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struct file *do_last(struct nameidata *nd, struct path *path,			    int open_flag, int acc_mode,			    int mode, const char *pathname){	if (!(open_flag & O_CREAT)) {		error = do_lookup(nd, &nd->last, path);		if (error)			goto exit;		error = -ENOENT;//不让以创建方式打开，那么如果找不到则返回		if (!path->dentry->d_inode)			goto exit_dput;		goto ok;	}	/* Negative dentry, just create the file */	if (!path->dentry->d_inode) {		error = mnt_want_write(nd->path.mnt);		=>int mnt_want_write(struct vfsmount *mnt)		{			if (__mnt_is_readonly(mnt)) {				dec_mnt_writers(mnt);				ret = -EROFS;//打开失败，由于是只读模式				goto out;			}		out:			return ret;		}		if (error)			goto exit_mutex_unlock;	}ok:	filp = finish_open(nd, open_flag, acc_mode);	=>struct file *finish_open(struct nameidata *nd,				int open_flag, int acc_mode)	{		error = may_open(&nd->path, acc_mode, open_flag);		=>int may_open(struct path *path, int acc_mode, int flag)		{			error = inode_permission(inode, acc_mode);			=>int inode_permission(struct inode *inode, int mask)			{				if (mask & MAY_WRITE) {//写模式下权限的判断					umode_t mode = inode->i_mode;					/*					 * Nobody gets write access to a read-only fs.					 */					if (IS_RDONLY(inode) &&						(S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))						return -EROFS;//只读					/*					 * Nobody gets write access to an immutable file.					 */					if (IS_IMMUTABLE(inode))						return -EACCES;				}			}			if (error)				return error;		}		if (error) {			if (will_truncate)				mnt_drop_write(nd->path.mnt);			goto exit;		}	}	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);	}

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