1 initramfs buffer format
2 -----------------------
3
4 Al Viro, H. Peter Anvin
5 Last revision: 2002-01-13
6
7 Starting with kernel 2.5.x, the old "initial ramdisk" protocol is
8 getting {replaced/complemented} with the new "initial ramfs"
9 (initramfs) protocol. The initramfs contents is passed using the same
10 memory buffer protocol used by the initrd protocol, but the contents
11 is different. The initramfs buffer contains an archive which is
12 expanded into a ramfs filesystem; this document details the format of
13 the initramfs buffer format.
14
15 The initramfs buffer format is based around the "newc" or "crc" CPIO
16 formats, and can be created with the cpio(1) utility. The cpio
17 archive can be compressed using gzip(1). One valid version of an
18 initramfs buffer is thus a single .cpio.gz file.
19
20 The full format of the initramfs buffer is defined by the following
21 grammar, 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
39 In human terms, the initramfs buffer contains a collection of
40 compressed and/or uncompressed cpio archives (in the "newc" or "crc"
41 formats); arbitrary amounts zero bytes (for padding) can be added
42 between members.
43
44 The cpio "TRAILER!!!" entry (cpio end-of-archive) is optional, but is
45 not ignored; see "handling of hard links" below.
46
47 The structure of the cpio_header is as follows (all fields contain
48 hexadecimal ASCII numbers fully padded with '0' on the left to the
49 full width of the field, for example, the integer 4780 is represented
50 by the ASCII string "000012ac"):
51
52 Field name Field size Meaning
53 c_magic 6 bytes The string "070701" or "070702"
54 c_ino 8 bytes File inode number
55 c_mode 8 bytes File mode and permissions
56 c_uid 8 bytes File uid
57 c_gid 8 bytes File gid
58 c_nlink 8 bytes Number of links
59 c_mtime 8 bytes Modification time
60 c_filesize 8 bytes Size of data field
61 c_maj 8 bytes Major part of file device number
62 c_min 8 bytes Minor part of file device number
63 c_rmaj 8 bytes Major part of device node reference
64 c_rmin 8 bytes Minor part of device node reference
65 c_namesize 8 bytes Length of filename, including final \0
66 c_chksum 8 bytes Checksum of data field if c_magic is 070702;
67 otherwise zero
68
69 The c_mode field matches the contents of st_mode returned by stat(2)
70 on Linux, and encodes the file type and file permissions.
71
72 The c_filesize should be zero for any file which is not a regular file
73 or symlink.
74
75 The c_chksum field contains a simple 32-bit unsigned sum of all the
76 bytes in the data field. cpio(1) refers to this as "crc", which is
77 clearly incorrect (a cyclic redundancy check is a different and
78 significantly stronger integrity check), however, this is the
79 algorithm used.
80
81 If the filename is "TRAILER!!!" this is actually an end-of-archive
82 marker; the c_filesize for an end-of-archive marker must be zero.
83
84
85 *** Handling of hard links
86
87 When a nondirectory with c_nlink > 1 is seen, the (c_maj,c_min,c_ino)
88 tuple is looked up in a tuple buffer. If not found, it is entered in
89 the tuple buffer and the entry is created as usual; if found, a hard
90 link rather than a second copy of the file is created. It is not
91 necessary (but permitted) to include a second copy of the file
92 contents; if the file contents is not included, the c_filesize field
93 should be set to zero to indicate no data section follows. If data is
94 present, the previous instance of the file is overwritten; this allows
95 the 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
97 file only.)
98
99 c_filesize must not be zero for a symlink.
100
101 When a "TRAILER!!!" end-of-archive marker is seen, the tuple buffer is
102 reset. This permits archives which are generated independently to be
103 concatenated.
104
105 To combine file data from different sources (without having to
106 regenerate the (c_maj,c_min,c_ino) fields), therefore, either one of
107 the following techniques can be used:
108
109 a) Separate the different file data sources with a "TRAILER!!!"
110 end-of-archive marker, or
111
112 b) Make sure c_nlink == 1 for all nondirectory entries.
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