dd conv=notrunc: Working around limitations of busybox

Busybox’s dd(1) as shipped in OpenBMC (as of Jan 21 2020) doesn’t support conv=notrunc, which is a mighty handy option if you’re trying to patch some binaries. Which I unfortunately was.

Many other parts of the OpenBMC busybox build are hamstrung as well. For instance grep(1)’s -b (--byte-offset) isn’t supported, nor is -a (--text), though interestingly -o (--only-matching) is. stat(1) is also hamstrung, with no option flags supported - generally stat --format=%s is a nice and concise way to find a file’s size.

Thankfully we have some substitutes - the problem at hand was that some file path strings in a binary no-longer matched the filesystem layout and thus needed some adjustment. Paths are just strings, so instead of grep -bao -F $string we can use strings -t d $file | grep -F "$string" | awk '{print $1}' to get the (decimal) offset. Similarly, wc -c $file | awk '{print $1}' is an indirect way of saying stat --format=%s $file for any file (not just text).

Unfortunately there’s not an obvious substitute for dd conv=notrunc. To fix that, here’s a minimally tested wrapper script that can be installed at /usr/local/bin/dd to add support for conv=notrunc:

#!/bin/sh -x

# SPDX-License-Identifier: Apache-2.0
# Copyright 2020 IBM Corp.

set -eu

ARG_IF=
ARG_OF=
ARG_BS=
ARG_COUNT=
ARG_SKIP=
ARG_SEEK=
ARG_APPEND=
ARG_CONV_NOTRUNC=
DD=/bin/dd

for arg in "$@"
do
	case "$arg" in
		if=*)
			ARG_IF="${arg#*=}"
			;;
		of=*)
			ARG_OF="${arg#*=}"
			;;
		bs=*)
			ARG_BS="${arg#*=}"
			;;
		count=*)
			ARG_COUNT="${arg#*=}"
			;;
		skip=*)
			ARG_SKIP="${arg#*=}"
			;;
		seek=*)
			ARG_SEEK="${arg#*=}"
			;;
		conv=notrunc)
			ARG_CONV_NOTRUNC=1
			;;
		*)
			ARG_APPEND+="${arg#*=}"
			;;
	esac
done

# If conv=notrunc isn't supplied, invoke system dd
if [ -z "$ARG_CONV_NOTRUNC" ]
then
	exec $DD "$@"
fi

# If conv=notrunc is supplied and system dd supports it, invoke system dd
if $DD conv=notrunc < /dev/null > /dev/null 2> /dev/null
then
	exec $DD "$@"
fi

# Otherwise, emulate conv=notrunc support

# Examples:
#
# Inject foo into bar at offset 100:
#
# $ dd if=foo of=bar bs=1 seek=100 conv=notrunc
if [ "$ARG_IF" = "$ARG_OF" ]
then
	>&2 echo "Add support for self-modification"
	exit 1
fi

cleanup()
{
	rm -f $OF_FILE
}

filesize()
{
	# busybox stat(1) doesn't support --format=%s
	wc -c "$1" | awk '{print $1}'
}

trap cleanup EXIT

if [ -z "$ARG_OF" ]
then
	DATA_SOURCE=
else
	DATA_SOURCE='if='$ARG_OF
fi

# Intermediate output file
OF_FILE=$(mktemp)

# Generate the prefix
if [ -z "$ARG_SEEK" ]
then
	PREFIX_COUNT=
else
	PREFIX_COUNT='count='$ARG_SEEK
fi

if [ -z "$ARG_BS" ]
then
	PREFIX_BS=
else
	PREFIX_BS='bs='$ARG_BS
fi
$DD $DATA_SOURCE of="$OF_FILE" $PREFIX_BS $PREFIX_COUNT $ARG_APPEND
OF_UPDATE_OFF=$(filesize "$OF_FILE")

# Append the source data
if [ -z "$ARG_IF" ]
then
	cat >> "$OF_FILE"
else
	cat < "$ARG_IF" >> "$OF_FILE"
fi
OF_SUFFIX_OFF=$(filesize "$OF_FILE")

if [ -z "$ARG_OF" ]
then
	OF_SUFFIX_OFF=$(($OF_SUFFIX_OFF - $OF_UPDATE_OFF))
fi

# Append the suffix
$DD $DATA_SOURCE of="$OF_FILE" bs=1 skip=$OF_SUFFIX_OFF seek=$OF_SUFFIX_OFF $ARG_APPEND

mv "$OF_FILE" "$ARG_OF"