Many of the Linux security/integrity features are dependent on file
metadata, stored as extended attributes (xattrs), for making decisions.
These features need to be initialized during initcall and enabled as
early as possible for complete security coverage.
Initramfs (tmpfs) supports xattrs, but newc CPIO archive format does not
support including them into the archive.
This patch describes "extended" newc format (newcx) that is based on
newc and has following changes:
- extended attributes support
- increased size of filesize to support files >4GB.
- increased mtime field size to have usec precision and more than
32-bit of seconds.
- removed unused checksum field.
Signed-off-by: Taras Kondratiuk <takondra@xxxxxxxxx>
Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Victor Kamensky <kamensky@xxxxxxxxx>
---
Documentation/early-userspace/buffer-format.txt | 46 ++++++++++++++++++++++---
1 file changed, 41 insertions(+), 5 deletions(-)
diff --git a/Documentation/early-userspace/buffer-format.txt b/Documentation/early-userspace/buffer-format.txt
index e1fd7f9dad16..d818df4f72dc 100644
--- a/Documentation/early-userspace/buffer-format.txt
+++ b/Documentation/early-userspace/buffer-format.txt
@@ -24,6 +24,7 @@ grammar, where:
+ indicates concatenation
GZIP() indicates the gzip(1) of the operand
ALGN(n) means padding with null bytes to an n-byte boundary
+ [n] means size of field is n bytes
initramfs := ("\0" | cpio_archive | cpio_gzip_archive)*
@@ -31,20 +32,29 @@ grammar, where:
cpio_archive := cpio_file* + (<nothing> | cpio_trailer)
- cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data
+ cpio_file := (cpio_newc_file | cpio_newcx_file)
+
+ cpio_newc_file := ALGN(4) + cpio_newc_header + filename + "\0" + \
+ ALGN(4) + data
+
+ cpio_newcx_file := ALGN(4) + cpio_newcx_header + filename + "\0" + \
+ ALGN(4) + xattrs + ALGN(4) + data
+
+ xattrs := xattr_entry*
+
+ xattr_entry := xattr_size[8] + xattr_name + "\0" + xattr_value
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.
+compressed and/or uncompressed cpio archives; 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
+The structure of the cpio_newc_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"):
@@ -81,6 +91,32 @@ 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.
+"Extended" newc format (newcx)
+"newcx" cpio format extends "newc" by increasing size of some fields
+and adding extended attributes support. cpio_newcx_header structure:
+
+Field name Field size Meaning
+c_magic 6 bytes The string "070703"
+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 16 bytes Modification time (microseconds)
+c_filesize 16 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_xattrs_size 8 bytes Size of xattrs field
+
+Most of the fields match cpio_newc_header except c_mtime that contains
+microseconds. c_chksum field is dropped.
+
+xattr_size is a total size of xattr_entry including 8 bytes of
+xattr_size. xattr_size has the same hexadecimal ASCII encoding as other
+fields of cpio header.
*** Handling of hard links
--
2.10.3.dirty
--
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