namei.c

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/*
 *  linux/fs/namei.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

/*
 * Some corrections by tytso.
 *
 * Aug 97 - cevans - fix security problem with O_TRUNC and append only files
 */

#include <asm/segment.h>

#include <linux/errno.h>
#include <linux/sched.h>
#include <linux/kernel.h>
#include <linux/string.h>
#include <linux/fcntl.h>
#include <linux/stat.h>
#include <linux/mm.h>

#define ACC_MODE(x) ("\000\004\002\006"[(x)&O_ACCMODE])

/*
 * How long a filename can we get from user space?
 *  -EFAULT if invalid area
 *  0 if ok (ENAMETOOLONG before EFAULT)
 *  >0 EFAULT after xx bytes
 */
static inline int get_max_filename(unsigned long address)
{
#ifndef NO_MM
	struct vm_area_struct * vma;
#endif

	if (get_fs() == KERNEL_DS)
		return 0;
#ifndef NO_MM
	vma = find_vma(current->mm, address);
	if (!vma || vma->vm_start > address || !(vma->vm_flags & VM_READ))
		return -EFAULT;
	address = vma->vm_end - address;
	if (address > PAGE_SIZE)
		return 0;
	if (vma->vm_next && vma->vm_next->vm_start == vma->vm_end &&
	   (vma->vm_next->vm_flags & VM_READ))
		return 0;
	return address;
#endif
	return PAGE_SIZE;
}

/*
 * In order to reduce some races, while at the same time doing additional
 * checking and hopefully speeding things up, we copy filenames to the
 * kernel data space before using them..
 *
 * POSIX.1 2.4: an empty pathname is invalid (ENOENT).
 */
int getname(const char * filename, char **result)
{
	int i, error;
	unsigned long page;
	char * tmp, c;

	i = get_max_filename((unsigned long) filename);
	if (i < 0)
		return i;
	error = -EFAULT;
	if (!i) {
		error = -ENAMETOOLONG;
		i = PAGE_SIZE;
	}
	c = get_user(filename++);
	if (!c)
		return -ENOENT;
	if(!(page = __get_free_page(GFP_KERNEL)))
		return -ENOMEM;
	*result = tmp = (char *) page;
	while (--i) {
		*(tmp++) = c;
		c = get_user(filename++);
		if (!c) {
			*tmp = '\0';
			return 0;
		}
	}
	free_page(page);
	return error;
}

void putname(char * name)
{
	free_page((unsigned long) name);
}

/*
 *	permission()
 *
 * is used to check for read/write/execute permissions on a file.
 * We use "fsuid" for this, letting us set arbitrary permissions
 * for filesystem access without changing the "normal" uids which
 * are used for other things..
 */
int permission(struct inode * inode,int mask)
{
	int mode = inode->i_mode;

	if (inode->i_op && inode->i_op->permission)
		return inode->i_op->permission(inode, mask);
	else if ((mask & S_IWOTH) && IS_RDONLY(inode) &&
		 (S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))
		return -EROFS; /* Nobody gets write access to a read-only fs */
	else if ((mask & S_IWOTH) && IS_IMMUTABLE(inode))
		return -EACCES; /* Nobody gets write access to an immutable file */
	else if (current->fsuid == inode->i_uid)
		mode >>= 6;
	else if (in_group_p(inode->i_gid))
		mode >>= 3;
	if (((mode & mask & 0007) == mask) || fsuser())
		return 0;
	return -EACCES;
}

/*
 * get_write_access() gets write permission for a file.
 * put_write_access() releases this write permission.
 * This is used for regular files.
 * We cannot support write (and maybe mmap read-write shared) accesses and
 * MAP_DENYWRITE mmappings simultaneously.
 */
int get_write_access(struct inode * inode)
{
#ifndef NO_MM
	struct task_struct * p;

	if ((inode->i_count > 1) && S_ISREG(inode->i_mode)) /* shortcut */
		for_each_task(p) {
		        struct vm_area_struct * mpnt;
			if (!p->mm)
				continue;
			for(mpnt = p->mm->mmap; mpnt; mpnt = mpnt->vm_next) {
				if (inode != mpnt->vm_inode)
					continue;
				if (mpnt->vm_flags & VM_DENYWRITE)
					return -ETXTBSY;
			}
		}
#endif
	inode->i_writecount++;
	return 0;
}

void put_write_access(struct inode * inode)
{
	inode->i_writecount--;
}

/*
 * lookup() looks up one part of a pathname, using the fs-dependent
 * routines (currently minix_lookup) for it. It also checks for
 * fathers (pseudo-roots, mount-points)
 */
int lookup(struct inode * dir,const char * name, int len,
           struct inode ** result)
{
	struct super_block * sb;
	int perm;

	*result = NULL;
	if (!dir)
		return -ENOENT;
/* check permissions before traversing mount-points */
	perm = permission(dir,MAY_EXEC);
	if (len==2 && name[0] == '.' && name[1] == '.') {
		if (dir == current->fs->root) {
			*result = dir;
			return 0;
		} else if ((sb = dir->i_sb) && (dir == sb->s_mounted)) {
			iput(dir);
			dir = sb->s_covered;
			if (!dir)
				return -ENOENT;
			dir->i_count++;
		}
	}
	if (!dir->i_op || !dir->i_op->lookup) {
		iput(dir);
		return -ENOTDIR;
	}
 	if (perm != 0) {
		iput(dir);
		return perm;
	}
	if (!len) {
		*result = dir;
		return 0;
	}
	return dir->i_op->lookup(dir, name, len, result);
}

int follow_link(struct inode * dir, struct inode * inode,
	int flag, int mode, struct inode ** res_inode)
{
	if (!dir || !inode) {
		iput(dir);
		iput(inode);
		*res_inode = NULL;
		return -ENOENT;
	}
	if (!inode->i_op || !inode->i_op->follow_link) {
		iput(dir);
		*res_inode = inode;
		return 0;
	}
	return inode->i_op->follow_link(dir,inode,flag,mode,res_inode);
}

/*
 *	dir_namei()
 *
 * dir_namei() returns the inode of the directory of the
 * specified name, and the name within that directory.
 */
static int dir_namei(const char *pathname, int *namelen, const char **name,
                     struct inode * base, struct inode **res_inode)
{
	char c;
	const char * thisname;
	int len,error;
	struct inode * inode;

	*res_inode = NULL;
	if (!base) {
		base = current->fs->pwd;
		base->i_count++;
	}
	if ((c = *pathname) == '/') {
		iput(base);
		base = current->fs->root;
		pathname++;
		base->i_count++;
	}
	while (1) {
		thisname = pathname;
		for(len=0;(c = *(pathname++))&&(c != '/');len++)
			/* nothing */ ;
		if (!c)
			break;
		base->i_count++;
		error = lookup(base, thisname, len, &inode);
		if (error) {
			iput(base);
			return error;
		}
		error = follow_link(base,inode,0,0,&base);
		if (error)
			return error;
	}
	if (!base->i_op || !base->i_op->lookup) {
		iput(base);
		return -ENOTDIR;
	}
	*name = thisname;
	*namelen = len;
	*res_inode = base;
	return 0;
}

static int _namei(const char * pathname, struct inode * base,
                  int follow_links, struct inode ** res_inode)
{
	const char *basename;
	int namelen,error;
	struct inode * inode;

	*res_inode = NULL;
	error = dir_namei(pathname, &namelen, &basename, base, &base);
	if (error)
		return error;
	base->i_count++;	/* lookup uses up base */
	error = lookup(base, basename, namelen, &inode);
	if (error) {
		iput(base);
		return error;
	}
	if (follow_links) {
		error = follow_link(base, inode, 0, 0, &inode);
		if (error)
			return error;
	} else
		iput(base);
	if ((inode->i_flags & S_BAD_INODE) != 0) {
		iput(inode);
		return -EIO;
	}
	*res_inode = inode;
	return 0;
}

int lnamei(const char *pathname, struct inode **res_inode)
{
	int error;
	char * tmp;

	error = getname(pathname, &tmp);
	if (!error) {
		error = _namei(tmp, NULL, 0, res_inode);
		putname(tmp);
	}
	return error;
}

/*
 *	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.
 */
int namei(const char *pathname, struct inode **res_inode)
{
	int error;
	char * tmp;

	error = getname(pathname, &tmp);
	if (!error) {
		error = _namei(tmp, NULL, 1, res_inode);
		putname(tmp);
	}
	return error;
}

/*
 *	open_namei()
 *
 * namei for open - this is in fact almost the whole open-routine.
 *
 * Note that the low bits of "flag" aren't the same as in the open
 * system call - they are 00 - no permissions needed
 *			  01 - read permission needed
 *			  10 - write permission needed
 *			  11 - read/write permissions needed
 * which is a lot more logical, and also allows the "no perm" needed
 * for symlinks (where the permissions are checked later).
 */
int open_namei(const char * pathname, int flag, int mode,
               struct inode ** res_inode, struct inode * base)
{
	const char * basename;
	int namelen,error;
	struct inode * dir, *inode;

	mode &= S_IALLUGO & ~current->fs->umask;
	mode |= S_IFREG;
	error = dir_namei(pathname, &namelen, &basename, base, &dir);
	if (error)
		return error;
	if (!namelen) {			/* special case: '/usr/' etc */
		if (flag & 2) {
			iput(dir);
			return -EISDIR;
		}
		/* thanks to Paul Pluzhnikov for noticing this was missing.. */
		if ((error = permission(dir,ACC_MODE(flag))) != 0) {
			iput(dir);
			return error;
		}
		*res_inode=dir;
		return 0;
	}
	dir->i_count++;		/* lookup eats the dir */
	if (flag & O_CREAT) {
		down(&dir->i_sem);
		error = lookup(dir, basename, namelen, &inode);
		if (!error) {
			if (flag & O_EXCL) {
				iput(inode);
				error = -EEXIST;
			}
		} else if (IS_RDONLY(dir))
			error = -EROFS;
		else if (!dir->i_op || !dir->i_op->create)
			error = -EACCES;
		else if ((error = permission(dir,MAY_WRITE | MAY_EXEC)) != 0)
			;	/* error is already set! */
		else {
			dir->i_count++;		/* create eats the dir */
			if (dir->i_sb && dir->i_sb->dq_op)
				dir->i_sb->dq_op->initialize(dir, -1);
			error = dir->i_op->create(dir, basename, namelen, mode, res_inode);
			up(&dir->i_sem);
			iput(dir);
			return error;
		}
		up(&dir->i_sem);
	} else
		error = lookup(dir, basename, namelen, &inode);
	if (error) {
		iput(dir);
		return error;
	}
	error = follow_link(dir,inode,flag,mode,&inode);
	if (error)
		return error;
	if (S_ISDIR(inode->i_mode) && (flag & 2)) {
		iput(inode);
		return -EISDIR;
	}
	if ((error = permission(inode,ACC_MODE(flag))) != 0) {
		iput(inode);
		return error;
	}
	if (S_ISFIFO(inode->i_mode) || S_ISSOCK(inode->i_mode)) {
		/*
		 * 2-Feb-1995 Bruce Perens <[e-mail]>
		 * Allow opens of Unix domain sockets and FIFOs for write on
		 * read-only filesystems. Their data does not live on the disk.
		 *
		 * If there was something like IS_NODEV(inode) for
		 * pipes and/or sockets I'd check it here.
		 */
	    	flag &= ~O_TRUNC;
	}
	else if (S_ISBLK(inode->i_mode) || S_ISCHR(inode->i_mode)) {
		if (IS_NODEV(inode)) {
			iput(inode);
			return -EACCES;
		}
		flag &= ~O_TRUNC;
	} else {
		if (IS_RDONLY(inode) && (flag & 2)) {
			iput(inode);
			return -EROFS;
		}
	}
	/*
	 * An append-only file must be opened in append mode for writing
	 * Additionally, we must disallow O_TRUNC -- cevans
	 */
	if (IS_APPEND(inode) && (((flag & FMODE_WRITE) && !(flag & O_APPEND)) || (flag & O_TRUNC))) {
		iput(inode);
		return -EPERM;
	}
	if (flag & O_TRUNC) {
		if ((error = get_write_access(inode))) {
			iput(inode);
			return error;
		}
		/*
		 * Refuse to truncate files with mandatory locks held on them
		 */
		error = locks_verify_locked(inode);
		if (error) {
			iput(inode);
			return error;
		}
		if (inode->i_sb && inode->i_sb->dq_op)
			inode->i_sb->dq_op->initialize(inode, -1);
			
		error = do_truncate(inode, 0);
		put_write_access(inode);
		if (error) {
			iput(inode);
			return error;
		}
	} else
		if (flag & FMODE_WRITE)
			if (inode->i_sb && inode->i_sb->dq_op)
				inode->i_sb->dq_op->initialize(inode, -1);