1 =======================
2 initramfs buffer format
3 =======================
4
5 Al Viro, H. Peter Anvin
6
7 Last revision: 2002-01-13
8
9 Starting with kernel 2.5.x, the old "initial ramdisk" protocol is
10 getting {replaced/complemented} with the new "initial ramfs"
11 (initramfs) protocol. The initramfs contents is passed using the same
12 memory buffer protocol used by the initrd protocol, but the contents
13 is different. The initramfs buffer contains an archive which is
14 expanded into a ramfs filesystem; this document details the format of
15 the initramfs buffer format.
16
17 The initramfs buffer format is based around the "newc" or "crc" CPIO
18 formats, and can be created with the cpio(1) utility. The cpio
19 archive can be compressed using gzip(1). One valid version of an
20 initramfs buffer is thus a single .cpio.gz file.
21
22 The full format of the initramfs buffer is defined by the following
23 grammar, 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
42 In human terms, the initramfs buffer contains a collection of
43 compressed and/or uncompressed cpio archives (in the "newc" or "crc"
44 formats); arbitrary amounts zero bytes (for padding) can be added
45 between members.
46
47 The cpio "TRAILER!!!" entry (cpio end-of-archive) is optional, but is
48 not ignored; see "handling of hard links" below.
49
50 The structure of the cpio_header is as follows (all fields contain
51 hexadecimal ASCII numbers fully padded with '0' on the left to the
52 full width of the field, for example, the integer 4780 is represented
53 by the ASCII string "000012ac"):
54
55 ============= ================== ==============================================
56 Field name Field size Meaning
57 ============= ================== ==============================================
58 c_magic 6 bytes The string "070701" or "070702"
59 c_ino 8 bytes File inode number
60 c_mode 8 bytes File mode and permissions
61 c_uid 8 bytes File uid
62 c_gid 8 bytes File gid
63 c_nlink 8 bytes Number of links
64 c_mtime 8 bytes Modification time
65 c_filesize 8 bytes Size of data field
66 c_maj 8 bytes Major part of file device number
67 c_min 8 bytes Minor part of file device number
68 c_rmaj 8 bytes Major part of device node reference
69 c_rmin 8 bytes Minor part of device node reference
70 c_namesize 8 bytes Length of filename, including final \0
71 c_chksum 8 bytes Checksum of data field if c_magic is 070702;
72 otherwise zero
73 ============= ================== ==============================================
74
75 The c_mode field matches the contents of st_mode returned by stat(2)
76 on Linux, and encodes the file type and file permissions.
77
78 The c_filesize should be zero for any file which is not a regular file
79 or symlink.
80
81 The c_chksum field contains a simple 32-bit unsigned sum of all the
82 bytes in the data field. cpio(1) refers to this as "crc", which is
83 clearly incorrect (a cyclic redundancy check is a different and
84 significantly stronger integrity check), however, this is the
85 algorithm used.
86
87 If the filename is "TRAILER!!!" this is actually an end-of-archive
88 marker; the c_filesize for an end-of-archive marker must be zero.
89
90
91 Handling of hard links
92 ======================
93
94 When a nondirectory with c_nlink > 1 is seen, the (c_maj,c_min,c_ino)
95 tuple is looked up in a tuple buffer. If not found, it is entered in
96 the tuple buffer and the entry is created as usual; if found, a hard
97 link rather than a second copy of the file is created. It is not
98 necessary (but permitted) to include a second copy of the file
99 contents; if the file contents is not included, the c_filesize field
100 should be set to zero to indicate no data section follows. If data is
101 present, the previous instance of the file is overwritten; this allows
102 the 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
104 file only.)
105
106 c_filesize must not be zero for a symlink.
107
108 When a "TRAILER!!!" end-of-archive marker is seen, the tuple buffer is
109 reset. This permits archives which are generated independently to be
110 concatenated.
111
112 To combine file data from different sources (without having to
113 regenerate the (c_maj,c_min,c_ino) fields), therefore, either one of
114 the following techniques can be used:
115
116 a) Separate the different file data sources with a "TRAILER!!!"
117 end-of-archive marker, or
118
119 b) Make sure c_nlink == 1 for all nondirectory entries.
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