.. SPDX-License-Identifier: GPL-2.0
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=======================
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ROMFS - ROM File System
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=======================
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This is a quite dumb, read only filesystem, mainly for initial RAM
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disks of installation disks. It has grown up by the need of having
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modules linked at boot time. Using this filesystem, you get a very
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similar feature, and even the possibility of a small kernel, with a
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file system which doesn't take up useful memory from the router
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functions in the basement of your office.
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For comparison, both the older minix and xiafs (the latter is now
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defunct) filesystems, compiled as module need more than 20000 bytes,
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while romfs is less than a page, about 4000 bytes (assuming i586
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code). Under the same conditions, the msdos filesystem would need
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about 30K (and does not support device nodes or symlinks), while the
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nfs module with nfsroot is about 57K. Furthermore, as a bit unfair
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comparison, an actual rescue disk used up 3202 blocks with ext2, while
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with romfs, it needed 3079 blocks.
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To create such a file system, you'll need a user program named
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genromfs. It is available on http://romfs.sourceforge.net/
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As the name suggests, romfs could be also used (space-efficiently) on
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various read-only media, like (E)EPROM disks if someone will have the
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motivation.. :)
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However, the main purpose of romfs is to have a very small kernel,
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which has only this filesystem linked in, and then can load any module
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later, with the current module utilities. It can also be used to run
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some program to decide if you need SCSI devices, and even IDE or
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floppy drives can be loaded later if you use the "initrd"--initial
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RAM disk--feature of the kernel. This would not be really news
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flash, but with romfs, you can even spare off your ext2 or minix or
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maybe even affs filesystem until you really know that you need it.
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For example, a distribution boot disk can contain only the cd disk
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drivers (and possibly the SCSI drivers), and the ISO 9660 filesystem
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module. The kernel can be small enough, since it doesn't have other
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filesystems, like the quite large ext2fs module, which can then be
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loaded off the CD at a later stage of the installation. Another use
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would be for a recovery disk, when you are reinstalling a workstation
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from the network, and you will have all the tools/modules available
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from a nearby server, so you don't want to carry two disks for this
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purpose, just because it won't fit into ext2.
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romfs operates on block devices as you can expect, and the underlying
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structure is very simple. Every accessible structure begins on 16
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byte boundaries for fast access. The minimum space a file will take
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is 32 bytes (this is an empty file, with a less than 16 character
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name). The maximum overhead for any non-empty file is the header, and
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the 16 byte padding for the name and the contents, also 16+14+15 = 45
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bytes. This is quite rare however, since most file names are longer
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than 3 bytes, and shorter than 15 bytes.
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The layout of the filesystem is the following::
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offset content
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+---+---+---+---+
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0 | - | r | o | m | \
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+---+---+---+---+ The ASCII representation of those bytes
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4 | 1 | f | s | - | / (i.e. "-rom1fs-")
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+---+---+---+---+
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8 | full size | The number of accessible bytes in this fs.
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+---+---+---+---+
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12 | checksum | The checksum of the FIRST 512 BYTES.
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+---+---+---+---+
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16 | volume name | The zero terminated name of the volume,
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: : padded to 16 byte boundary.
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+---+---+---+---+
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xx | file |
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: headers :
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Every multi byte value (32 bit words, I'll use the longwords term from
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now on) must be in big endian order.
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The first eight bytes identify the filesystem, even for the casual
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inspector. After that, in the 3rd longword, it contains the number of
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bytes accessible from the start of this filesystem. The 4th longword
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is the checksum of the first 512 bytes (or the number of bytes
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accessible, whichever is smaller). The applied algorithm is the same
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as in the AFFS filesystem, namely a simple sum of the longwords
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(assuming bigendian quantities again). For details, please consult
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the source. This algorithm was chosen because although it's not quite
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reliable, it does not require any tables, and it is very simple.
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The following bytes are now part of the file system; each file header
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must begin on a 16 byte boundary::
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offset content
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+---+---+---+---+
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0 | next filehdr|X| The offset of the next file header
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+---+---+---+---+ (zero if no more files)
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4 | spec.info | Info for directories/hard links/devices
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+---+---+---+---+
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8 | size | The size of this file in bytes
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+---+---+---+---+
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12 | checksum | Covering the meta data, including the file
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+---+---+---+---+ name, and padding
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16 | file name | The zero terminated name of the file,
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: : padded to 16 byte boundary
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+---+---+---+---+
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xx | file data |
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: :
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Since the file headers begin always at a 16 byte boundary, the lowest
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4 bits would be always zero in the next filehdr pointer. These four
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bits are used for the mode information. Bits 0..2 specify the type of
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the file; while bit 4 shows if the file is executable or not. The
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permissions are assumed to be world readable, if this bit is not set,
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and world executable if it is; except the character and block devices,
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they are never accessible for other than owner. The owner of every
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file is user and group 0, this should never be a problem for the
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intended use. The mapping of the 8 possible values to file types is
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the following:
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== =============== ============================================
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mapping spec.info means
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== =============== ============================================
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0 hard link link destination [file header]
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1 directory first file's header
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2 regular file unused, must be zero [MBZ]
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3 symbolic link unused, MBZ (file data is the link content)
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4 block device 16/16 bits major/minor number
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5 char device - " -
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6 socket unused, MBZ
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7 fifo unused, MBZ
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== =============== ============================================
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Note that hard links are specifically marked in this filesystem, but
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they will behave as you can expect (i.e. share the inode number).
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Note also that it is your responsibility to not create hard link
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loops, and creating all the . and .. links for directories. This is
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normally done correctly by the genromfs program. Please refrain from
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using the executable bits for special purposes on the socket and fifo
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special files, they may have other uses in the future. Additionally,
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please remember that only regular files, and symlinks are supposed to
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have a nonzero size field; they contain the number of bytes available
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directly after the (padded) file name.
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Another thing to note is that romfs works on file headers and data
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aligned to 16 byte boundaries, but most hardware devices and the block
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device drivers are unable to cope with smaller than block-sized data.
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To overcome this limitation, the whole size of the file system must be
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padded to an 1024 byte boundary.
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If you have any problems or suggestions concerning this file system,
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please contact me. However, think twice before wanting me to add
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features and code, because the primary and most important advantage of
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this file system is the small code. On the other hand, don't be
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alarmed, I'm not getting that much romfs related mail. Now I can
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understand why Avery wrote poems in the ARCnet docs to get some more
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feedback. :)
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romfs has also a mailing list, and to date, it hasn't received any
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traffic, so you are welcome to join it to discuss your ideas. :)
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It's run by ezmlm, so you can subscribe to it by sending a message
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to romfs-subscribe@shadow.banki.hu, the content is irrelevant.
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Pending issues:
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- Permissions and owner information are pretty essential features of a
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Un*x like system, but romfs does not provide the full possibilities.
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I have never found this limiting, but others might.
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- The file system is read only, so it can be very small, but in case
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one would want to write _anything_ to a file system, he still needs
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a writable file system, thus negating the size advantages. Possible
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solutions: implement write access as a compile-time option, or a new,
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similarly small writable filesystem for RAM disks.
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- Since the files are only required to have alignment on a 16 byte
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boundary, it is currently possibly suboptimal to read or execute files
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from the filesystem. It might be resolved by reordering file data to
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have most of it (i.e. except the start and the end) laying at "natural"
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boundaries, thus it would be possible to directly map a big portion of
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the file contents to the mm subsystem.
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- Compression might be an useful feature, but memory is quite a
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limiting factor in my eyes.
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- Where it is used?
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- Does it work on other architectures than intel and motorola?
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Have fun,
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Janos Farkas <chexum@shadow.banki.hu>
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