diff options
Diffstat (limited to 'Documentation/early-userspace/buffer-format.txt')
| -rw-r--r-- | Documentation/early-userspace/buffer-format.txt | 112 |
1 files changed, 0 insertions, 112 deletions
diff --git a/Documentation/early-userspace/buffer-format.txt b/Documentation/early-userspace/buffer-format.txt deleted file mode 100644 index e1fd7f9dad16..000000000000 --- a/Documentation/early-userspace/buffer-format.txt +++ /dev/null @@ -1,112 +0,0 @@ - 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. |