namei.c.bak

一份精简的linux内核源代码

/*
 *  linux/fs/namei.c
 *
 *  (C) 1991  Linus Torvalds
 */

/*
 * Some corrections by tytso.
 */

#include <linux/sched.h>
#include <linux/kernel.h>
#include <asm/segment.h>

#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <const.h>
#include <sys/stat.h>

#define ACC_MODE(x) ("\004\002\006\377"[(x)&O_ACCMODE])/*根据x获得读写权限（x=0->004,x=1->002....）；O_ACCMODE＝000011为屏蔽码；/002等代表一个字符*/

/*
 * comment out this line if you want names > NAME_LEN chars to be
 * truncated. Else they will be disallowed.
 */
/* #define NO_TRUNCATE */

#define MAY_EXEC 1	/* 可执行 */
#define MAY_WRITE 2/* 可写 */
#define MAY_READ 4/* 可读 */

/*
 *	permission()
 *
 * is used to check for read/write/execute permissions on a file.
 * I don't know if we should look at just the euid or both euid and
 * uid, but that should be easily changed.
 */
static int permission(struct m_inode * inode,int mask)/* 检查文件访问许可权，mask为属性屏蔽码，返回1则访问许可*/
{
	int mode = inode->i_mode;

/* special case: not even root can read/write a deleted file */
	if (inode->i_dev && !inode->i_nlinks)/* i节点有对应设备，但引用计数为0，表示该文件已被删除 */
		return 0;
	else if (current->euid==inode->i_uid)/* 进程的有效用户id(euid)与i节点的用户id相同，则取文件宿主的访问权限 */
		mode >>= 6;
	else if (current->egid==inode->i_gid)/*进程的有效组id（egid）与i节点的组id相同，则取组用户的访问权限 */
		mode >>= 3;
	if (((mode & mask & 0007) == mask) || suser())/* 若所取的访问权限与屏蔽码相同或是超级用户则返回1 */
		return 1;
	return 0;
}

/*
 * ok, we cannot use strncmp, as the name is not in our data space.
 * Thus we'll have to use match. No big problem. Match also makes
 * some sanity tests.
 *
 * NOTE! unlike strncmp, match returns 1 for success, 0 for failure.
 */
static int match(int len,const char * name,struct dir_entry * de)/*目录项中的文件名同指定的文件名比较 */
{
	register int same __asm__("ax");

	if (!de || !de->inode || len > NAME_LEN)
		return 0;
	if (len < NAME_LEN && de->name[len])
		return 0;
	__asm__("cld\n\t"
		"fs ; repe ; cmpsb\n\t"/*在用户空间进行循环比较[edi][esi]指向的字符 */
		"setz %%al"
		:"=a" (same)
		:"0" (0),"S" ((long) name),"D" ((long) de->name),"c" (len)
		:"cx","di","si");
	return same;
}

/*
 *	find_entry()
 *
 * finds an entry in the specified directory with the wanted name. It
 * returns the cache buffer in which the entry was found, and the entry
 * itself (as a parameter - res_dir). It does NOT read the inode of the
 * entry - you'll have to do that yourself if you want to.
 *
 * This also takes care of the few special cases due to '..'-traversal
 * over a pseudo-root and a mount point.
 */
static struct buffer_head * find_entry(struct m_inode ** dir,/* 在指定目录中寻找一个与名字匹配的目录项，返回一个含有目录相的高速缓冲块及目录项本身 */
	const char * name, int namelen, struct dir_entry ** res_dir)/*该函数并不读取目录项i节点 */
{
	int entries;
	int block,i;
	struct buffer_head * bh;
	struct dir_entry * de;
	struct super_block * sb;

#ifdef NO_TRUNCATE /* 当文件名长度过长时是否截断文件名 */
	if (namelen > NAME_LEN)
		return NULL;
#else
	if (namelen > NAME_LEN)
		namelen = NAME_LEN;
#endif
	entries = (*dir)->i_size / (sizeof (struct dir_entry));/* 获得目录中含有的项数（目录的i_size中存放着此目录文件的数据长度） */
	*res_dir = NULL;
	if (!namelen)
		return NULL;
/* check for '..', as we might have to do some "magic" for it */
	if (namelen==2 && get_fs_byte(name)=='.' && get_fs_byte(name+1)=='.') {
/* '..' in a pseudo-root results in a faked '.' (just change namelen) */
		if ((*dir) == current->root)/*若当前进程的根目录为dir，则不能访问dir的父目录 */
			namelen=1;
		else if ((*dir)->i_num == ROOT_INO) {/*若目录为文件系统的根目录，则dir=文件系统被安装到的i节点*/
/* '..' over a mount-point results in 'dir' being exchanged for the mounted
   directory-inode. NOTE! We set mounted, so that we can iput the new dir */
			sb=get_super((*dir)->i_dev);
			if (sb->s_imount) {
				iput(*dir);
				(*dir)=sb->s_imount;
				(*dir)->i_count++;
			}
		}
	}
	if (!(block = (*dir)->i_zone[0]))
		return NULL;
	if (!(bh = bread((*dir)->i_dev,block)))
		return NULL;
	i = 0;
	de = (struct dir_entry *) bh->b_data;
	while (i < entries) {
		if ((char *)de >= BLOCK_SIZE+bh->b_data) {
			brelse(bh);
			bh = NULL;
			if (!(block = bmap(*dir,i/DIR_ENTRIES_PER_BLOCK)) ||
			    !(bh = bread((*dir)->i_dev,block))) {
				i += DIR_ENTRIES_PER_BLOCK;
				continue;
			}
			de = (struct dir_entry *) bh->b_data;
		}
		if (match(namelen,name,de)) {
			*res_dir = de;
			return bh;
		}
		de++;
		i++;
	}
	brelse(bh);
	return NULL;
}

/*
 *	add_entry()
 *
 * adds a file entry to the specified directory, using the same
 * semantics as find_entry(). It returns NULL if it failed.
 *
 * NOTE!! The inode part of 'de' is left at 0 - which means you
 * may not sleep between calling this and putting something into
 * the entry, as someone else might have used it while you slept.
 */
static struct buffer_head * add_entry(struct m_inode * dir,/*向指定的目录中添加一个目录项（只添加了目录项的名字，未添加该名字对应的i节点） */
	const char * name, int namelen, struct dir_entry ** res_dir)/*读入一块并寻找空项，找到则插入，未找到则读入下一块（若该块为空则新建），若无法在插入到中间则插入到在最后块的最后目录项 */
{                                                              
	int block,i;
	struct buffer_head * bh;
	struct dir_entry * de;

	*res_dir = NULL;
#ifdef NO_TRUNCATE
	if (namelen > NAME_LEN)
		return NULL;
#else
	if (namelen > NAME_LEN)
		namelen = NAME_LEN;
#endif
	if (!namelen)
		return NULL;
	if (!(block = dir->i_zone[0]))
		return NULL;
	if (!(bh = bread(dir->i_dev,block)))
		return NULL;
	i = 0;
	de = (struct dir_entry *) bh->b_data;
	while (1) {
		if ((char *)de >= BLOCK_SIZE+bh->b_data) {
			brelse(bh);
			bh = NULL;
			block = create_block(dir,i/DIR_ENTRIES_PER_BLOCK);
			if (!block)
				return NULL;
			if (!(bh = bread(dir->i_dev,block))) {
				i += DIR_ENTRIES_PER_BLOCK;
				continue;
			}
			de = (struct dir_entry *) bh->b_data;
		}
		if (i*sizeof(struct dir_entry) >= dir->i_size) {
			de->inode=0;
			dir->i_size = (i+1)*sizeof(struct dir_entry);
			dir->i_dirt = 1;
			dir->i_ctime = CURRENT_TIME;
		}
		if (!de->inode) {
			dir->i_mtime = CURRENT_TIME;
			for (i=0; i < NAME_LEN ; i++)
				de->name[i]=(i<namelen)?get_fs_byte(name+i):0;
			bh->b_dirt = 1;
			*res_dir = de;
			return bh;
		}
		de++;
		i++;
	}
	brelse(bh);
	return NULL;
}

/*
 *	get_dir()
 *
 * Getdir traverses the pathname until it hits the topmost directory.
 * It returns NULL on failure.
 */
static struct m_inode * get_dir(const char * pathname)/*搜索指定路径名的末端目录i节点（最后一个“/”之前的目录i节点）并返回该i节点 */
{
	char c;
	const char * thisname;
	struct m_inode * inode;
	struct buffer_head * bh;
	int namelen,inr,idev;
	struct dir_entry * de;

	if (!current->root || !current->root->i_count)/*当前进程根目录或工作目录为空或空闲，则出错 */
		panic("No root inode");
	if (!current->pwd || !current->pwd->i_count)
		panic("No cwd inode");
	if ((c=get_fs_byte(pathname))=='/') {/*若路径名的首字符为“/”则 路径名为绝对路径名，从进程根目录开始搜索*/
		inode = current->root;
		pathname++;
	} else if (c) /*若路径名的首字符非“/”则 路径名为相对路径名，从进程工作目录开始搜索*/
		inode = current->pwd;
	else
		return NULL;	/* empty name is bad */
	inode->i_count++;
	while (1) {
		thisname = pathname;
		if (!S_ISDIR(inode->i_mode) || !permission(inode,MAY_EXEC)) {
			iput(inode);
			return NULL;
		}
		for(namelen=0;(c=get_fs_byte(pathname++))&&(c!='/');namelen++)
			/* nothing */ ;
		if (!c)
			return inode;
		if (!(bh = find_entry(&inode,thisname,namelen,&de))) {
			iput(inode);
			return NULL;
		}
		inr = de->inode;
		idev = inode->i_dev;
		brelse(bh);
		iput(inode);
		if (!(inode = iget(idev,inr)))
			return NULL;
	}
}

/*
 *	dir_namei()
 *
 * dir_namei() returns the inode of the directory of the
 * specified name, and the name within that directory.
 */
static struct m_inode * dir_namei(const char * pathname,/*返回指定路径下的末端目录i节点及最后一个“/”后的字符 */
	int * namelen, const char ** name)
{
	char c;
	const char * basename;
	struct m_inode * dir;

	if (!(dir = get_dir(pathname)))
		return NULL;
	basename = pathname;
	while (c=get_fs_byte(pathname++))
		if (c=='/')
			basename=pathname;
	*namelen = pathname-basename-1;
	*name = basename;
	return dir;
}

/*
 *	namei()
 *
 * is used by most simple commands to get the inode of a specified name.
 * Open, link etc use their own routines, but this is enough for things
 * like 'chmod' etc.
 */
struct m_inode * namei(const char * pathname)/*搜索指定路径名的末端i节点并返回该i节点 */
{
	const char * basename;
	int inr,dev,namelen;
	struct m_inode * dir;
	struct buffer_head * bh;
	struct dir_entry * de;

	if (!(dir = dir_namei(pathname,&namelen,&basename)))
		return NULL;
	if (!namelen)			/* special case: '/usr/' etc */
		return dir;
	bh = find_entry(&dir,basename,namelen,&de);
	if (!bh) {
		iput(dir);
		return NULL;
	}
	inr = de->inode;
	dev = dir->i_dev;
	brelse(bh);
	iput(dir);
	dir=iget(dev,inr);
	if (dir) {
		dir->i_atime=CURRENT_TIME;
		dir->i_dirt=1;
	}
	return dir;
}

/*
 *	open_namei()
 *
 * namei for open - this is in fact almost the whole open-routine.
 */
int open_namei(const char * pathname, int flag, int mode,/*打开指定路径的文件（实质是从设备上读入该文件的i节点，并返回该i节点的指针）*/
	struct m_inode ** res_inode)
{
	const char * basename;
	int inr,dev,namelen;
	struct m_inode * dir, *inode;
	struct buffer_head * bh;
	struct dir_entry * de;

	if ((flag & O_TRUNC) && !(flag & O_ACCMODE))/*标志的初始化 */
		flag |= O_WRONLY;
	mode &= 0777 & ~current->umask;
	mode |= I_REGULAR;
	if (!(dir = dir_namei(pathname,&namelen,&basename)))/* 获取指路径的i节点，若路径名以"/"结尾，则返回目录i节点（若对目录执行非法操作如读写则出错退出） */
		return -ENOENT;
	if (!namelen) {			/* special case: '/usr/' etc */ 
		if (!(flag & (O_ACCMODE|O_CREAT|O_TRUNC))) {
			*res_inode=dir;
			return 0;
		}
		iput(dir);
		return -EISDIR;
	}
	bh = find_entry(&dir,basename,namelen,&de);
	if (!bh) {/*创建文件（末端文件不存在（bh==nulll），且创建文件标志置位） */
		if (!(flag & O_CREAT)) {
			iput(dir);
			return -ENOENT;
		}
		if (!permission(dir,MAY_WRITE)) {/*创建文件时还需判断当前进程是否有写权限*/
			iput(dir);
			return -EACCES;
		}
		inode = new_inode(dir->i_dev);/*创建文件 */
		if (!inode) {
			iput(dir);
			return -ENOSPC;
		}
		inode->i_uid = current->euid;
		inode->i_mode = mode;
		inode->i_dirt = 1;
		bh = add_entry(dir,basename,namelen,&de);
		if (!bh) {
			inode->i_nlinks--;
			iput(inode);
			iput(dir);
			return -ENOSPC;
		}
		de->inode = inode->i_num;
		bh->b_dirt = 1;
		brelse(bh);
		iput(dir);
		*res_inode = inode;
		return 0;