)指定了O_CREAT，而文件已經(jīng)存在，則出錯(cuò)???

#endif??

#ifndef?O_NOCTTY??

#define?O_NOCTTY????00000400????//如果pathname代表終端設(shè)備，則不將此設(shè)備分配作為此進(jìn)程的控制終端??

#endif??

#ifndef?O_TRUNC??

#define?O_TRUNC?????00001000????//如果此文件存在，而且為只讀或只寫成功打開，則將其長(zhǎng)度截短為0???

#endif??

#ifndef?O_APPEND??

#define?O_APPEND????00002000????//每次寫時(shí)都加到文件的尾端??

#endif??

#ifndef?O_NONBLOCK??

#define?O_NONBLOCK??00004000?????//如果pathname指的是一個(gè)FIFO、一個(gè)塊特殊文件或一個(gè)字符特殊文件，則此選擇項(xiàng)為此文件的本次打開操作和后續(xù)的I?/?O操作設(shè)置非阻塞??

#endif??

#ifndef?O_SYNC??

#define?O_SYNC??????00010000????//使每次write都等到物理I/O操作完成??

#endif??

#ifndef?FASYNC??

#define?FASYNC??????00020000????//兼容BSD的fcntl同步操作??

#endif??

#ifndef?O_DIRECT??

#define?O_DIRECT????00040000????//直接磁盤操作標(biāo)識(shí)，每次讀寫都不使用內(nèi)核提供的緩存，直接讀寫磁盤設(shè)備??

#endif??

#ifndef?O_LARGEFILE??

#define?O_LARGEFILE?00100000????//?大文件標(biāo)識(shí)??

#endif??

#ifndef?O_DIRECTORY??

#define?O_DIRECTORY?00200000????//必須是目錄??

#endif??

#ifndef?O_NOFOLLOW??

#define?O_NOFOLLOW??00400000????//不獲取連接文件??

#endif??

#ifndef?O_NOATIME??

#define?O_NOATIME???01000000??

#endif??

#ifndef?O_CLOEXEC??

#define?O_CLOEXEC???02000000????/*?set?close_on_exec?*/??

#endif??

#ifndef?O_NDELAY??

#define?O_NDELAY????O_NONBLOCK??

#endif??

當(dāng)新創(chuàng)建一個(gè)文件時(shí)，需要指定mode參數(shù)，以下說(shuō)明的格式如宏定義名稱<實(shí)際常數(shù)值>:
?描述如下(includelinuxstat.h)：

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#define?S_IRWXU?00700???//文件擁有者有讀寫執(zhí)行權(quán)限??

#define?S_IRUSR?00400???//文件擁有者僅有讀權(quán)限??????

#define?S_IWUSR?00200???//文件擁有者僅有寫權(quán)限??

#define?S_IXUSR?00100???//文件擁有者僅有執(zhí)行權(quán)限??

#define?S_IRWXG?00070???//組用戶有讀寫執(zhí)行權(quán)限??

#define?S_IRGRP?00040???//組用戶僅有讀權(quán)限??

#define?S_IWGRP?00020???//組用戶僅有寫權(quán)限??

#define?S_IXGRP?00010???//組用戶僅有執(zhí)行權(quán)限??

#define?S_IRWXO?00007???//其他用戶有讀寫執(zhí)行權(quán)限??

#define?S_IROTH?00004???//其他用戶僅有讀權(quán)限??

#define?S_IWOTH?00002???//其他用戶僅有寫權(quán)限??

#define?S_IXOTH?00001???//其他用戶僅有執(zhí)行權(quán)限??

系統(tǒng)調(diào)用號(hào)定義在arch/x86/include/asm/unistd_32.h中：

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#define?__NR_restart_syscall??????0??

#define?__NR_exit?????????1??

#define?__NR_fork?????????2??

#define?__NR_read?????????3??

#define?__NR_write????????4??

#define?__NR_open?????????5??

#define?__NR_close????????6??

#define?__NR_waitpid??????????7??

#define?__NR_creat????????8??

#define?__NR_link?????????9??

#define?__NR_unlink??????10??

#define?__NR_execve??????11??

#define?__NR_chdir???????12??

#define?__NR_time????????13??

#define?__NR_mknod???????14??

#define?__NR_chmod???????15??

當(dāng)open系統(tǒng)調(diào)用產(chǎn)生時(shí)，就會(huì)進(jìn)入下面這個(gè)函數(shù)（）：

[cpp]?view plain?copy

SYSCALL_DEFINE3(open,?const?char?__user?*,?filename,?int,?flags,?int,?mode)??

{??

long?ret;??

/*檢查是否應(yīng)該不考慮用戶層傳遞的標(biāo)志、總是強(qiáng)行設(shè)置??

O_LARGEFILE標(biāo)志。如果底層處理器的字長(zhǎng)不是32位，就是這種??

情況*/???

if?(force_o_largefile())??

flags?|=?O_LARGEFILE;??

/*實(shí)際工作*/???

ret?=?do_sys_open(AT_FDCWD,?filename,?flags,?mode);??

/*?avoid?REGPARM?breakage?on?x86:?*/??

asmlinkage_protect(3,?ret,?filename,?flags,?mode);??

return?ret;??

}??

我們看下SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, int, mode) 展開是怎么樣的

首先看下宏SYSCALL_DEFINE3

[cpp]?view plain?copy

#define?SYSCALL_DEFINE3(name,?...)?SYSCALL_DEFINEx(3,?_##name,?__VA_ARGS__)??

再看下SYSCALL_DEFINEx

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#define?SYSCALL_DEFINEx(x,?sname,?...)????????????????

__SYSCALL_DEFINEx(x,?sname,?__VA_ARGS__)??

再看下__SYSCALL_DEFINEx ??

[cpp]?view plain?copy

#define?__SYSCALL_DEFINEx(x,?name,?...)???????????????????

asmlinkage?long?sys##name(__SC_DECL##x(__VA_ARGS__))??

這里對(duì)對(duì)應(yīng)__SC_DECL3

[cpp]?view plain?copy

#define?__SC_DECL1(t1,?a1)??t1?a1??

#define?__SC_DECL2(t2,?a2,?...)?t2?a2,?__SC_DECL1(__VA_ARGS__)??

#define?__SC_DECL3(t3,?a3,?...)?t3?a3,?__SC_DECL2(__VA_ARGS__)??

這們一步步展開SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, int, mode)代替進(jìn)去，可以得到?

[cpp]?view plain?copy

SYSCALL_DEFINE3(open,?const?char?__user?*,?filename,?int,?flags,?int,?mode)??

=?SYSCALL_DEFINEx(3,?_##name,?__VA_ARGS__)???

=asmlinkage?long?sys_open(__SC_DECL3(__VA_ARGS__))??

=asmlinkage?long?sys_open(const?char?__user*?filename,?int?flags,?int?mode)??

這個(gè)才是真正的函數(shù)原型

在sys_open里面繼續(xù)調(diào)用do_sys_open完成 open操作

[cpp]?view plain?copy

long?do_sys_open(int?dfd,?const?char?__user?*filename,?int?flags,?int?mode)??

{??

/*從進(jìn)程地址空間讀取該文件的路徑名*/????

char?*tmp?=?getname(filename);??

int?fd?=?PTR_ERR(tmp);??

if?(!IS_ERR(tmp))?{??

/*在內(nèi)核中，每個(gè)打開的文件由一個(gè)文件描述符表示??

該描述符在特定于進(jìn)程的數(shù)組中充當(dāng)位置索引(數(shù)組是??

task_struct->files->fd_arry)，該數(shù)組的元素包含了file結(jié)構(gòu)，其中??

包括每個(gè)打開文件的所有必要信息。因此，調(diào)用下面??

函數(shù)查找一個(gè)未使用的文件描述符,返回的是上面??

說(shuō)的數(shù)組的下標(biāo)*/????

fd?=?get_unused_fd_flags(flags);??

if?(fd?>=?0)?{??

/*fd獲取成功則開始打開文件，此函數(shù)是主要完成打開功能的函數(shù)*/??

//如果分配fd成功，則創(chuàng)建一個(gè)file對(duì)象??

struct?file?*f?=?do_filp_open(dfd,?tmp,?flags,?mode,?0);??

if?(IS_ERR(f))?{??

put_unused_fd(fd);??

fd?=?PTR_ERR(f);??

}?else?{??

/*文件如果打開成功，調(diào)用fsnotify_open()函數(shù)，根據(jù)inode所指定的信息進(jìn)行打開?

函數(shù)（參數(shù)為f）將該文件加入到文件監(jiān)控的系統(tǒng)中。該系統(tǒng)是用來(lái)監(jiān)控文件被打開，創(chuàng)建，?

讀寫，關(guān)閉，修改等操作的*/???

fsnotify_open(f->f_path.dentry);??

/*將文件指針安裝在fd數(shù)組中?

將struct?file?*f加入到fd索引位置處的數(shù)組中。如果后續(xù)過(guò)程中，有對(duì)該文件描述符的?

操作的話，就會(huì)通過(guò)查找該數(shù)組得到對(duì)應(yīng)的文件結(jié)構(gòu)，而后在進(jìn)行相關(guān)操作。*/??

fd_install(fd,?f);??

}??

}??

putname(tmp);??

}??

return?fd;??

}??

該函數(shù)主要分為如下幾個(gè)步驟來(lái)完成打開文件的操作：
1.將文件名參數(shù)從用戶態(tài)拷貝至內(nèi)核，調(diào)用函數(shù)get_name()；
2.從進(jìn)程的文件表中找到一個(gè)空閑的文件表指針，調(diào)用了函數(shù)get_unused_fd_flgas()；
3.完成真正的打開操作，調(diào)用函數(shù)do_filp_open()；
4.將打開的文件添加到進(jìn)程的文件表數(shù)組中，調(diào)用函數(shù)fd_install()；

getname函數(shù)主要的任務(wù)是將文件名filename從用戶態(tài)拷貝至內(nèi)核態(tài)

[cpp]?view plain?copy

char?*?getname(const?char?__user?*?filename)??

{??

char?*tmp,?*result;??

result?=?ERR_PTR(-ENOMEM);??

tmp?=?__getname();?//從內(nèi)核緩存中分配空間；????

if?(tmp)??{??

//將文件名從用戶態(tài)拷貝至內(nèi)核態(tài)；???

int?retval?=?do_getname(filename,?tmp);??

result?=?tmp;??

if?(retval?

__putname(tmp);??

result?=?ERR_PTR(retval);??

}??

}??

audit_getname(result);??

return?result;??

}??

get_unused_fd_flags實(shí)際調(diào)用的是alloc_fd

[cpp]?view plain?copy

#define?get_unused_fd_flags(flags)?alloc_fd(0,?(flags))??

[cpp]?view plain?copy

/*?

*?allocate?a?file?descriptor,?mark?it?busy.?

*/??

int?alloc_fd(unsigned?start,?unsigned?flags)??

{??

struct?files_struct?*files?=?current->files;//獲得當(dāng)前進(jìn)程的files_struct?結(jié)構(gòu)??

unsigned?int?fd;??

int?error;??

struct?fdtable?*fdt;??

spin_lock(&files->file_lock);??

repeat:??

fdt?=?files_fdtable(files);??

fd?=?start;??

if?(fd?next_fd)?//從上一次打開的fd的下一個(gè)fd開始搜索空閑的fd????

fd?=?files->next_fd;??

if?(fd?max_fds)//尋找空閑的fd，返回值為空閑的fd????

fd?=?find_next_zero_bit(fdt->open_fds->fds_bits,??

fdt->max_fds,?fd);??

//如果有必要，即打開的fd超過(guò)max_fds,則需要expand當(dāng)前進(jìn)程的fd表；????

//返回值error<0表示出錯(cuò)，error=0表示無(wú)需expand，error=1表示進(jìn)行了expand；??

error?=?expand_files(files,?fd);??

if?(error?

goto?out;??

/*?

*?If?we?needed?to?expand?the?fs?array?we?

*?might?have?blocked?-?try?again.?

*/??

//error=1表示進(jìn)行了expand，那么此時(shí)需要重新去查找空閑的fd；????

if?(error)??

goto?repeat;??

//設(shè)置下一次查找的起始fd，即本次找到的空閑的fd的下一個(gè)fd，記錄在files->next_fd中；????

if?(start?<=?files->next_fd)??

files->next_fd?=?fd?+?1;??

FD_SET(fd,?fdt->open_fds);??

if?(flags?&?O_CLOEXEC)??

FD_SET(fd,?fdt->close_on_exec);??

else??

FD_CLR(fd,?fdt->close_on_exec);??

error?=?fd;??

#if?1??

/*?Sanity?check?*/??

if?(rcu_dereference(fdt->fd[fd])?!=?NULL)?{??

printk(KERN_WARNING?"alloc_fd:?slot?%d?not?NULL! ",?fd);??

rcu_assign_pointer(fdt->fd[fd],?NULL);??

}??

#endif??

out:??

spin_unlock(&files->file_lock);??

return?error;??

}??

該函數(shù)為需要打開的文件在當(dāng)前進(jìn)程內(nèi)分配一個(gè)空閑的文件描述符fd，該fd就是open()系統(tǒng)調(diào)用的返回值

do_filp_open函數(shù)的一個(gè)重要作用就是根據(jù)傳遞近來(lái)的權(quán)限進(jìn)行分析，并且分析傳遞近來(lái)的路徑名字，根據(jù)路徑名逐個(gè)解析成dentry，并且通過(guò)dentry找到inode，inode就是記錄著該文件相關(guān)的信息， 包括文件的創(chuàng)建時(shí)間和文件屬性所有者等等信息，根據(jù)這些信息就可以找到對(duì)應(yīng)的文件操作方法。在這個(gè)過(guò)程當(dāng)中有一個(gè)臨時(shí)的結(jié)構(gòu)體用于保存在查找過(guò)程中的相關(guān)信息，就是

[cpp]?view plain?copy

struct?nameidata?{??

struct?path?path;//當(dāng)前目錄的dentry數(shù)據(jù)結(jié)構(gòu)??

struct?qstr?last;//這個(gè)結(jié)構(gòu)體也是臨時(shí)性的，主要用來(lái)保存當(dāng)前目錄的名稱，雜湊值。??

unsigned?int????flags;??

int?????last_type;??

unsigned????depth;//連接文件的深度（可能一個(gè)連接文件跟到最后還是一個(gè)了連接文件）??

//用來(lái)保存連接文件的一些信息，下標(biāo)表示連接文件的深度??

char?*saved_names[MAX_NESTED_LINKS?+?1];??

/*?Intent?data?*/??

union?{??

struct?open_intent?open;??

}?intent;??

};??

[cpp]?view plain?copy

struct?file?*do_filp_open(int?dfd,?const?char?*pathname,??

int?open_flag,?int?mode,?int?acc_mode)??

{??

struct?file?*filp;??

struct?nameidata?nd;??

int?error;??

struct?path?path;??

int?count?=?0;??

int?flag?=?open_to_namei_flags(open_flag);?/*改變參數(shù)flag的值，具體做法是flag+1*/???

int?force_reval?=?0;??

if?(!(open_flag?&?O_CREAT))??

mode?=?0;??

/*?

*?O_SYNC?is?implemented?as?__O_SYNC|O_DSYNC.??As?many?places?only?

*?check?for?O_DSYNC?if?the?need?any?syncing?at?all?we?enforce?it's?

*?always?set?instead?of?having?to?deal?with?possibly?weird?behaviour?

*?for?malicious?applications?setting?only?__O_SYNC.?

*/??

if?(open_flag?&?__O_SYNC)/*根據(jù)__O_SYNC標(biāo)志來(lái)設(shè)置O_DSYNC?標(biāo)志，用以防止惡意破壞程序*/???

open_flag?|=?O_DSYNC;??

if?(!acc_mode)/*設(shè)置訪問(wèn)權(quán)限*/???

acc_mode?=?MAY_OPEN?|?ACC_MODE(open_flag);??

/*?O_TRUNC?implies?we?need?access?checks?for?write?permissions?*/??

if?(open_flag?&?O_TRUNC)/*根據(jù)?O_TRUNC標(biāo)志設(shè)置寫權(quán)限?*/???

acc_mode?|=?MAY_WRITE;??

/*?Allow?the?LSM?permission?hook?to?distinguish?append??

access?from?general?write?access.?*/??

if?(open_flag?&?O_APPEND)/*?設(shè)置O_APPEND?標(biāo)志*/????

acc_mode?|=?MAY_APPEND;??

/*?find?the?parent?*/??

reval:??

error?=?path_init(dfd,?pathname,?LOOKUP_PARENT,?&nd);//初始化nd??

if?(error)??

return?ERR_PTR(error);??

if?(force_reval)??

nd.flags?|=?LOOKUP_REVAL;??

current->total_link_count?=?0;??

error?=?link_path_walk(pathname,?&nd);//路徑名解析函數(shù)，將一個(gè)路徑名最終轉(zhuǎn)化為一個(gè)dentry??

if?(error)?{??

filp?=?ERR_PTR(error);??

goto?out;??

}??

if?(unlikely(!audit_dummy_context())?&&?(open_flag?&?O_CREAT))??

audit_inode(pathname,?nd.path.dentry);??

/*?

*?We?have?the?parent?and?last?component.?

*/??

error?=?-ENFILE;??

filp?=?get_empty_filp();//?從進(jìn)程文件表中獲取一個(gè)未使用的文件結(jié)構(gòu)指針，空則出錯(cuò)返回??

if?(filp?==?NULL)??

goto?exit_parent;??

nd.intent.open.file?=?filp;??

filp->f_flags?=?open_flag;??

nd.intent.open.flags?=?flag;??

nd.intent.open.create_mode?=?mode;??

nd.flags?&=?~LOOKUP_PARENT;??

nd.flags?|=?LOOKUP_OPEN;??

if?(open_flag?&?O_CREAT)?{??

nd.flags?|=?LOOKUP_CREATE;??

if?(open_flag?&?O_EXCL)??

nd.flags?|=?LOOKUP_EXCL;??

}??

if?(open_flag?&?O_DIRECTORY)??

nd.flags?|=?LOOKUP_DIRECTORY;??

if?(!(open_flag?&?O_NOFOLLOW))??

nd.flags?|=?LOOKUP_FOLLOW;??

filp?=?do_last(&nd,?&path,?open_flag,?acc_mode,?mode,?pathname);//返回一個(gè)file結(jié)構(gòu)??

while?(unlikely(!filp))?{?/*?trailing?symlink?*///符號(hào)鏈接??

struct?path?holder;??

struct?inode?*inode?=?path.dentry->d_inode;??

void?*cookie;??

error?=?-ELOOP;??

/*?S_ISDIR?part?is?a?temporary?automount?kludge?*/??

if?(!(nd.flags?&?LOOKUP_FOLLOW)?&&?!S_ISDIR(inode->i_mode))??

goto?exit_dput;??

if?(count++?==?32)??

goto?exit_dput;??

/*?

*?This?is?subtle.?Instead?of?calling?do_follow_link()?we?do?

*?the?thing?by?hands.?The?reason?is?that?this?way?we?have?zero?

*?link_count?and?path_walk()?(called?from?->follow_link)?

*?honoring?LOOKUP_PARENT.??After?that?we?have?the?parent?and?

*?last?component,?i.e.?we?are?in?the?same?situation?as?after?

*?the?first?path_walk().??Well,?almost?-?if?the?last?component?

*?is?normal?we?get?its?copy?stored?in?nd->last.name?and?we?will?

*?have?to?putname()?it?when?we?are?done.?Procfs-like?symlinks?

*?just?set?LAST_BIND.?

*/??

nd.flags?|=?LOOKUP_PARENT;??

error?=?security_inode_follow_link(path.dentry,?&nd);??

if?(error)??

goto?exit_dput;??

error?=?__do_follow_link(&path,?&nd,?&cookie);//查找符號(hào)鏈接對(duì)應(yīng)的目錄中的最后一項(xiàng)??

if?(unlikely(error))?{??

/*?nd.path?had?been?dropped?*/??

if?(!IS_ERR(cookie)?&&?inode->i_op->put_link)??

inode->i_op->put_link(path.dentry,?&nd,?cookie);??

path_put(&path);??

release_open_intent(&nd);??

filp?=?ERR_PTR(error);??

goto?out;??

}??

holder?=?path;??

nd.flags?&=?~LOOKUP_PARENT;??

filp?=?do_last(&nd,?&path,?open_flag,?acc_mode,?mode,?pathname);??

if?(inode->i_op->put_link)??

inode->i_op->put_link(holder.dentry,?&nd,?cookie);??

path_put(&holder);??

}??

out:??

if?(nd.root.mnt)??

path_put(&nd.root);??

if?(filp?==?ERR_PTR(-ESTALE)?&&?!force_reval)?{??

force_reval?=?1;??

goto?reval;??

}??

return?filp;//成功，返回??

exit_dput:??

path_put_conditional(&path,?&nd);??

if?(!IS_ERR(nd.intent.open.file))??

release_open_intent(&nd);??

exit_parent:??

path_put(&nd.path);??

filp?=?ERR_PTR(error);??

goto?out;??

}??

當(dāng)內(nèi)核要訪問(wèn)一個(gè)文件的時(shí)候，第一步要做的是找到這個(gè)文件，而查找文件的過(guò)程在vfs里面是由link_path_walk函數(shù)來(lái)完成的,在path_init的時(shí)候我們可以看到傳進(jìn)去的參數(shù)有一個(gè)LOOKUP_PARENT，它的含義是查找最后一個(gè)分量名所在的目錄。也就是當(dāng)這個(gè)函數(shù)返回的時(shí)候，我們得到了一個(gè)路徑名中最后一個(gè)分量所在的目錄。

接著調(diào)用do_last返回最后一個(gè)分量對(duì)應(yīng)的file指針，我們關(guān)注一下這個(gè)函數(shù)

[cpp]?view plain?copy

static?struct?file?*do_last(struct?nameidata?*nd,?struct?path?*path,??

int?open_flag,?int?acc_mode,??

int?mode,?const?char?*pathname)??

{??

struct?dentry?*dir?=?nd->path.dentry;??

struct?file?*filp;??

int?error?=?-EISDIR;??

switch?(nd->last_type)?{//?檢查最后一段文件或目錄名的屬性情況??

case?LAST_DOTDOT:??

follow_dotdot(nd);??

dir?=?nd->path.dentry;??

case?LAST_DOT:??

if?(nd->path.mnt->mnt_sb->s_type->fs_flags?&?FS_REVAL_DOT)?{??

if?(!dir->d_op->d_revalidate(dir,?nd))?{??

error?=?-ESTALE;??

goto?exit;??

}??

}??

/*?fallthrough?*/??

case?LAST_ROOT:??

if?(open_flag?&?O_CREAT)??

goto?exit;??

/*?fallthrough?*/??

case?LAST_BIND:??

audit_inode(pathname,?dir);??

goto?ok;??

}??

/*?trailing?slashes??*/??

if?(nd->last.name[nd->last.len])?{??

if?(open_flag?&?O_CREAT)??

goto?exit;??

nd->flags?|=?LOOKUP_DIRECTORY?|?LOOKUP_FOLLOW;??

}??

/*?just?plain?open??*/??

if?(!(open_flag?&?O_CREAT))?{//沒有創(chuàng)建標(biāo)志，即文件存在??

error?=?do_lookup(nd,?&nd->last,?path);//找到路徑中最后一項(xiàng)對(duì)應(yīng)的目錄項(xiàng)??

if?(error)??

goto?exit;??

error?=?-ENOENT;??

if?(!path->dentry->d_inode)??

goto?exit_dput;??

if?(path->dentry->d_inode->i_op->follow_link)??

return?NULL;??

error?=?-ENOTDIR;??

if?(nd->flags?&?LOOKUP_DIRECTORY)?{??

if?(!path->dentry->d_inode->i_op->lookup)??

goto?exit_dput;??

}??

path_to_nameidata(path,?nd);//賦值到nd結(jié)構(gòu)??

audit_inode(pathname,?nd->path.dentry);??

goto?ok;??

}??

/*?OK,?it's?O_CREAT?*/??

//文件不存在，需要?jiǎng)?chuàng)建??

mutex_lock(&dir->d_inode->i_mutex);??

path->dentry?=?lookup_hash(nd);//獲取最后路徑名中最后一項(xiàng)對(duì)應(yīng)的目錄項(xiàng)??

path->mnt?=?nd->path.mnt;??

error?=?PTR_ERR(path->dentry);??

if?(IS_ERR(path->dentry))?{??

mutex_unlock(&dir->d_inode->i_mutex);??

goto?exit;??

}??

if?(IS_ERR(nd->intent.open.file))?{??

error?=?PTR_ERR(nd->intent.open.file);??

goto?exit_mutex_unlock;??

}??

/*?Negative?dentry,?just?create?the?file?*/??

if?(!path->dentry->d_inode)?{//沒有索引節(jié)點(diǎn)與目錄項(xiàng)關(guān)聯(lián)??

/*?

*?This?write?is?needed?to?ensure?that?a?

*?ro->rw?transition?does?not?occur?between?

*?the?time?when?the?file?is?created?and?when?

*?a?permanent?write?count?is?taken?through?

*?the?'struct?file'?in?nameidata_to_filp().?

*/??

error?=?mnt_want_write(nd->path.mnt);??

if?(error)??

goto?exit_mutex_unlock;??

error?=?__open_namei_create(nd,?path,?open_flag,?mode);//創(chuàng)建相應(yīng)的索引節(jié)點(diǎn)??

if?(error)?{??

mnt_drop_write(nd->path.mnt);??

goto?exit;??

}??

filp?=?nameidata_to_filp(nd);/*根據(jù)nameidata?得到相應(yīng)的file結(jié)構(gòu)*/??

mnt_drop_write(nd->path.mnt);??

if?(!IS_ERR(filp))?{??

error?=?ima_file_check(filp,?acc_mode);??

if?(error)?{??

fput(filp);??

filp?=?ERR_PTR(error);??

}??

}??

return?filp;??

}??

/*?

*?It?already?exists.?

*/??

mutex_unlock(&dir->d_inode->i_mutex);??

audit_inode(pathname,?path->dentry);??

error?=?-EEXIST;??

if?(open_flag?&?O_EXCL)??

goto?exit_dput;??

if?(__follow_mount(path))?{??

error?=?-ELOOP;??

if?(open_flag?&?O_NOFOLLOW)??

goto?exit_dput;??

}??

error?=?-ENOENT;??

if?(!path->dentry->d_inode)??

goto?exit_dput;??

if?(path->dentry->d_inode->i_op->follow_link)??

return?NULL;??

path_to_nameidata(path,?nd);??

error?=?-EISDIR;??

if?(S_ISDIR(path->dentry->d_inode->i_mode))??

goto?exit;??

ok:??

filp?=?finish_open(nd,?open_flag,?acc_mode);//完成文件打開操作??

return?filp;??

exit_mutex_unlock:??

mutex_unlock(&dir->d_inode->i_mutex);??

exit_dput:??

path_put_conditional(path,?nd);??

exit:??

if?(!IS_ERR(nd->intent.open.file))??

release_open_intent(nd);??

path_put(&nd->path);??

return?ERR_PTR(error);??

}??

首先進(jìn)行一些判斷，然后看是否需要?jiǎng)?chuàng)建文件，如果需要?jiǎng)?chuàng)建的，則創(chuàng)建文件。如果文件存在的話，直接調(diào)用finish_open完成文件打開，我們這里關(guān)注下打開文件的

[cpp]?view plain?copy

static?struct?file?*finish_open(struct?nameidata?*nd,??

int?open_flag,?int?acc_mode)??

{??

struct?file?*filp;??

int?will_truncate;??

int?error;??

/*檢測(cè)是否截?cái)辔募?biāo)志*/??

will_truncate?=?open_will_truncate(open_flag,?nd->path.dentry->d_inode);??

if?(will_truncate)?{/*要截?cái)嗟脑捑鸵@取寫權(quán)限*/??

error?=?mnt_want_write(nd->path.mnt);??

if?(error)??

goto?exit;??

}??

//may_open執(zhí)行權(quán)限檢測(cè)、文件打開和truncate的操作??

error?=?may_open(&nd->path,?acc_mode,?open_flag);??

if?(error)?{??

if?(will_truncate)??

mnt_drop_write(nd->path.mnt);??

goto?exit;??

}??

filp?=?nameidata_to_filp(nd);???/*根據(jù)nameidata?得到相應(yīng)的file結(jié)構(gòu)*/??

if?(!IS_ERR(filp))?{??

error?=?ima_file_check(filp,?acc_mode);??

if?(error)?{??

fput(filp);??

filp?=?ERR_PTR(error);??

}??

}??

if?(!IS_ERR(filp))?{??

if?(will_truncate)?{//?//處理截?cái)??

error?=?handle_truncate(&nd->path);??

if?(error)?{??

fput(filp);??

filp?=?ERR_PTR(error);??

}??

}??

}??

/*?

*?It?is?now?safe?to?drop?the?mnt?write?

*?because?the?filp?has?had?a?write?taken?

*?on?its?behalf.?

*/??

if?(will_truncate)??//安全的放棄寫權(quán)限??

mnt_drop_write(nd->path.mnt);??

return?filp;??

exit:??

if?(!IS_ERR(nd->intent.open.file))??

release_open_intent(nd);??

path_put(&nd->path);??

return?ERR_PTR(error);??

}??

這里主要調(diào)用nameidata_to_filp得到相應(yīng)的file結(jié)構(gòu)

[cpp]?view plain?copy

struct?file?*nameidata_to_filp(struct?nameidata?*nd)??

{??

const?struct?cred?*cred?=?current_cred();??

struct?file?*filp;??

/*?Pick?up?the?filp?from?the?open?intent?*/??

filp?=?nd->intent.open.file;///?把相關(guān)?file結(jié)構(gòu)的指針賦予?filp??

/*?Has?the?filesystem?initialised?the?file?for?us??*/??

if?(filp->f_path.dentry?==?NULL)??

filp?=?__dentry_open(nd->path.dentry,?nd->path.mnt,?filp,??

NULL,?cred);??

else??

path_put(&nd->path);??

return?filp;??

}??

調(diào)用__dentry_open

[cpp]?view plain?copy

static?struct?file?*__dentry_open(struct?dentry?*dentry,?struct?vfsmount?*mnt,??

struct?file?*f,??

int?(*open)(struct?inode?*,?struct?file?*),??

const?struct?cred?*cred)??

{??

struct?inode?*inode;??

int?error;??

f->f_mode?=?OPEN_FMODE(f->f_flags)?|?FMODE_LSEEK?|//初始化f_mode??

FMODE_PREAD?|?FMODE_PWRITE;??

inode?=?dentry->d_inode;??

if?(f->f_mode?&?FMODE_WRITE)?{??

error?=?__get_file_write_access(inode,?mnt);??

if?(error)??

goto?cleanup_file;??

if?(!special_file(inode->i_mode))??

file_take_write(f);??

}??

f->f_mapping?=?inode->i_mapping;??

f->f_path.dentry?=?dentry;//初始化目錄項(xiàng)對(duì)象??

f->f_path.mnt?=?mnt;//初始化文件系統(tǒng)對(duì)象??

f->f_pos?=?0;??

f->f_op?=?fops_get(inode->i_fop);//為文件操作建立起所有方法??

file_move(f,?&inode->i_sb->s_files);//把文件對(duì)象插入到文件系統(tǒng)超級(jí)塊的s_files字段所指向的打開文件的鏈表。??

error?=?security_dentry_open(f,?cred);??

if?(error)??

goto?cleanup_all;??

if?(!open?&&?f->f_op)//傳進(jìn)來(lái)的open為NULL??

open?=?f->f_op->open;??

if?(open)?{??

error?=?open(inode,?f);??

if?(error)??

goto?cleanup_all;??

}??

ima_counts_get(f);??

f->f_flags?&=?~(O_CREAT?|?O_EXCL?|?O_NOCTTY?|?O_TRUNC);//初始化f_f_flags??

file_ra_state_init(&f->f_ra,?f->f_mapping->host->i_mapping);//初始化預(yù)讀的數(shù)據(jù)結(jié)構(gòu)??

/*?NB:?we're?sure?to?have?correct?a_ops?only?after?f_op->open?*/??

if?(f->f_flags?&?O_DIRECT)?{//檢查直接IO操作是否可以作用于文件??

if?(!f->f_mapping->a_ops?||??

((!f->f_mapping->a_ops->direct_IO)?&&??

(!f->f_mapping->a_ops->get_xip_mem)))?{??

fput(f);??

f?=?ERR_PTR(-EINVAL);??

}??

}??

return?f;??

cleanup_all:??

fops_put(f->f_op);??

if?(f->f_mode?&?FMODE_WRITE)?{??

put_write_access(inode);??

if?(!special_file(inode->i_mode))?{??

/*?

*?We?don't?consider?this?a?real?

*?mnt_want/drop_write()?pair?

*?because?it?all?happenend?right?

*?here,?so?just?reset?the?state.?

*/??

file_reset_write(f);??

mnt_drop_write(mnt);??

}??

}??

file_kill(f);??

f->f_path.dentry?=?NULL;??

f->f_path.mnt?=?NULL;??

cleanup_file:??

put_filp(f);??

dput(dentry);??

mntput(mnt);??

return?ERR_PTR(error);??

}??

這里主要是進(jìn)行一些賦值操作

對(duì)應(yīng)于這里，傳進(jìn)來(lái)的open指針為NULL， 如果相應(yīng)file_operations結(jié)構(gòu)存在的話就調(diào)用它的open函數(shù)

對(duì)于每個(gè)文件在創(chuàng)建 的時(shí)候會(huì)賦值對(duì)其進(jìn)行操作的file_operations結(jié)構(gòu)，這個(gè)結(jié)構(gòu)對(duì)于一類文件是一樣的，例如 對(duì)應(yīng)于字符設(shè)備是chrdev_open

[cpp]?view plain?copy

const?struct?file_operations?def_chr_fops?=?{??

.open?=?chrdev_open,??

};??

但打開之后，我們可以重新獲取它們的file_operations結(jié)構(gòu)，這個(gè)是在注冊(cè)設(shè)備驅(qū)動(dòng)的時(shí)候?yàn)樵擃愒O(shè)備賦予的，也就是我們?cè)隍?qū)動(dòng)里面實(shí)現(xiàn)的，而前面的缺省file_operations就是為了完成這個(gè)轉(zhuǎn)換的，def_chr_fops只起過(guò)渡作用，它的open方法要去尋找硬件驅(qū)動(dòng)的支撐。

[cpp]?view plain?copy

static?int?chrdev_open(struct?inode?*inode,?struct?file?*filp)??

{??

struct?cdev?*p;??

struct?cdev?*new?=?NULL;??

int?ret?=?0;??

spin_lock(&cdev_lock);??

p?=?inode->i_cdev;??

if?(!p)?{?/*?很顯然，第一次打開的時(shí)候是NULL?*/??

struct?kobject?*kobj;??

int?idx;??

spin_unlock(&cdev_lock);??

kobj?=?kobj_lookup(cdev_map,?inode->i_rdev,?&idx);/*?找到和設(shè)備號(hào)i_rdev對(duì)應(yīng)的kobj，其實(shí)就是cdev了，因?yàn)閏dev中包含kobj；idx保存的是次設(shè)備號(hào)，后面會(huì)分析kobj_lookup()函數(shù)?*/??

if?(!kobj)??

return?-ENXIO;??

new?=?container_of(kobj,?struct?cdev,?kobj);//得到cdev??

spin_lock(&cdev_lock);??

/*?Check?i_cdev?again?in?case?somebody?beat?us?to?it?while?

we?dropped?the?lock.?*/??

p?=?inode->i_cdev;??

if?(!p)?{??

inode->i_cdev?=?p?=?new;/*?把找到的cdev保存到inode的icdev中?*/??

list_add(&inode->i_devices,?&p->list);?/*?inode加入到cdev的鏈表中?*/??

new?=?NULL;??

}?else?if?(!cdev_get(p))??

ret?=?-ENXIO;??

}?else?if?(!cdev_get(p))??

ret?=?-ENXIO;??

spin_unlock(&cdev_lock);??

cdev_put(new);??

if?(ret)??

return?ret;??

ret?=?-ENXIO;??

/*??

保存用戶的fops，以后你再調(diào)用read,?write,?ioctl系統(tǒng)調(diào)用的時(shí)候就直接使用了，你懂的??

*/??

filp->f_op?=?fops_get(p->ops);??

if?(!filp->f_op)?//?如果你沒有注冊(cè)fops??

goto?out_cdev_put;??

if?(filp->f_op->open)?{//判斷open函數(shù)是否存在??

ret?=?filp->f_op->open(inode,filp);//*?調(diào)用用戶的open函數(shù)，我們前面寫的驅(qū)動(dòng)??

if?(ret)??

goto?out_cdev_put;??

}??

return?0;??

out_cdev_put:??

cdev_put(p);??

return?ret;??

}??

在這個(gè)函數(shù)里，我們重新 對(duì)f_op賦值了，這里的f_op就是我們?cè)趯戲?qū)動(dòng)時(shí)寫的系統(tǒng)調(diào)用函數(shù)了。后面還調(diào)用了open方法

這里調(diào)用?kobj_lookup找到前面我們?cè)谧?cè)驅(qū)動(dòng)添加設(shè)備時(shí)添加的相應(yīng)的kobj

[cpp]?view plain?copy

struct?kobject?*kobj_lookup(struct?kobj_map?*domain,?dev_t?dev,?int?*index)??

{??

struct?kobject?*kobj;??

struct?probe?*p;??

unsigned?long?best?=?~0UL;??

retry:??

mutex_lock(domain->lock);??

/*?根據(jù)主設(shè)備號(hào)和設(shè)備號(hào)查找它的一畝三分地。因?yàn)橐С?^12次方也就是4096個(gè)主設(shè)備號(hào)，?

但只使用了前255個(gè)主設(shè)備號(hào)索引，所以這255個(gè)索引對(duì)應(yīng)的probe結(jié)構(gòu)都有一個(gè)單向?

鏈表保存著大于255的主設(shè)備號(hào)（被255整除后的索引相等）??*/??

for?(p?=?domain->probes[MAJOR(dev)?%?255];?p;?p?=?p->next)?{??

struct?kobject?*(*probe)(dev_t,?int?*,?void?*);??

struct?module?*owner;??

void?*data;??

//?比較，看是否真找到了，因?yàn)橛墟湵泶嬖??

if?(p->dev?>?dev?||?p->dev?+?p->range?-?1?

continue;??

if?(p->range?-?1?>=?best)??

break;??

if?(!try_module_get(p->owner))??

continue;??

owner?=?p->owner;??

data?=?p->data;//data就是cdev??

probe?=?p->get;??

best?=?p->range?-?1;??

*index?=?dev?-?p->dev;//得到次設(shè)備號(hào)??

/*?調(diào)用的lock就是exact_lock()函數(shù)，增加對(duì)該字符設(shè)備驅(qū)動(dòng)的引用，防止被卸載什么的?*/??

if?(p->lock?&&?p->lock(dev,?data)?

module_put(owner);??

continue;??

}??

mutex_unlock(domain->lock);??

/*調(diào)用的probe就是exact_match()函數(shù)，獲取cdev的kobj指針?*/??

kobj?=?probe(dev,?index,?data);??

/*?Currently?->owner?protects?_only_?->probe()?itself.?*/??

module_put(owner);??

if?(kobj)??

return?kobj;??

goto?retry;??

}??

mutex_unlock(domain->lock);??

return?NULL; ?

} ?

到這里do_filp_open的流程就基本完成了，即返回了一個(gè)file結(jié)構(gòu)

[cpp]?view plain?copy

void?fd_install(unsigned?int?fd,?struct?file?*file)??

{??

struct?files_struct?*files?=?current->files;??

struct?fdtable?*fdt;??

spin_lock(&files->file_lock);??

fdt?=?files_fdtable(files);//獲取fdtbale??

BUG_ON(fdt->fd[fd]?!=?NULL);??

rcu_assign_pointer(fdt->fd[fd],?file);//fd和file關(guān)系到fdtbale??

spin_unlock(&files->file_lock);??

}??

這樣，我們的open系統(tǒng)調(diào)用就基本上完成了，我們得到一個(gè)fd，這個(gè)fd關(guān)聯(lián)著一個(gè)file結(jié)構(gòu)，這樣以后，我們就可以通過(guò)這個(gè)fd結(jié)構(gòu)操作相應(yīng)的文件了

最后還是一樣看下整個(gè)簡(jiǎn)略的流程圖

?