Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 112 insertions, 0 deletions

diff --git a/Documentation/early-userspace/buffer-format.txt b/Documentation/early-userspace/buffer-format.txt
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+		       initramfs buffer format
+		       -----------------------
+
+		       Al Viro, H. Peter Anvin
+		      Last revision: 2002-01-13
+
+Starting with kernel 2.5.x, the old "initial ramdisk" protocol is
+getting {replaced/complemented} with the new "initial ramfs"
+(initramfs) protocol.  The initramfs contents is passed using the same
+memory buffer protocol used by the initrd protocol, but the contents
+is different.  The initramfs buffer contains an archive which is
+expanded into a ramfs filesystem; this document details the format of
+the initramfs buffer format.
+
+The initramfs buffer format is based around the "newc" or "crc" CPIO
+formats, and can be created with the cpio(1) utility.  The cpio
+archive can be compressed using gzip(1).  One valid version of an
+initramfs buffer is thus a single .cpio.gz file.
+
+The full format of the initramfs buffer is defined by the following
+grammar, where:
+	*	is used to indicate "0 or more occurrences of"
+	(|)	indicates alternatives
+	+	indicates concatenation
+	GZIP()	indicates the gzip(1) of the operand
+	ALGN(n)	means padding with null bytes to an n-byte boundary
+
+	initramfs  := ("\0" | cpio_archive | cpio_gzip_archive)*
+
+	cpio_gzip_archive := GZIP(cpio_archive)
+
+	cpio_archive := cpio_file* + (<nothing> | cpio_trailer)
+
+	cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data
+
+	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.
+
+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
+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"):
+
+Field name    Field size	 Meaning
+c_magic	      6 bytes		 The string "070701" or "070702"
+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	      8 bytes		 Modification time
+c_filesize    8 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_chksum      8 bytes		 Checksum of data field if c_magic is 070702;
+				 otherwise zero
+
+The c_mode field matches the contents of st_mode returned by stat(2)
+on Linux, and encodes the file type and file permissions.
+
+The c_filesize should be zero for any file which is not a regular file
+or symlink.
+
+The c_chksum field contains a simple 32-bit unsigned sum of all the
+bytes in the data field.  cpio(1) refers to this as "crc", which is
+clearly incorrect (a cyclic redundancy check is a different and
+significantly stronger integrity check), however, this is the
+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.
+
+
+*** Handling of hard links
+
+When a nondirectory with c_nlink > 1 is seen, the (c_maj,c_min,c_ino)
+tuple is looked up in a tuple buffer.  If not found, it is entered in
+the tuple buffer and the entry is created as usual; if found, a hard
+link rather than a second copy of the file is created.  It is not
+necessary (but permitted) to include a second copy of the file
+contents; if the file contents is not included, the c_filesize field
+should be set to zero to indicate no data section follows.  If data is
+present, the previous instance of the file is overwritten; this allows
+the data-carrying instance of a file to occur anywhere in the sequence
+(GNU cpio is reported to attach the data to the last instance of a
+file only.)
+
+c_filesize must not be zero for a symlink.
+
+When a "TRAILER!!!" end-of-archive marker is seen, the tuple buffer is
+reset.  This permits archives which are generated independently to be
+concatenated.
+
+To combine file data from different sources (without having to
+regenerate the (c_maj,c_min,c_ino) fields), therefore, either one of
+the following techniques can be used:
+
+a) Separate the different file data sources with a "TRAILER!!!"
+   end-of-archive marker, or
+
+b) Make sure c_nlink == 1 for all nondirectory entries.