Create unmkinitramfs command

This command supports extracting files from an initramfs image.  It is
designed to be a drop-in for cpio wherever an initramfs may be present.

It is implemented using the code from lsinitramfs, which now delegates
to uninitramfs instead of cpio.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
[bwh: Rename it from uninitramfs to unmkinitramfs; add to .install list]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

debian/initramfs-tools-core.install

@@ -1,4 +1,5 @@
 lsinitramfs		usr/bin
+unmkinitramfs		usr/bin
 init			usr/share/initramfs-tools
 scripts			usr/share/initramfs-tools
 conf/initramfs.conf	etc/initramfs-tools

lsinitramfs

@@ -12,7 +12,7 @@ if [ "$#" -eq 0 ] ; then
 	exit 1
 fi
 
-cpio_args="--extract --quiet --list"
+cpio_args="--quiet --list"
 
 OPTIONS=`getopt -o hl --long help,long -n "$0" -- "$@"`
 # Check for non-GNU getopt
@@ -40,73 +40,6 @@ while true; do
 	esac
 done
 
-listarchive()
-{
-	archive="$1"
-	if zcat -t "${archive}" >/dev/null 2>&1 ; then
-		zcat "${archive}" | cpio ${cpio_args}
-	elif xzcat -t "${archive}" >/dev/null 2>&1 ; then
-		xzcat "${archive}" | cpio ${cpio_args}
-	elif bzip2 -t "${archive}" >/dev/null 2>&1 ; then
-		bzip2 -c -d "${archive}" | cpio ${cpio_args}
-	elif lzop -t "${archive}" >/dev/null 2>&1 ; then
-		lzop -c -d "${archive}" | cpio ${cpio_args}
-	fi
-}
-
-# Read bytes out of a file, checking that they are valid hex digits
-readhex()
-{
-	dd < "$1" bs=1 skip="$2" count="$3" 2> /dev/null | \
-		LANG=C grep -E "^[0-9A-Fa-f]{$3}\$"
-}
-
-# Check for a zero byte in a file
-checkzero()
-{
-	dd < "$1" bs=1 skip="$2" count=1 2> /dev/null | \
-		LANG=C grep -q -z '^$'
-}
-
 for initramfs in "$@" ; do
-	if ! [ -r "${initramfs}" ] ; then
-		echo "Specified file could not be read." >&2
-		exit 1
-	else
-		echo "${initramfs}"
-
-		# There may be a prepended uncompressed archive.  cpio
-		# won't tell us the true size of this so we have to
-		# parse the headers and padding ourselves.  This is
-		# very roughly based on linux/lib/earlycpio.c
-		offset=0
-		while true; do
-			if checkzero "$initramfs" $offset; then
-				offset=$((offset + 4))
-				continue
-			fi
-			magic="$(readhex "$initramfs" $offset 6)" || break
-			test $magic = 070701 || test $magic = 070702 || break
-			namesize=0x$(readhex "$initramfs" $((offset + 94)) 8)
-			filesize=0x$(readhex "$initramfs" $((offset + 54)) 8)
-			offset=$(((offset + 110)))
-			offset=$(((offset + $namesize + 3) & ~3))
-			offset=$(((offset + $filesize + 3) & ~3))
-		done
-
-		if [ $offset -ne 0 ]; then
-			# List uncompressed archive
-			cpio ${cpio_args} < "$initramfs"
-
-			# List main archive
-			subarchive=$(mktemp ${TMPDIR:-/var/tmp}/lsinitramfs_XXXXXX)
-			trap "rm -f $subarchive" EXIT
-			dd < "$initramfs" bs="$offset" skip=1 2> /dev/null \
-				> $subarchive
-			listarchive $subarchive
-		else
-			listarchive "${initramfs}"
-		fi
-
-	fi
+	unmkinitramfs $cpio_args < "$initramfs"
 done

lsinitramfs.8

@@ -50,4 +50,5 @@ and numerous others.
 .BR
 .IR initramfs-tools (8),
 .IR mkinitramfs (8),
+.IR unmkinitramfs (8),
 .IR update-initramfs (8).

mkinitramfs.8

@@ -155,4 +155,5 @@ Jeff Bailey <jbailey@raspberryginger.com> and numerous others.
 .IR initramfs.conf (5),
 .IR initramfs-tools (8),
 .IR update-initramfs (8),
-.IR lsinitramfs (8).
+.IR lsinitramfs (8),
+.IR unmkinitramfs (8).

unmkinitramfs

@@ -0,0 +1,99 @@
+#!/bin/sh
+
+set -eu
+
+usage()
+{
+	echo "Usage: $(basename "$0") [cpio options ...] < <initramfs file>"
+}
+
+# Abort if stdin is a terminal.
+#
+# Although cpio supports this, the chance of someone actually keying in a cpio
+# archive on the terminal is negligible compared to the risk of someone who
+# forgot cpio's arcane argument convention.
+if [ -t 0 ] ; then
+	echo "Error: stdin must be an initramfs" >&2
+	usage >&2
+	exit 1
+fi
+
+# Extract a compressed cpio archive
+xcpio()
+{
+	archive="$1" ; shift
+
+	if zcat -t "$archive" >/dev/null 2>&1 ; then
+		zcat "$archive" | cpio "$@"
+	elif xzcat -t "$archive" >/dev/null 2>&1 ; then
+		xzcat "$archive" | cpio "$@"
+	elif bzip2 -t "$archive" >/dev/null 2>&1 ; then
+		bzip2 -c -d "$archive" | cpio "$@"
+	elif lzop -t "$archive" >/dev/null 2>&1 ; then
+		lzop -c -d "$archive" | cpio "$@"
+	# Ignoring other data, which may be garbage at the end of the file
+	fi
+}
+
+# Read bytes out of a file, checking that they are valid hex digits
+readhex()
+{
+	dd < "$1" bs=1 skip="$2" count="$3" 2> /dev/null | \
+		LANG=C grep -E "^[0-9A-Fa-f]{$3}\$"
+}
+
+# Check for a zero byte in a file
+checkzero()
+{
+	dd < "$1" bs=1 skip="$2" count=1 2> /dev/null | \
+		LANG=C grep -q -z '^$'
+}
+
+# Split an initramfs into archives and call xcpio on each
+splitinitramfs()
+{
+	initramfs="$1" ; shift
+
+	# There may be a prepended uncompressed archive.  cpio
+	# won't tell us the true size of this so we have to
+	# parse the headers and padding ourselves.  This is
+	# very roughly based on linux/lib/earlycpio.c
+	offset=0
+	while true; do
+		if checkzero "$initramfs" $offset; then
+			offset=$((offset + 4))
+			continue
+		fi
+		magic="$(readhex "$initramfs" $offset 6)" || break
+		test $magic = 070701 || test $magic = 070702 || break
+		namesize=0x$(readhex "$initramfs" $((offset + 94)) 8)
+		filesize=0x$(readhex "$initramfs" $((offset + 54)) 8)
+		offset=$(((offset + 110)))
+		offset=$(((offset + $namesize + 3) & ~3))
+		offset=$(((offset + $filesize + 3) & ~3))
+	done
+
+	if [ $offset -ne 0 ]; then
+		# uncompressed archive
+		cpio -i "$@" < "$initramfs"
+
+		# main archive
+		subarchive=$(mktemp ${TMPDIR:-/var/tmp}/unmkinitramfs_XXXXXX)
+		trap "rm -f '$subarchive'" EXIT
+		dd < "$initramfs" bs="$offset" skip=1 2> /dev/null \
+			> $subarchive
+		xcpio "$subarchive" -i "$@"
+	else
+		xcpio "$initramfs" -i "$@"
+	fi
+}
+
+# If we can get the name of the file passed in, use it.  Otherwise, create one.
+stdin_filename=$(readlink /proc/$$/fd/0 2>/dev/null || true)
+if ! [ -r "$stdin_filename" ] ; then
+	stdin_filename=$(mktemp ${TMPDIR:-/var/tmp}/uninitramfs_stdin_XXXXXX)
+	trap "rm -f '$stdin_filename'" EXIT
+	cat > "$stdin_filename"
+fi
+
+splitinitramfs "$stdin_filename" "$@"

unmkinitramfs.8

@@ -0,0 +1,59 @@
+.TH UNMKINITRAMFS 8  "2016/02/09" "Linux" "unmkinitramfs manual"
+
+.SH NAME
+unmkinitramfs \- extract content from an initramfs image
+
+.SH SYNOPSIS
+.B unmkinitramfs
+.RI "" "[cpio options ...]" " < " <initramfsfile>
+.br
+
+.SH DESCRIPTION
+The
+.B unmkinitramfs
+command extracts the content of a given initramfs image using
+.BR cpio .
+If the image contains multiple segments, each are passed to
+.B cpio
+in order.
+
+.SH OPTIONS
+
+All options are passed through to
+.B cpio
+, after an initial
+.B \-i
+option which puts
+.B cpio
+into \*(lqextract\*(rq or \*(lqCopy-in\*(rq mode.
+
+.SH USAGE EXAMPLES
+
+Extract initramfs content of current running kernel:
+
+.PP
+.B unmkinitramfs < /boot/initrd.img-$(uname -r)
+
+Extract
+.I init
+from the initramfs verbosely and keep its mtime:
+
+.PP
+.B unmkinitramfs -m -v init < /boot/initrd.img-$(uname -r)
+
+.SH BUGS
+.BR unmkinitramfs
+cannot deal with multiple-segmented initramfs images, except where an
+early (uncompressed) initramfs with system firmware is prepended to
+the regular compressed initramfs.
+
+.SH AUTHOR
+The initramfs-tools are written by Maximilian Attems <maks@debian.org>
+and numerous others.
+
+.SH SEE ALSO
+.BR
+.IR initramfs-tools (8),
+.IR lsinitramfs (8),
+.IR mkinitramfs (8),
+.IR update-initramfs (8).