1 initramfs buffer format 2 ----------------------- 3 4 Al Viro, H. Peter Anvin 5 Last revision: 2002-01-13 6 7Starting with kernel 2.5.x, the old "initial ramdisk" protocol is 8getting {replaced/complemented} with the new "initial ramfs" 9(initramfs) protocol. The initramfs contents is passed using the same 10memory buffer protocol used by the initrd protocol, but the contents 11is different. The initramfs buffer contains an archive which is 12expanded into a ramfs filesystem; this document details the format of 13the initramfs buffer format. 14 15The initramfs buffer format is based around the "newc" or "crc" CPIO 16formats, and can be created with the cpio(1) utility. The cpio 17archive can be compressed using gzip(1). One valid version of an 18initramfs buffer is thus a single .cpio.gz file. 19 20The full format of the initramfs buffer is defined by the following 21grammar, where: 22 * is used to indicate "0 or more occurrences of" 23 (|) indicates alternatives 24 + indicates concatenation 25 GZIP() indicates the gzip(1) of the operand 26 ALGN(n) means padding with null bytes to an n-byte boundary 27 28 initramfs := ("\0" | cpio_archive | cpio_gzip_archive)* 29 30 cpio_gzip_archive := GZIP(cpio_archive) 31 32 cpio_archive := cpio_file* + (<nothing> | cpio_trailer) 33 34 cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data 35 36 cpio_trailer := ALGN(4) + cpio_header + "TRAILER!!!\0" + ALGN(4) 37 38 39In human terms, the initramfs buffer contains a collection of 40compressed and/or uncompressed cpio archives (in the "newc" or "crc" 41formats); arbitrary amounts zero bytes (for padding) can be added 42between members. 43 44The cpio "TRAILER!!!" entry (cpio end-of-archive) is optional, but is 45not ignored; see "handling of hard links" below. 46 47The structure of the cpio_header is as follows (all fields contain 48hexadecimal ASCII numbers fully padded with '0' on the left to the 49full width of the field, for example, the integer 4780 is represented 50by the ASCII string "000012ac"): 51 52Field name Field size Meaning 53c_magic 6 bytes The string "070701" or "070702" 54c_ino 8 bytes File inode number 55c_mode 8 bytes File mode and permissions 56c_uid 8 bytes File uid 57c_gid 8 bytes File gid 58c_nlink 8 bytes Number of links 59c_mtime 8 bytes Modification time 60c_filesize 8 bytes Size of data field 61c_maj 8 bytes Major part of file device number 62c_min 8 bytes Minor part of file device number 63c_rmaj 8 bytes Major part of device node reference 64c_rmin 8 bytes Minor part of device node reference 65c_namesize 8 bytes Length of filename, including final \0 66c_chksum 8 bytes Checksum of data field if c_magic is 070702; 67 otherwise zero 68 69The c_mode field matches the contents of st_mode returned by stat(2) 70on Linux, and encodes the file type and file permissions. 71 72The c_filesize should be zero for any file which is not a regular file 73or symlink. 74 75The c_chksum field contains a simple 32-bit unsigned sum of all the 76bytes in the data field. cpio(1) refers to this as "crc", which is 77clearly incorrect (a cyclic redundancy check is a different and 78significantly stronger integrity check), however, this is the 79algorithm used. 80 81If the filename is "TRAILER!!!" this is actually an end-of-archive 82marker; the c_filesize for an end-of-archive marker must be zero. 83 84 85*** Handling of hard links 86 87When a nondirectory with c_nlink > 1 is seen, the (c_maj,c_min,c_ino) 88tuple is looked up in a tuple buffer. If not found, it is entered in 89the tuple buffer and the entry is created as usual; if found, a hard 90link rather than a second copy of the file is created. It is not 91necessary (but permitted) to include a second copy of the file 92contents; if the file contents is not included, the c_filesize field 93should be set to zero to indicate no data section follows. If data is 94present, the previous instance of the file is overwritten; this allows 95the data-carrying instance of a file to occur anywhere in the sequence 96(GNU cpio is reported to attach the data to the last instance of a 97file only.) 98 99c_filesize must not be zero for a symlink. 100 101When a "TRAILER!!!" end-of-archive marker is seen, the tuple buffer is 102reset. This permits archives which are generated independently to be 103concatenated. 104 105To combine file data from different sources (without having to 106regenerate the (c_maj,c_min,c_ino) fields), therefore, either one of 107the following techniques can be used: 108 109a) Separate the different file data sources with a "TRAILER!!!" 110 end-of-archive marker, or 111 112b) Make sure c_nlink == 1 for all nondirectory entries. 113