buffer-format.rst (4857B)
1======================= 2initramfs buffer format 3======================= 4 5Al Viro, H. Peter Anvin 6 7Last revision: 2002-01-13 8 9Starting with kernel 2.5.x, the old "initial ramdisk" protocol is 10getting {replaced/complemented} with the new "initial ramfs" 11(initramfs) protocol. The initramfs contents is passed using the same 12memory buffer protocol used by the initrd protocol, but the contents 13is different. The initramfs buffer contains an archive which is 14expanded into a ramfs filesystem; this document details the format of 15the initramfs buffer format. 16 17The initramfs buffer format is based around the "newc" or "crc" CPIO 18formats, and can be created with the cpio(1) utility. The cpio 19archive can be compressed using gzip(1). One valid version of an 20initramfs buffer is thus a single .cpio.gz file. 21 22The full format of the initramfs buffer is defined by the following 23grammar, where:: 24 25 * is used to indicate "0 or more occurrences of" 26 (|) indicates alternatives 27 + indicates concatenation 28 GZIP() indicates the gzip(1) of the operand 29 ALGN(n) means padding with null bytes to an n-byte boundary 30 31 initramfs := ("\0" | cpio_archive | cpio_gzip_archive)* 32 33 cpio_gzip_archive := GZIP(cpio_archive) 34 35 cpio_archive := cpio_file* + (<nothing> | cpio_trailer) 36 37 cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data 38 39 cpio_trailer := ALGN(4) + cpio_header + "TRAILER!!!\0" + ALGN(4) 40 41 42In human terms, the initramfs buffer contains a collection of 43compressed and/or uncompressed cpio archives (in the "newc" or "crc" 44formats); arbitrary amounts zero bytes (for padding) can be added 45between members. 46 47The cpio "TRAILER!!!" entry (cpio end-of-archive) is optional, but is 48not ignored; see "handling of hard links" below. 49 50The structure of the cpio_header is as follows (all fields contain 51hexadecimal ASCII numbers fully padded with '0' on the left to the 52full width of the field, for example, the integer 4780 is represented 53by the ASCII string "000012ac"): 54 55============= ================== ============================================== 56Field name Field size Meaning 57============= ================== ============================================== 58c_magic 6 bytes The string "070701" or "070702" 59c_ino 8 bytes File inode number 60c_mode 8 bytes File mode and permissions 61c_uid 8 bytes File uid 62c_gid 8 bytes File gid 63c_nlink 8 bytes Number of links 64c_mtime 8 bytes Modification time 65c_filesize 8 bytes Size of data field 66c_maj 8 bytes Major part of file device number 67c_min 8 bytes Minor part of file device number 68c_rmaj 8 bytes Major part of device node reference 69c_rmin 8 bytes Minor part of device node reference 70c_namesize 8 bytes Length of filename, including final \0 71c_chksum 8 bytes Checksum of data field if c_magic is 070702; 72 otherwise zero 73============= ================== ============================================== 74 75The c_mode field matches the contents of st_mode returned by stat(2) 76on Linux, and encodes the file type and file permissions. 77 78The c_filesize should be zero for any file which is not a regular file 79or symlink. 80 81The c_chksum field contains a simple 32-bit unsigned sum of all the 82bytes in the data field. cpio(1) refers to this as "crc", which is 83clearly incorrect (a cyclic redundancy check is a different and 84significantly stronger integrity check), however, this is the 85algorithm used. 86 87If the filename is "TRAILER!!!" this is actually an end-of-archive 88marker; the c_filesize for an end-of-archive marker must be zero. 89 90 91Handling of hard links 92====================== 93 94When a nondirectory with c_nlink > 1 is seen, the (c_maj,c_min,c_ino) 95tuple is looked up in a tuple buffer. If not found, it is entered in 96the tuple buffer and the entry is created as usual; if found, a hard 97link rather than a second copy of the file is created. It is not 98necessary (but permitted) to include a second copy of the file 99contents; if the file contents is not included, the c_filesize field 100should be set to zero to indicate no data section follows. If data is 101present, the previous instance of the file is overwritten; this allows 102the data-carrying instance of a file to occur anywhere in the sequence 103(GNU cpio is reported to attach the data to the last instance of a 104file only.) 105 106c_filesize must not be zero for a symlink. 107 108When a "TRAILER!!!" end-of-archive marker is seen, the tuple buffer is 109reset. This permits archives which are generated independently to be 110concatenated. 111 112To combine file data from different sources (without having to 113regenerate the (c_maj,c_min,c_ino) fields), therefore, either one of 114the following techniques can be used: 115 116a) Separate the different file data sources with a "TRAILER!!!" 117 end-of-archive marker, or 118 119b) Make sure c_nlink == 1 for all nondirectory entries.