namei.c

在VC平台下调试通过的linux 0.11文件系统代码！ 对学习文件系统实现的朋友很有帮助

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

/*
 * Some corrections by tytso.
 */


#include <string.h>

#include "errno.h"
#include "fs.h"

#define ACC_MODE(x) ("\004\002\006\377"[(x)&O_ACCMODE])

/*
 * 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
#define MAY_WRITE 2
#define MAY_READ 4

/*
 *	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)
{
	int mode = inode->i_mode;

/* special case: not even root can read/write a deleted file */
	if (inode->i_dev && !inode->i_nlinks)
		return 0;

	//我不需要用户检测
	/*else if (current->euid==inode->i_uid)
		mode >>= 6;
	else if (current->egid==inode->i_gid)
		mode >>= 3;
		*/

	/*suser 应该是判断是否是超级权限的用户*/
	/*if (((mode & mask & 0007) == mask) || suser())
		return 1;*/

	//xly
	if (((mode & mask & 0007) == mask))
		return 1;
	return 1;
}

/*
 * 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)
{
	/*register int same __asm__("ax");*/

	
	int i;
	if (!de || !de->inode || len > NAME_LEN)
		return 0;
	if (len < NAME_LEN && de->name[len])
		return 0;

	//xly
	for (i = 0;i < len; i++)
		if (name[i] != de->name[i])
			return 0;

	/*
	__asm__("cld\n\t"
		"fs ; repe ; cmpsb\n\t"
		"setz %%al"
		:"=a" (same)
		:"0" (0),"S" ((long) name),"D" ((long) de->name),"c" (len)
		:"cx","di","si");
		*/
    
	//return same;
		return 1;
}

/*
 *	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,
	const char * name, int namelen, struct dir_entry ** res_dir)
{
	int entries;
	int block,i;
	struct buffer_head * bh;
	struct dir_entry * de;
	struct super_block * sb;

#ifdef NO_TRUNCATE
	if (namelen > NAME_LEN)
		return NULL;
#else
	if (namelen > NAME_LEN)
		namelen = NAME_LEN;
#endif
	entries = (*dir)->i_size / (sizeof (struct dir_entry));
	*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)
			namelen=1;
		else if ((*dir)->i_num == ROOT_INO) {
/* '..' 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++;
			}
		}
	}

	//目录项为空,退出去
	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) {

		//如果当前目录项数据块已经搜索完，还没有找到匹配的目录项，则释放当前目录项数据块。
		if ((char *)de >= BLOCK_SIZE+bh->b_data) {
			brelse(bh);
			bh = NULL;

			// 在读入下一目录项数据块。若这块为空，则只要还没有搜索完目录中的所有目录项，就跳过该块，
			// 继续读下一目录项数据块。若该块不空，就让de 指向该目录项数据块，继续搜索。
			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;
		}

		//是否找到

		if (match(namelen,name,de)) {
			*res_dir = de;
			return bh;
		}
		de++;
		i++;
	}
	brelse(bh);
	return NULL;
}

char * iname(struct m_inode * inode, char * name)
{
	/**/
	struct m_inode * dir;
	int  entries;
	int block, i;
	struct buffer_head * bh;
	struct dir_entry * de;

	//偷懒,换一下pwd
	struct m_inode * old_pwd = current->pwd = inode;
	current->pwd = inode;

	if (!(dir = namei("..")))
		return NULL;
		//目录项为空,退出去
	entries = dir->i_size / (sizeof (struct dir_entry));
	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) {

		//如果当前目录项数据块已经搜索完，还没有找到匹配的目录项，则释放当前目录项数据块。
		if ((char *)de >= BLOCK_SIZE+bh->b_data) {
			brelse(bh);
			bh = NULL;

			// 在读入下一目录项数据块。若这块为空，则只要还没有搜索完目录中的所有目录项，就跳过该块，
			// 继续读下一目录项数据块。若该块不空，就让de 指向该目录项数据块，继续搜索。
			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;
		}

		//是否找到

		if (de->inode == inode->i_num) {
			memcpy(name, de->name, NAME_LEN-1);
			brelse(bh);
			current->pwd = old_pwd;
			return de->name;
		}
		de++;
		i++;
	}

	brelse(bh);

	current->pwd = old_pwd;
	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,
	const char * name, int namelen, struct dir_entry ** res_dir)
{
	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)
{
	char c;
	const char * thisname;
	struct m_inode * inode;
	struct buffer_head * bh;
	int namelen,inr,idev;
	struct dir_entry * de;

	//和进程相关,判断权限之类操作
	if (!current->root || !current->root->i_count)
		panic("No root inode");
	if (!current->pwd || !current->pwd->i_count)
		panic("No cwd inode");


	//如果pathname第一个字符为'/',表示给出的是绝对路径
	if ((c=get_fs_byte(pathname))=='/') {
		inode = current->root;
		pathname++;

		//否则相对路径
	} else if (c)
		inode = current->pwd;
	   //空名
	else
		return NULL;	/* empty name is bad */



	inode->i_count++;
	while (1) {
		thisname = pathname;

		//判断该节点是否为目录,有访问权限
		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,
	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)
{
	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.
 */