forked from mirrors/linux
1da177e4c3
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
112 lines
4.5 KiB
Text
112 lines
4.5 KiB
Text
initramfs buffer format
|
|
-----------------------
|
|
|
|
Al Viro, H. Peter Anvin
|
|
Last revision: 2002-01-13
|
|
|
|
Starting with kernel 2.5.x, the old "initial ramdisk" protocol is
|
|
getting {replaced/complemented} with the new "initial ramfs"
|
|
(initramfs) protocol. The initramfs contents is passed using the same
|
|
memory buffer protocol used by the initrd protocol, but the contents
|
|
is different. The initramfs buffer contains an archive which is
|
|
expanded into a ramfs filesystem; this document details the format of
|
|
the initramfs buffer format.
|
|
|
|
The initramfs buffer format is based around the "newc" or "crc" CPIO
|
|
formats, and can be created with the cpio(1) utility. The cpio
|
|
archive can be compressed using gzip(1). One valid version of an
|
|
initramfs buffer is thus a single .cpio.gz file.
|
|
|
|
The full format of the initramfs buffer is defined by the following
|
|
grammar, where:
|
|
* is used to indicate "0 or more occurrences of"
|
|
(|) indicates alternatives
|
|
+ indicates concatenation
|
|
GZIP() indicates the gzip(1) of the operand
|
|
ALGN(n) means padding with null bytes to an n-byte boundary
|
|
|
|
initramfs := ("\0" | cpio_archive | cpio_gzip_archive)*
|
|
|
|
cpio_gzip_archive := GZIP(cpio_archive)
|
|
|
|
cpio_archive := cpio_file* + (<nothing> | cpio_trailer)
|
|
|
|
cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data
|
|
|
|
cpio_trailer := ALGN(4) + cpio_header + "TRAILER!!!\0" + ALGN(4)
|
|
|
|
|
|
In human terms, the initramfs buffer contains a collection of
|
|
compressed and/or uncompressed cpio archives (in the "newc" or "crc"
|
|
formats); arbitrary amounts zero bytes (for padding) can be added
|
|
between members.
|
|
|
|
The cpio "TRAILER!!!" entry (cpio end-of-archive) is optional, but is
|
|
not ignored; see "handling of hard links" below.
|
|
|
|
The structure of the cpio_header is as follows (all fields contain
|
|
hexadecimal ASCII numbers fully padded with '0' on the left to the
|
|
full width of the field, for example, the integer 4780 is represented
|
|
by the ASCII string "000012ac"):
|
|
|
|
Field name Field size Meaning
|
|
c_magic 6 bytes The string "070701" or "070702"
|
|
c_ino 8 bytes File inode number
|
|
c_mode 8 bytes File mode and permissions
|
|
c_uid 8 bytes File uid
|
|
c_gid 8 bytes File gid
|
|
c_nlink 8 bytes Number of links
|
|
c_mtime 8 bytes Modification time
|
|
c_filesize 8 bytes Size of data field
|
|
c_maj 8 bytes Major part of file device number
|
|
c_min 8 bytes Minor part of file device number
|
|
c_rmaj 8 bytes Major part of device node reference
|
|
c_rmin 8 bytes Minor part of device node reference
|
|
c_namesize 8 bytes Length of filename, including final \0
|
|
c_chksum 8 bytes Checksum of data field if c_magic is 070702;
|
|
otherwise zero
|
|
|
|
The c_mode field matches the contents of st_mode returned by stat(2)
|
|
on Linux, and encodes the file type and file permissions.
|
|
|
|
The c_filesize should be zero for any file which is not a regular file
|
|
or symlink.
|
|
|
|
The c_chksum field contains a simple 32-bit unsigned sum of all the
|
|
bytes in the data field. cpio(1) refers to this as "crc", which is
|
|
clearly incorrect (a cyclic redundancy check is a different and
|
|
significantly stronger integrity check), however, this is the
|
|
algorithm used.
|
|
|
|
If the filename is "TRAILER!!!" this is actually an end-of-archive
|
|
marker; the c_filesize for an end-of-archive marker must be zero.
|
|
|
|
|
|
*** Handling of hard links
|
|
|
|
When a nondirectory with c_nlink > 1 is seen, the (c_maj,c_min,c_ino)
|
|
tuple is looked up in a tuple buffer. If not found, it is entered in
|
|
the tuple buffer and the entry is created as usual; if found, a hard
|
|
link rather than a second copy of the file is created. It is not
|
|
necessary (but permitted) to include a second copy of the file
|
|
contents; if the file contents is not included, the c_filesize field
|
|
should be set to zero to indicate no data section follows. If data is
|
|
present, the previous instance of the file is overwritten; this allows
|
|
the data-carrying instance of a file to occur anywhere in the sequence
|
|
(GNU cpio is reported to attach the data to the last instance of a
|
|
file only.)
|
|
|
|
c_filesize must not be zero for a symlink.
|
|
|
|
When a "TRAILER!!!" end-of-archive marker is seen, the tuple buffer is
|
|
reset. This permits archives which are generated independently to be
|
|
concatenated.
|
|
|
|
To combine file data from different sources (without having to
|
|
regenerate the (c_maj,c_min,c_ino) fields), therefore, either one of
|
|
the following techniques can be used:
|
|
|
|
a) Separate the different file data sources with a "TRAILER!!!"
|
|
end-of-archive marker, or
|
|
|
|
b) Make sure c_nlink == 1 for all nondirectory entries.
|