初探Linux文件和文件系统

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drwxrwxr-x 3 mi mi 4096 4月  27 19:32 .
drwxrwxr-x 7 mi mi 4096 4月  26 10:02 ..
-rw-rw-r-- 1 mi mi    0 4月  26 10:03 file_attr
drwxrwxr-x 2 mi mi 4096 4月  27 19:32 file_dic

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chmod 000 ./filesystem/

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$ ls ./filesystem/
ls: cannot open directory './filesystem/': Permission denied

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chmod 600 ./filesystem/

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$ ls  ./filesystem/
ls: cannot access './filesystem/file_attr': Permission denied
ls: cannot access './filesystem/file_dic': Permission denied
file_attr  file_dic

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$ ls -la filesystem/
total 12
drwxrwxrwx  3 mi mi 4096 5月  11 20:36 .
drwxrwxrwx 11 mi mi 4096 5月   8 20:32 ..
-rw-rw-r--  1 mi mi    0 5月   8 20:32 file_attr
drwxrwxr-x  2 mi mi 4096 5月  11 20:36 file_dic

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$ stat .
  File: .
  Size: 4096            Blocks: 8          IO Block: 4096   directory
Device: 802h/2050d      Inode: 136185988   Links: 3
Access: (0775/drwxrwxr-x)  Uid: ( 1000/      mi)   Gid: ( 1000/      mi)
Access: 2021-05-11 20:36:13.625994262 +0800
Modify: 2021-05-11 20:36:12.533997328 +0800
Change: 2021-05-11 20:36:12.533997328 +0800
 Birth: -

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$ stat ./file_attr
  File: ./file_attr
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: 802h/2050d      Inode: 136185989   Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/      mi)   Gid: ( 1000/      mi)
Access: 2021-05-12 13:04:02.996823303 +0800
Modify: 2021-05-08 20:32:55.623371621 +0800
Change: 2021-05-08 20:32:55.623371621 +0800
 Birth: -

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/*
 * Keep mostly read-only and often accessed (especially for
 * the RCU path lookup and 'stat' data) fields at the beginning
 * of the 'struct inode'
 */
struct inode {
	umode_t				i_mode;               // 文件权限, rwx等
	unsigned short		i_opflags;
	kuid_t				i_uid;                // 文件所属用户id, ls可以看到
	kgid_t				i_gid;                // 文件所属用户组id, ls可以看到
	unsigned int		i_flags;
#ifdef CONFIG_FS_POSIX_ACL
	struct posix_acl	*i_acl;
	struct posix_acl	*i_default_acl;
#endif
	const struct inode_operations	*i_op;
	struct super_block				*i_sb;        // 指向了super block, 对同一个文件系统是唯一的
	struct address_space			*i_mapping;
//......
	/* Stat data, not accessed from path walking */
	unsigned long		i_ino;
	/*
	 * Filesystems may only read i_nlink directly.  They shall use the
	 * following functions for modification:
	 *
	 *    (set|clear|inc|drop)_nlink
	 *    inode_(inc|dec)_link_count
	 */
	union {
		const unsigned int 	i_nlink;
		unsigned int 		__i_nlink;
	};
	dev_t				i_rdev;
	loff_t				i_size;         // 文件大小
	struct timespec64	i_atime;        // 操作时间相关
	struct timespec64	i_mtime;        // 操作时间相关
	struct timespec64	i_ctime;        // 操作时间相关
	spinlock_t			i_lock;	/* i_blocks, i_bytes, maybe i_size */
	unsigned short      i_bytes;
	u8					i_blkbits;
	u8					i_write_hint;
	blkcnt_t			i_blocks;
//......
	union {
		struct pipe_inode_info	*i_pipe;
		struct block_device		*i_bdev;
		struct cdev				*i_cdev;
		char					*i_link;
		unsigned				i_dir_seq;
	};                        	// inode的类型, 比如可以是一个pipe或者link等, 这时候可以不需要磁盘上具体的文件内容, 仅inode结构就可以了
//......
} __randomize_layout;

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/*
 * Structure of an inode on the disk
 */
struct ext4_inode {
	__le16	i_mode;		/* File mode */
	__le16	i_uid;		/* Low 16 bits of Owner Uid */
	__le32	i_size_lo;	/* Size in bytes */
	__le32	i_atime;	/* Access time */
	__le32	i_ctime;	/* Inode Change time */
	__le32	i_mtime;	/* Modification time */
	__le32	i_dtime;	/* Deletion Time */
	__le16	i_gid;		/* Low 16 bits of Group Id */
	__le16	i_links_count;	/* Links count */
	__le32	i_blocks_lo;	/* Blocks count */
	__le32	i_flags;	/* File flags */
//.......
	__le32	i_block[EXT4_N_BLOCKS];/* Pointers to blocks */
	__le32	i_generation;	/* File version (for NFS) */
	__le32	i_file_acl_lo;	/* File ACL */
	__le32	i_size_high;
	__le32	i_obso_faddr;	/* Obsoleted fragment address */
//......
};

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/**
 *	struct pipe_inode_info - a linux kernel pipe
 *	@mutex: mutex protecting the whole thing
 *	@wait: reader/writer wait point in case of empty/full pipe
 *	@nrbufs: the number of non-empty pipe buffers in this pipe
 *	@buffers: total number of buffers (should be a power of 2)
 *	@curbuf: the current pipe buffer entry
 *	@tmp_page: cached released page
 *	@readers: number of current readers of this pipe
 *	@writers: number of current writers of this pipe
 *	@files: number of struct file referring this pipe (protected by ->i_lock)
 *	@waiting_writers: number of writers blocked waiting for room
 *	@r_counter: reader counter
 *	@w_counter: writer counter
 *	@fasync_readers: reader side fasync
 *	@fasync_writers: writer side fasync
 *	@bufs: the circular array of pipe buffers
 *	@user: the user who created this pipe
 **/
struct pipe_inode_info {
	struct mutex mutex;
	wait_queue_head_t wait;
	unsigned int nrbufs, curbuf, buffers;
	unsigned int readers;
	unsigned int writers;
	unsigned int files;
	unsigned int waiting_writers;
	unsigned int r_counter;
	unsigned int w_counter;
	struct page *tmp_page;
	struct fasync_struct *fasync_readers;
	struct fasync_struct *fasync_writers;
	struct pipe_buffer *bufs;
	struct user_struct *user;
};

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/**
 *	struct pipe_buffer - a linux kernel pipe buffer
 *	@page: the page containing the data for the pipe buffer
 *	@offset: offset of data inside the @page
 *	@len: length of data inside the @page
 *	@ops: operations associated with this buffer. See @pipe_buf_operations.
 *	@flags: pipe buffer flags. See above.
 *	@private: private data owned by the ops.
 **/
struct pipe_buffer {
	struct page *page;
	unsigned int offset, len;
	const struct pipe_buf_operations *ops;
	unsigned int flags;
	unsigned long private;
};

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struct super_block {
	struct list_head	s_list;		/* Keep this first */
	dev_t				s_dev;		/* search index; _not_ kdev_t */
	unsigned char		s_blocksize_bits;
	unsigned long		s_blocksize;
	loff_t				s_maxbytes;	/* Max file size */
	struct file_system_type			*s_type;
	const struct super_operations	*s_op;
	const struct dquot_operations	*dq_op;
	const struct quotactl_ops		*s_qcop;
	const struct export_operations 	*s_export_op;
	unsigned long		s_flags;
	unsigned long		s_iflags;	/* internal SB_I_* flags */
	unsigned long		s_magic;
	struct dentry		*s_root;    // 根结点dentry
	struct rw_semaphore	s_umount;
	int					s_count;
	atomic_t			s_active;
//......
	struct hlist_bl_head	s_roots;	/* alternate root dentries for NFS */
	struct list_head		s_mounts;	/* list of mounts; _not_ for fs use */
	struct block_device		*s_bdev;
	struct backing_dev_info *s_bdi;
	struct mtd_info			*s_mtd;
	struct hlist_node		s_instances;
	unsigned int			s_quota_types;	/* Bitmask of supported quota types */
	struct quota_info		s_dquot;		/* Diskquota specific options */
	struct sb_writers		s_writers;
//......
	char			s_id[32];	/* Informational name */
	uuid_t			s_uuid;		/* UUID */
	unsigned int	s_max_links;
	fmode_t			s_mode;
	/*
	 * The next field is for VFS *only*. No filesystems have any business
	 * even looking at it. You had been warned.
	 */
	struct mutex s_vfs_rename_mutex;	/* Kludge */
	/*
	 * Filesystem subtype.  If non-empty the filesystem type field
	 * in /proc/mounts will be "type.subtype"
	 */
	const char *s_subtype;
	const struct dentry_operations *s_d_op; /* default d_op for dentries */
	/*
	 * Saved pool identifier for cleancache (-1 means none)
	 */
	int cleancache_poolid;
	struct shrinker s_shrink;	/* per-sb shrinker handle */
	/* Number of inodes with nlink == 0 but still referenced */
	atomic_long_t s_remove_count;
	/* Pending fsnotify inode refs */
	atomic_long_t s_fsnotify_inode_refs;
	/* Being remounted read-only */
	int s_readonly_remount;
	/* AIO completions deferred from interrupt context */
	struct workqueue_struct *s_dio_done_wq;
	struct hlist_head s_pins;
	/*
	 * Owning user namespace and default context in which to
	 * interpret filesystem uids, gids, quotas, device nodes,
	 * xattrs and security labels.
	 */
	struct user_namespace *s_user_ns;
	/*
	 * The list_lru structure is essentially just a pointer to a table
	 * of per-node lru lists, each of which has its own spinlock.
	 * There is no need to put them into separate cachelines.
	 */
	struct list_lru		s_dentry_lru;
	struct list_lru		s_inode_lru;
	struct rcu_head		rcu;
	struct work_struct	destroy_work;
	struct mutex		s_sync_lock;	/* sync serialisation lock */
	/*
	 * Indicates how deep in a filesystem stack this SB is
	 */
	int s_stack_depth;
	/* s_inode_list_lock protects s_inodes */
	spinlock_t		s_inode_list_lock ____cacheline_aligned_in_smp;
	struct list_head	s_inodes;	/* all inodes */
	spinlock_t		s_inode_wblist_lock;
	struct list_head	s_inodes_wb;	/* writeback inodes */
} __randomize_layout;

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struct dentry {
	/* RCU lookup touched fields */
	unsigned int 	d_flags;		/* protected by d_lock */
	seqcount_t 		d_seq;			/* per dentry seqlock */
	struct hlist_bl_node d_hash;	/* lookup hash list */
	struct dentry	 	*d_parent;		/* parent directory */
	struct qstr 		d_name;
	struct inode 		*d_inode;		/* Where the name belongs to - NULL is
					 	* negative */
	unsigned char d_iname[DNAME_INLINE_LEN];	/* small names */
	/* Ref lookup also touches following */
	struct lockref d_lockref;					/* per-dentry lock and refcount */
	const struct dentry_operations *d_op;
	struct super_block *d_sb;	/* The root of the dentry tree */
	unsigned long d_time;		/* used by d_revalidate */
	void *d_fsdata;				/* fs-specific data */
	union {
		struct list_head d_lru;		/* LRU list */
		wait_queue_head_t *d_wait;	/* in-lookup ones only */
	};
	struct list_head d_child;	/* child of parent list */
	struct list_head d_subdirs;	/* our children */
	/*
	 * d_alias and d_rcu can share memory
	 */
	union {
		struct hlist_node d_alias;	/* inode alias list */
		struct hlist_bl_node d_in_lookup_hash;	/* only for in-lookup ones */
	 	struct rcu_head d_rcu;
	} d_u;
} __randomize_layout;

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ls -ia /
2 .  2 ..  2 dev  2 run
1 proc  1 sys