文件系统--mknod系统调用

在前面 介绍android的init进程的时候，我们看到其中有如下代码

    mkdir("/dev", 0755);  
    mkdir("/proc", 0755);  
    mkdir("/sys", 0755);  
  
    mount("tmpfs", "/dev", "tmpfs", 0, "mode=0755");  
    mkdir("/dev/pts", 0755);  
    mkdir("/dev/socket", 0755);  
    mount("devpts", "/dev/pts", "devpts", 0, NULL);  
    mount("proc", "/proc", "proc", 0, NULL);  
    mount("sysfs", "/sys", "sysfs", 0, NULL);  
  

在挂载完根文件系统后/dev , /proc...一般都是有的，这里主要是进行了挂载，注意这里挂载了tmpfs到/dev目录下面

我们来看下mknod系统调用的实现

SYSCALL_DEFINE3(mknod, const char __user *, filename, int, mode, unsigned, dev)
{
	return sys_mknodat(AT_FDCWD, filename, mode, dev);
}

直接调用sys_mknodat函数实现

SYSCALL_DEFINE4(mknodat, int, dfd, const char __user *, filename, int, mode,
		unsigned, dev)
{
	int error;
	char *tmp;
	struct dentry *dentry;
	struct nameidata nd;

	if (S_ISDIR(mode))
		return -EPERM;
	printk("filename = %s.\n", filename);
	printk("mode = 0x%x.\n", mode);
	error = user_path_parent(dfd, filename, &nd, &tmp);//查找路径中的最后一个项的父目录项
	if (error)
		return error;
	printk("before lookup_create.\n");
	dentry = lookup_create(&nd, 0);//创建需要创建的最后一项的目录项
	printk("after lookup_create.\n");
	if (IS_ERR(dentry)) {
		error = PTR_ERR(dentry);
		goto out_unlock;
	}
	if (!IS_POSIXACL(nd.path.dentry->d_inode))
		mode &= ~current_umask();
	error = may_mknod(mode);//进行模式检查
	if (error)
		goto out_dput;
	error = mnt_want_write(nd.path.mnt);
	if (error)
		goto out_dput;
	error = security_path_mknod(&nd.path, dentry, mode, dev);
	if (error)
		goto out_drop_write;
	switch (mode & S_IFMT) {
		case 0: case S_IFREG:
			printk("S_IFREG .\n");
			error = vfs_create(nd.path.dentry->d_inode,dentry,mode,&nd);
			break;
		case S_IFCHR: case S_IFBLK:
			printk("S_IFCHR: case S_IFBLK.\n");
			error = vfs_mknod(nd.path.dentry->d_inode,dentry,mode,
					new_decode_dev(dev));
			break;
		case S_IFIFO: case S_IFSOCK:
			printk("S_IFIFO: case S_IFSOCK.\n");
			error = vfs_mknod(nd.path.dentry->d_inode,dentry,mode,0);
			break;
	}
out_drop_write:
	mnt_drop_write(nd.path.mnt);
out_dput:
	dput(dentry);
out_unlock:
	mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
	path_put(&nd.path);
	putname(tmp);

	return error;
}

这里首先查找要创建文件路径的最后一项的父目录的目录项，然后建立要创建文件的目录项最后根据相应的类型调用相应的vfs函数，以字符设备为例，这里会调用

		case S_IFCHR: case S_IFBLK:
			printk("S_IFCHR: case S_IFBLK.\n");
			error = vfs_mknod(nd.path.dentry->d_inode,dentry,mode,
					new_decode_dev(dev));

跟踪vfs_mknod

int vfs_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
{
	int error = may_create(dir, dentry);
	printk(" vfs_mknod.\n");
	printk(" vfs_mknod.filesystem = %s.\n",dir->i_sb->s_type->name);
	if (error)
		return error;

	if ((S_ISCHR(mode) || S_ISBLK(mode)) && !capable(CAP_MKNOD))
		return -EPERM;

	if (!dir->i_op->mknod)
		return -EPERM;

	error = devcgroup_inode_mknod(mode, dev);
	if (error)
		return error;

	error = security_inode_mknod(dir, dentry, mode, dev);
	if (error)
		return error;
	printk(" before dir->i_op->mknod.\n");
	error = dir->i_op->mknod(dir, dentry, mode, dev);//调用具体的mknod函数
	if (!error)
		fsnotify_create(dir, dentry);
	return error;
}

在这里做一些检查之后，就调用相应文件系统的mknod函数了，前面我们知道/dev目录属于tmpfs文件系统，我们看在挂载tmpfs的时候，会调用它的get_sb函数

static struct file_system_type tmpfs_fs_type = {
	.owner		= THIS_MODULE,
	.name		= "tmpfs",
	.get_sb		= shmem_get_sb,
	.kill_sb	= kill_litter_super,
};

这里对应shmem_get_sb

static int shmem_get_sb(struct file_system_type *fs_type,
	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
{
	return get_sb_nodev(fs_type, flags, data, shmem_fill_super, mnt);
}

对应的填充超级块的函数是shmem_fill_super

int shmem_fill_super(struct super_block *sb, void *data, int silent)
{
	struct inode *inode;
	struct dentry *root;
	struct shmem_sb_info *sbinfo;
	int err = -ENOMEM;

	/* Round up to L1_CACHE_BYTES to resist false sharing */
	sbinfo = kzalloc(max((int)sizeof(struct shmem_sb_info),
				L1_CACHE_BYTES), GFP_KERNEL);
	if (!sbinfo)
		return -ENOMEM;

	sbinfo->mode = S_IRWXUGO | S_ISVTX;
	sbinfo->uid = current_fsuid();
	sbinfo->gid = current_fsgid();
	sb->s_fs_info = sbinfo;

#ifdef CONFIG_TMPFS
	/*
	 * Per default we only allow half of the physical ram per
	 * tmpfs instance, limiting inodes to one per page of lowmem;
	 * but the internal instance is left unlimited.
	 */
	if (!(sb->s_flags & MS_NOUSER)) {
		sbinfo->max_blocks = shmem_default_max_blocks();
		sbinfo->max_inodes = shmem_default_max_inodes();
		if (shmem_parse_options(data, sbinfo, false)) {
			err = -EINVAL;
			goto failed;
		}
	}
	sb->s_export_op = &shmem_export_ops;
#else
	sb->s_flags |= MS_NOUSER;
#endif

	spin_lock_init(&sbinfo->stat_lock);
	sbinfo->free_blocks = sbinfo->max_blocks;
	sbinfo->free_inodes = sbinfo->max_inodes;

	sb->s_maxbytes = SHMEM_MAX_BYTES;
	sb->s_blocksize = PAGE_CACHE_SIZE;
	sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
	sb->s_magic = TMPFS_MAGIC;
	sb->s_op = &shmem_ops;
	sb->s_time_gran = 1;
#ifdef CONFIG_TMPFS_POSIX_ACL
	sb->s_xattr = shmem_xattr_handlers;
	sb->s_flags |= MS_POSIXACL;
#endif

	inode = shmem_get_inode(sb, NULL, S_IFDIR | sbinfo->mode, 0, VM_NORESERVE);
	if (!inode)
		goto failed;
	inode->i_uid = sbinfo->uid;
	inode->i_gid = sbinfo->gid;
	root = d_alloc_root(inode);
	if (!root)
		goto failed_iput;
	sb->s_root = root;
	return 0;

failed_iput:
	iput(inode);
failed:
	shmem_put_super(sb);
	return err;
}

再来看一下shmem_get_inode，对应填充它的inode节点

static struct inode *shmem_get_inode(struct super_block *sb, const struct inode *dir,
				     int mode, dev_t dev, unsigned long flags)
{
	struct inode *inode;
	struct shmem_inode_info *info;
	struct shmem_sb_info *sbinfo = SHMEM_SB(sb);

	if (shmem_reserve_inode(sb))
		return NULL;

	inode = new_inode(sb);
	if (inode) {
		inode_init_owner(inode, dir, mode);
		inode->i_blocks = 0;
		inode->i_mapping->backing_dev_info = &shmem_backing_dev_info;
		inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
		inode->i_generation = get_seconds();
		info = SHMEM_I(inode);
		memset(info, 0, (char *)inode - (char *)info);
		spin_lock_init(&info->lock);
		info->flags = flags & VM_NORESERVE;
		INIT_LIST_HEAD(&info->swaplist);
		cache_no_acl(inode);

		switch (mode & S_IFMT) {
		default:
			inode->i_op = &shmem_special_inode_operations;
			init_special_inode(inode, mode, dev);
			break;
		case S_IFREG:
			inode->i_mapping->a_ops = &shmem_aops;
			inode->i_op = &shmem_inode_operations;
			inode->i_fop = &shmem_file_operations;
			mpol_shared_policy_init(&info->policy,
						 shmem_get_sbmpol(sbinfo));
			break;
		case S_IFDIR:
			inc_nlink(inode);
			/* Some things misbehave if size == 0 on a directory */
			inode->i_size = 2 * BOGO_DIRENT_SIZE;
			inode->i_op = &shmem_dir_inode_operations;
			inode->i_fop = &simple_dir_operations;
			break;
		case S_IFLNK:
			/*
			 * Must not load anything in the rbtree,
			 * mpol_free_shared_policy will not be called.
			 */
			mpol_shared_policy_init(&info->policy, NULL);
			break;
		}
	} else
		shmem_free_inode(sb);
	return inode;
}

当我们创建/dev目录时，这里对应

		case S_IFDIR:
			inc_nlink(inode);
			/* Some things misbehave if size == 0 on a directory */
			inode->i_size = 2 * BOGO_DIRENT_SIZE;
			inode->i_op = &shmem_dir_inode_operations;
			inode->i_fop = &simple_dir_operations;
			break;

对应的i_op就是shmem_dir_inode_operations