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Linux Transparent Binary Compressor
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#!/bin/sh | |
# This file is licensed under the MIT license, | |
# see the bottom of the file for the full text. | |
# | |
# script dependencies: sed, xz-utils, gcc/clang | |
# generated binary dependencies: xz-util | |
# *note* if using gcc version < 12, every invocation of `cc -Oz` needs to be | |
# changed to `cc -Os` | |
# | |
# this script takes an executable and compresses it using `xz` into a binary | |
# blob and then embeds the blob into a small decompressor stub (whose source | |
# code can be seen inlined in this script). the resulting binary is placed in | |
# the same directory as the script with the same name as the original. | |
# | |
# USAGE: xzbin.sh EXECUTABLE_FILENAME | |
# | |
# TECHNICAL DISCLAIMER: | |
# the decompressor stub this program script uses utilizes the linux specific | |
# system call: memfd_create | |
# while in *theory* this could be avoided using POSIX compliant tools like | |
# shm_create, it adds significant complexity (and therefore size) to the | |
# decompressor stub, and I couldn't get it to behave correctly in some cases. | |
# an additional consequence of using memfd_create is that the file stops | |
# exisiting on exec, so this programs doesn't work on shebang scripts. | |
set -e | |
# arugment sanity check | |
if [ ! $# -eq 1 ] || [ ! -x "$1" ]; then | |
echo "USAGE: $(basename "$0") EXECUTABLE_FILENAME" | |
exit 1 | |
fi | |
BIN_NAME="$(basename "$1")" | |
if [ "$BIN_NAME" = "xzdec" ]; then | |
echo "WARNING: xzdec is used by decompression stub at runtime" | |
echo "!! compressing the xzdec binary will create an inadvertent fork" \ | |
"bomb when running *ANY* compressed program !!" | |
fi | |
# cd to location of script and setup a temp directory | |
cd "$(dirname "$(realpath "$0")")" | |
TMPDIR="$(mktemp -d)" | |
trap 'rm -rf $TMPDIR' EXIT | |
# creates object file containing two symbols: | |
# BIN_BLOB (a byte array containing the xz compressed program) | |
# BIN_BLOB_LEN (the length, in bytes, of BIN_BLOB) | |
# this is accomplished through the hexdump program, some questionable regex, | |
# and a whole lot of pipes :) | |
{ | |
printf "#include <stddef.h>\n"; | |
printf "char BIN_BLOB[] = {\n"; | |
xz -z -c < "$1" |\ | |
hexdump -v |\ | |
sed -r 's/^[0-9a-f]+ ?//' |\ | |
sed -r 's/\s{2,}/ /g' |\ | |
sed -r 's/([0-9a-f]{2})([0-9a-f]{2})/0x\2, 0x\1,/g' |\ | |
sed -r 's/,\s+$//g'; | |
printf "};\nsize_t BIN_BLOB_LEN = sizeof(BIN_BLOB);\n"; | |
} | cc -Oz -c -x c - -o "$TMPDIR/blob.o" | |
# compile stub program from heredoc | |
cc -Oz -c -x c - -o "$TMPDIR/stub.o" << EOF | |
#include <errno.h> | |
#include <fcntl.h> | |
#include <stddef.h> | |
#include <stdio.h> | |
#include <sys/mman.h> | |
#include <sys/syscall.h> | |
#include <sys/wait.h> | |
#include <unistd.h> | |
extern size_t BIN_BLOB_LEN; | |
extern char BIN_BLOB; | |
int main(int argc, char** argv) { | |
int mem_fd = syscall(SYS_memfd_create, argv[0], 0); | |
int pipe_fd[2]; | |
if (pipe(pipe_fd) == -1) goto FAILURE; | |
pid_t pid = fork(); | |
if (pid == -1) goto FAILURE; | |
if (pid == 0) { | |
dup2(pipe_fd[0], STDIN_FILENO); | |
dup2(mem_fd, STDOUT_FILENO); | |
close(pipe_fd[0]); | |
close(pipe_fd[1]); | |
close(mem_fd); | |
execlp("xzdec", "xzdec", NULL); | |
goto FAILURE; | |
} | |
if (write(pipe_fd[1], (void*)&BIN_BLOB, BIN_BLOB_LEN) != BIN_BLOB_LEN) { | |
goto FAILURE; | |
} | |
close(pipe_fd[0]); | |
close(pipe_fd[1]); | |
char buf[30]; | |
snprintf(buf, 30, "/proc/self/fd/%i", mem_fd); | |
fcntl(mem_fd, F_SETFD, FD_CLOEXEC); | |
wait(NULL); | |
fsync(mem_fd); | |
execv(buf, argv); | |
FAILURE: | |
printf("decompressor stub encountered an error: %u", errno); | |
return -1; | |
} | |
EOF | |
# link stub program and binary blob into result | |
cc "$TMPDIR/stub.o" "$TMPDIR/blob.o" -o "./$BIN_NAME" | |
if [ \ | |
"$(du --apparent-size --block-size=1 "$1" | grep -o -E '^[0-9]+')" -le \ | |
"$(du --apparent-size --block-size=1 "./$BIN_NAME" | grep -o -E '^[0-9]+')" \ | |
]; then | |
echo "WARNING: output larger than input, compression ineffective" | |
fi | |
# MIT License | |
# | |
# Copyright 2023 Jenna Fligor | |
# | |
# Permission is hereby granted, free of charge, to any person obtaining a copy | |
# of this software and associated documentation files (the “Software”), to deal | |
# in the Software without restriction, including without limitation the rights | |
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | |
# copies of the Software, and to permit persons to whom the Software is | |
# furnished to do so, subject to the following conditions: | |
# | |
# The above copyright notice and this permission notice shall be included in all | |
# copies or substantial portions of the Software. | |
# | |
# THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | |
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | |
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | |
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | |
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | |
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE | |
# SOFTWARE. |
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