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diff --git a/Documentation/filesystems/ext4/inodes.rst b/Documentation/filesystems/ext4/inodes.rst new file mode 100644 index 000000000..a65baffb4 --- /dev/null +++ b/Documentation/filesystems/ext4/inodes.rst @@ -0,0 +1,578 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Index Nodes +----------- + +In a regular UNIX filesystem, the inode stores all the metadata +pertaining to the file (time stamps, block maps, extended attributes, +etc), not the directory entry. To find the information associated with a +file, one must traverse the directory files to find the directory entry +associated with a file, then load the inode to find the metadata for +that file. ext4 appears to cheat (for performance reasons) a little bit +by storing a copy of the file type (normally stored in the inode) in the +directory entry. (Compare all this to FAT, which stores all the file +information directly in the directory entry, but does not support hard +links and is in general more seek-happy than ext4 due to its simpler +block allocator and extensive use of linked lists.) + +The inode table is a linear array of ``struct ext4_inode``. The table is +sized to have enough blocks to store at least +``sb.s_inode_size * sb.s_inodes_per_group`` bytes. The number of the +block group containing an inode can be calculated as +``(inode_number - 1) / sb.s_inodes_per_group``, and the offset into the +group's table is ``(inode_number - 1) % sb.s_inodes_per_group``. There +is no inode 0. + +The inode checksum is calculated against the FS UUID, the inode number, +and the inode structure itself. + +The inode table entry is laid out in ``struct ext4_inode``. + +.. list-table:: + :widths: 8 8 24 40 + :header-rows: 1 + :class: longtable + + * - Offset + - Size + - Name + - Description + * - 0x0 + - \_\_le16 + - i\_mode + - File mode. See the table i_mode_ below. + * - 0x2 + - \_\_le16 + - i\_uid + - Lower 16-bits of Owner UID. + * - 0x4 + - \_\_le32 + - i\_size\_lo + - Lower 32-bits of size in bytes. + * - 0x8 + - \_\_le32 + - i\_atime + - Last access time, in seconds since the epoch. However, if the EA\_INODE + inode flag is set, this inode stores an extended attribute value and + this field contains the checksum of the value. + * - 0xC + - \_\_le32 + - i\_ctime + - Last inode change time, in seconds since the epoch. However, if the + EA\_INODE inode flag is set, this inode stores an extended attribute + value and this field contains the lower 32 bits of the attribute value's + reference count. + * - 0x10 + - \_\_le32 + - i\_mtime + - Last data modification time, in seconds since the epoch. However, if the + EA\_INODE inode flag is set, this inode stores an extended attribute + value and this field contains the number of the inode that owns the + extended attribute. + * - 0x14 + - \_\_le32 + - i\_dtime + - Deletion Time, in seconds since the epoch. + * - 0x18 + - \_\_le16 + - i\_gid + - Lower 16-bits of GID. + * - 0x1A + - \_\_le16 + - i\_links\_count + - Hard link count. Normally, ext4 does not permit an inode to have more + than 65,000 hard links. This applies to files as well as directories, + which means that there cannot be more than 64,998 subdirectories in a + directory (each subdirectory's '..' entry counts as a hard link, as does + the '.' entry in the directory itself). With the DIR\_NLINK feature + enabled, ext4 supports more than 64,998 subdirectories by setting this + field to 1 to indicate that the number of hard links is not known. + * - 0x1C + - \_\_le32 + - i\_blocks\_lo + - Lower 32-bits of “block” count. If the huge\_file feature flag is not + set on the filesystem, the file consumes ``i_blocks_lo`` 512-byte blocks + on disk. If huge\_file is set and EXT4\_HUGE\_FILE\_FL is NOT set in + ``inode.i_flags``, then the file consumes ``i_blocks_lo + (i_blocks_hi + << 32)`` 512-byte blocks on disk. If huge\_file is set and + EXT4\_HUGE\_FILE\_FL IS set in ``inode.i_flags``, then this file + consumes (``i_blocks_lo + i_blocks_hi`` << 32) filesystem blocks on + disk. + * - 0x20 + - \_\_le32 + - i\_flags + - Inode flags. See the table i_flags_ below. + * - 0x24 + - 4 bytes + - i\_osd1 + - See the table i_osd1_ for more details. + * - 0x28 + - 60 bytes + - i\_block[EXT4\_N\_BLOCKS=15] + - Block map or extent tree. See the section “The Contents of inode.i\_block”. + * - 0x64 + - \_\_le32 + - i\_generation + - File version (for NFS). + * - 0x68 + - \_\_le32 + - i\_file\_acl\_lo + - Lower 32-bits of extended attribute block. ACLs are of course one of + many possible extended attributes; I think the name of this field is a + result of the first use of extended attributes being for ACLs. + * - 0x6C + - \_\_le32 + - i\_size\_high / i\_dir\_acl + - Upper 32-bits of file/directory size. In ext2/3 this field was named + i\_dir\_acl, though it was usually set to zero and never used. + * - 0x70 + - \_\_le32 + - i\_obso\_faddr + - (Obsolete) fragment address. + * - 0x74 + - 12 bytes + - i\_osd2 + - See the table i_osd2_ for more details. + * - 0x80 + - \_\_le16 + - i\_extra\_isize + - Size of this inode - 128. Alternately, the size of the extended inode + fields beyond the original ext2 inode, including this field. + * - 0x82 + - \_\_le16 + - i\_checksum\_hi + - Upper 16-bits of the inode checksum. + * - 0x84 + - \_\_le32 + - i\_ctime\_extra + - Extra change time bits. This provides sub-second precision. See Inode + Timestamps section. + * - 0x88 + - \_\_le32 + - i\_mtime\_extra + - Extra modification time bits. This provides sub-second precision. + * - 0x8C + - \_\_le32 + - i\_atime\_extra + - Extra access time bits. This provides sub-second precision. + * - 0x90 + - \_\_le32 + - i\_crtime + - File creation time, in seconds since the epoch. + * - 0x94 + - \_\_le32 + - i\_crtime\_extra + - Extra file creation time bits. This provides sub-second precision. + * - 0x98 + - \_\_le32 + - i\_version\_hi + - Upper 32-bits for version number. + * - 0x9C + - \_\_le32 + - i\_projid + - Project ID. + +.. _i_mode: + +The ``i_mode`` value is a combination of the following flags: + +.. list-table:: + :widths: 16 64 + :header-rows: 1 + + * - Value + - Description + * - 0x1 + - S\_IXOTH (Others may execute) + * - 0x2 + - S\_IWOTH (Others may write) + * - 0x4 + - S\_IROTH (Others may read) + * - 0x8 + - S\_IXGRP (Group members may execute) + * - 0x10 + - S\_IWGRP (Group members may write) + * - 0x20 + - S\_IRGRP (Group members may read) + * - 0x40 + - S\_IXUSR (Owner may execute) + * - 0x80 + - S\_IWUSR (Owner may write) + * - 0x100 + - S\_IRUSR (Owner may read) + * - 0x200 + - S\_ISVTX (Sticky bit) + * - 0x400 + - S\_ISGID (Set GID) + * - 0x800 + - S\_ISUID (Set UID) + * - + - These are mutually-exclusive file types: + * - 0x1000 + - S\_IFIFO (FIFO) + * - 0x2000 + - S\_IFCHR (Character device) + * - 0x4000 + - S\_IFDIR (Directory) + * - 0x6000 + - S\_IFBLK (Block device) + * - 0x8000 + - S\_IFREG (Regular file) + * - 0xA000 + - S\_IFLNK (Symbolic link) + * - 0xC000 + - S\_IFSOCK (Socket) + +.. _i_flags: + +The ``i_flags`` field is a combination of these values: + +.. list-table:: + :widths: 16 64 + :header-rows: 1 + + * - Value + - Description + * - 0x1 + - This file requires secure deletion (EXT4\_SECRM\_FL). (not implemented) + * - 0x2 + - This file should be preserved, should undeletion be desired + (EXT4\_UNRM\_FL). (not implemented) + * - 0x4 + - File is compressed (EXT4\_COMPR\_FL). (not really implemented) + * - 0x8 + - All writes to the file must be synchronous (EXT4\_SYNC\_FL). + * - 0x10 + - File is immutable (EXT4\_IMMUTABLE\_FL). + * - 0x20 + - File can only be appended (EXT4\_APPEND\_FL). + * - 0x40 + - The dump(1) utility should not dump this file (EXT4\_NODUMP\_FL). + * - 0x80 + - Do not update access time (EXT4\_NOATIME\_FL). + * - 0x100 + - Dirty compressed file (EXT4\_DIRTY\_FL). (not used) + * - 0x200 + - File has one or more compressed clusters (EXT4\_COMPRBLK\_FL). (not used) + * - 0x400 + - Do not compress file (EXT4\_NOCOMPR\_FL). (not used) + * - 0x800 + - Encrypted inode (EXT4\_ENCRYPT\_FL). This bit value previously was + EXT4\_ECOMPR\_FL (compression error), which was never used. + * - 0x1000 + - Directory has hashed indexes (EXT4\_INDEX\_FL). + * - 0x2000 + - AFS magic directory (EXT4\_IMAGIC\_FL). + * - 0x4000 + - File data must always be written through the journal + (EXT4\_JOURNAL\_DATA\_FL). + * - 0x8000 + - File tail should not be merged (EXT4\_NOTAIL\_FL). (not used by ext4) + * - 0x10000 + - All directory entry data should be written synchronously (see + ``dirsync``) (EXT4\_DIRSYNC\_FL). + * - 0x20000 + - Top of directory hierarchy (EXT4\_TOPDIR\_FL). + * - 0x40000 + - This is a huge file (EXT4\_HUGE\_FILE\_FL). + * - 0x80000 + - Inode uses extents (EXT4\_EXTENTS\_FL). + * - 0x100000 + - Verity protected file (EXT4\_VERITY\_FL). + * - 0x200000 + - Inode stores a large extended attribute value in its data blocks + (EXT4\_EA\_INODE\_FL). + * - 0x400000 + - This file has blocks allocated past EOF (EXT4\_EOFBLOCKS\_FL). + (deprecated) + * - 0x01000000 + - Inode is a snapshot (``EXT4_SNAPFILE_FL``). (not in mainline) + * - 0x04000000 + - Snapshot is being deleted (``EXT4_SNAPFILE_DELETED_FL``). (not in + mainline) + * - 0x08000000 + - Snapshot shrink has completed (``EXT4_SNAPFILE_SHRUNK_FL``). (not in + mainline) + * - 0x10000000 + - Inode has inline data (EXT4\_INLINE\_DATA\_FL). + * - 0x20000000 + - Create children with the same project ID (EXT4\_PROJINHERIT\_FL). + * - 0x80000000 + - Reserved for ext4 library (EXT4\_RESERVED\_FL). + * - + - Aggregate flags: + * - 0x705BDFFF + - User-visible flags. + * - 0x604BC0FF + - User-modifiable flags. Note that while EXT4\_JOURNAL\_DATA\_FL and + EXT4\_EXTENTS\_FL can be set with setattr, they are not in the kernel's + EXT4\_FL\_USER\_MODIFIABLE mask, since it needs to handle the setting of + these flags in a special manner and they are masked out of the set of + flags that are saved directly to i\_flags. + +.. _i_osd1: + +The ``osd1`` field has multiple meanings depending on the creator: + +Linux: + +.. list-table:: + :widths: 8 8 24 40 + :header-rows: 1 + + * - Offset + - Size + - Name + - Description + * - 0x0 + - \_\_le32 + - l\_i\_version + - Inode version. However, if the EA\_INODE inode flag is set, this inode + stores an extended attribute value and this field contains the upper 32 + bits of the attribute value's reference count. + +Hurd: + +.. list-table:: + :widths: 8 8 24 40 + :header-rows: 1 + + * - Offset + - Size + - Name + - Description + * - 0x0 + - \_\_le32 + - h\_i\_translator + - ?? + +Masix: + +.. list-table:: + :widths: 8 8 24 40 + :header-rows: 1 + + * - Offset + - Size + - Name + - Description + * - 0x0 + - \_\_le32 + - m\_i\_reserved + - ?? + +.. _i_osd2: + +The ``osd2`` field has multiple meanings depending on the filesystem creator: + +Linux: + +.. list-table:: + :widths: 8 8 24 40 + :header-rows: 1 + + * - Offset + - Size + - Name + - Description + * - 0x0 + - \_\_le16 + - l\_i\_blocks\_high + - Upper 16-bits of the block count. Please see the note attached to + i\_blocks\_lo. + * - 0x2 + - \_\_le16 + - l\_i\_file\_acl\_high + - Upper 16-bits of the extended attribute block (historically, the file + ACL location). See the Extended Attributes section below. + * - 0x4 + - \_\_le16 + - l\_i\_uid\_high + - Upper 16-bits of the Owner UID. + * - 0x6 + - \_\_le16 + - l\_i\_gid\_high + - Upper 16-bits of the GID. + * - 0x8 + - \_\_le16 + - l\_i\_checksum\_lo + - Lower 16-bits of the inode checksum. + * - 0xA + - \_\_le16 + - l\_i\_reserved + - Unused. + +Hurd: + +.. list-table:: + :widths: 8 8 24 40 + :header-rows: 1 + + * - Offset + - Size + - Name + - Description + * - 0x0 + - \_\_le16 + - h\_i\_reserved1 + - ?? + * - 0x2 + - \_\_u16 + - h\_i\_mode\_high + - Upper 16-bits of the file mode. + * - 0x4 + - \_\_le16 + - h\_i\_uid\_high + - Upper 16-bits of the Owner UID. + * - 0x6 + - \_\_le16 + - h\_i\_gid\_high + - Upper 16-bits of the GID. + * - 0x8 + - \_\_u32 + - h\_i\_author + - Author code? + +Masix: + +.. list-table:: + :widths: 8 8 24 40 + :header-rows: 1 + + * - Offset + - Size + - Name + - Description + * - 0x0 + - \_\_le16 + - h\_i\_reserved1 + - ?? + * - 0x2 + - \_\_u16 + - m\_i\_file\_acl\_high + - Upper 16-bits of the extended attribute block (historically, the file + ACL location). + * - 0x4 + - \_\_u32 + - m\_i\_reserved2[2] + - ?? + +Inode Size +~~~~~~~~~~ + +In ext2 and ext3, the inode structure size was fixed at 128 bytes +(``EXT2_GOOD_OLD_INODE_SIZE``) and each inode had a disk record size of +128 bytes. Starting with ext4, it is possible to allocate a larger +on-disk inode at format time for all inodes in the filesystem to provide +space beyond the end of the original ext2 inode. The on-disk inode +record size is recorded in the superblock as ``s_inode_size``. The +number of bytes actually used by struct ext4\_inode beyond the original +128-byte ext2 inode is recorded in the ``i_extra_isize`` field for each +inode, which allows struct ext4\_inode to grow for a new kernel without +having to upgrade all of the on-disk inodes. Access to fields beyond +EXT2\_GOOD\_OLD\_INODE\_SIZE should be verified to be within +``i_extra_isize``. By default, ext4 inode records are 256 bytes, and (as +of August 2019) the inode structure is 160 bytes +(``i_extra_isize = 32``). The extra space between the end of the inode +structure and the end of the inode record can be used to store extended +attributes. Each inode record can be as large as the filesystem block +size, though this is not terribly efficient. + +Finding an Inode +~~~~~~~~~~~~~~~~ + +Each block group contains ``sb->s_inodes_per_group`` inodes. Because +inode 0 is defined not to exist, this formula can be used to find the +block group that an inode lives in: +``bg = (inode_num - 1) / sb->s_inodes_per_group``. The particular inode +can be found within the block group's inode table at +``index = (inode_num - 1) % sb->s_inodes_per_group``. To get the byte +address within the inode table, use +``offset = index * sb->s_inode_size``. + +Inode Timestamps +~~~~~~~~~~~~~~~~ + +Four timestamps are recorded in the lower 128 bytes of the inode +structure -- inode change time (ctime), access time (atime), data +modification time (mtime), and deletion time (dtime). The four fields +are 32-bit signed integers that represent seconds since the Unix epoch +(1970-01-01 00:00:00 GMT), which means that the fields will overflow in +January 2038. For inodes that are not linked from any directory but are +still open (orphan inodes), the dtime field is overloaded for use with +the orphan list. The superblock field ``s_last_orphan`` points to the +first inode in the orphan list; dtime is then the number of the next +orphaned inode, or zero if there are no more orphans. + +If the inode structure size ``sb->s_inode_size`` is larger than 128 +bytes and the ``i_inode_extra`` field is large enough to encompass the +respective ``i_[cma]time_extra`` field, the ctime, atime, and mtime +inode fields are widened to 64 bits. Within this “extra” 32-bit field, +the lower two bits are used to extend the 32-bit seconds field to be 34 +bit wide; the upper 30 bits are used to provide nanosecond timestamp +accuracy. Therefore, timestamps should not overflow until May 2446. +dtime was not widened. There is also a fifth timestamp to record inode +creation time (crtime); this field is 64-bits wide and decoded in the +same manner as 64-bit [cma]time. Neither crtime nor dtime are accessible +through the regular stat() interface, though debugfs will report them. + +We use the 32-bit signed time value plus (2^32 \* (extra epoch bits)). +In other words: + +.. list-table:: + :widths: 20 20 20 20 20 + :header-rows: 1 + + * - Extra epoch bits + - MSB of 32-bit time + - Adjustment for signed 32-bit to 64-bit tv\_sec + - Decoded 64-bit tv\_sec + - valid time range + * - 0 0 + - 1 + - 0 + - ``-0x80000000 - -0x00000001`` + - 1901-12-13 to 1969-12-31 + * - 0 0 + - 0 + - 0 + - ``0x000000000 - 0x07fffffff`` + - 1970-01-01 to 2038-01-19 + * - 0 1 + - 1 + - 0x100000000 + - ``0x080000000 - 0x0ffffffff`` + - 2038-01-19 to 2106-02-07 + * - 0 1 + - 0 + - 0x100000000 + - ``0x100000000 - 0x17fffffff`` + - 2106-02-07 to 2174-02-25 + * - 1 0 + - 1 + - 0x200000000 + - ``0x180000000 - 0x1ffffffff`` + - 2174-02-25 to 2242-03-16 + * - 1 0 + - 0 + - 0x200000000 + - ``0x200000000 - 0x27fffffff`` + - 2242-03-16 to 2310-04-04 + * - 1 1 + - 1 + - 0x300000000 + - ``0x280000000 - 0x2ffffffff`` + - 2310-04-04 to 2378-04-22 + * - 1 1 + - 0 + - 0x300000000 + - ``0x300000000 - 0x37fffffff`` + - 2378-04-22 to 2446-05-10 + +This is a somewhat odd encoding since there are effectively seven times +as many positive values as negative values. There have also been +long-standing bugs decoding and encoding dates beyond 2038, which don't +seem to be fixed as of kernel 3.12 and e2fsprogs 1.42.8. 64-bit kernels +incorrectly use the extra epoch bits 1,1 for dates between 1901 and +1970. At some point the kernel will be fixed and e2fsck will fix this +situation, assuming that it is run before 2310. |