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README.rst

dnload

Minimal binary generator for *nix operating systems

dnload.py

dnload.py is a script for generating minimal ELF binaries from C code. It serves no practical real-world use case, but can be utilized to aid in the creation of size-limited demoscene productions.

System requirements

The bridging header file can be created on any *nix or Windows platform with an up-to-date Python installation and a suitable compiler.

Generating a final binary is supported on *nix platforms only. Due to practical purposes (i.e. the OS of choice of the author), the primary target operating system is FreeBSD. Existence of binutils toolchain (as, ld, etc.) on the system is assumed.

The script is self-contained, and should require no external python packages to be installed.

For compiling without size optimizations, GLEW and SDL development files are needed. This is subject to change if/when other backends are added.

Note: Cross-compilation support is limited. You should be able to compile 32-bit binaries on a 64-bit host by using the --32 -command line argument. You will still need 32-bit libraries or chroot/jail environment to run the generated binary though.

Supported (last updated: 2017-10-12)

  • FreeBSD
  • Linux
  • Windows (preprocessing only)
  • amd64 (x86_64)
  • ia32 (i386)
  • arm32l (armel)
  • clang++
  • g++
  • cl.exe (preprocessing only)
  • python2.7
  • python3

Unsupported but should be supportable (last updated: 2017-10-12)

  • Linux-ia32 (crashes in linked libraries)
  • arm64 (AArch64)
  • ES2 GLSL support

Usage

The script is used for the following purposes purposes:

  • Building size-limited binaries directly from C/C++ source on systems, where compilation is supported.
  • Generating a header file to hide the complexities of size-limited linking. This can be also be on systems where compilation is not supported. The purpose of this feature is to allow developers to work on platforms besides the main targets. The generated header tries to preserve portability.
  • Minifying GLSL shaders. This may done as part of compilation or used separately.

Summary of operation

When invoked, the script will:

  • Probe for suitable compiler, usually gcc or clang, cl.exe on Windows.
  • Search for header file it was supposed to generate. By default this is called dnload.h.
  • Preprocess given source files with the compiler found earlier.
  • Examine preprocessor output and locate all function calls made with a specific prefix. By default this prefix is dnload_.
  • Generate a loader code block that locates pointers to given functions.
  • Write the header file.

If the script was invoked to additionally generate the binary:

  • Search source files for #include directives pointing to GLSL source, if found, minify them.
  • Search for usual binary utilities in addition to the compiler.
  • Compile given source file with flags aiming for small code footprint.
  • Perform a series on operations on the compiler output known to further reduce code footprint.
  • Link an output binary.
  • Possibly strip the generated binary.
  • Compress the produced binary and concatenate it with a shell script that will decompress and execute it.

Only one source file is supported when generating a binary. This is to enable whole program optimization. The user can #include other source files inside this main source for convenience.

Example program

To understand how to use the script, a simple example will clarify the operation with more ease than any lengthy explanation would. This tutorial will cover a traditional hello world program.

Use this command to clone the repository:

git clone https://github.com/faemiyah/dnload.git

The checked out repository will have the dnload.py script in the root folder. The minimal example is included in the src/ folder and called hello_world.cpp. The example looks like this (removing non-essential comments):

#include "dnload.h"

#if defined(USE_LD)
int main()
#else
void _start()
#endif
{
  dnload();
  dnload_puts("Hello World!");
#if defined(USE_LD)
  return 0;
#else
  asm_exit();
#endif
}

When beginning to work with a project, the first thing needed is to ensure that our header is up to date. To do this, run:

python dnload.py -v -E src/hello_world.cpp

This should produce output somewhat akin to this:

Looking for 'preprocessor' executable... 'cpp'
Found header file: 'src/dnload.h'
Selected GLSL swizzle: ('x', 'y', 'z', 'w') (0 vs. 0)
Executing command: cpp src/hello_world.cpp -I/usr/local/include -I/usr/local/include/freetype2/ -I/usr/local/include/SDL
Symbols found: ['puts']
Template substitutions not matched: ['INCLUDE_SNDFILE', 'UND_SYMBOLS', 'INCLUDE_SDL', 'INCLUDE_RAND', 'INCLUDE_OPENGL', 'INCLUDE_FREETYPE'] (6)
Wrote header file: 'src/dnload.h'

You should now have an up-to date header file, which can be used to build the program. You may take a look at the contents of the header, but it will be explained in detail [#The_quest_for_minimal_ELF_binaries later on].

Building the example without size optimizations

Even when developing an intro, the programmer is hardly interested in building a size-optimized binary every time. For this purpose, everything in the generated header file is wrapped to compile-time guards that allow us to compile the program as normal from Makefiles, Autotools scripts, CMake or even Visual Studio projects.

By default this separation is done with the preprocessor definition USE_LD not being present for size-limited builts. So, for normal operation, the user needs to compile with it enabled. For example (replace with your favorite compiler):

> clang++ -o src/hello_world src/hello_world.cpp -DUSE_LD -O2 -s -lc && ./src/hello_world

Hello World!

When USE_LD is turned on, all "tricks" will essentially evaluate to NOP, and all calls made with the reserved dnload_ prefix will simply call the functions as normal. You can change this definition to anything you want.

Compiling the example as a size-optimized binary

To invoke the script and perform full compilation, use:

python dnload.py -v src/hello_world.cpp -o src/hello_world

You might notice the flags are similar to the conventions used in other binary utilities. This is intentional. The command should produce output somewhat similar to this:

Looking for 'preprocessor' executable... 'cpp'
Found header file: 'src/dnload.h'
Analyzing source files: ['src/hello_world.cpp']
Executing command: cpp src/hello_world.cpp -I/usr/local/include -I/usr/local/include/freetype2/ -I/usr/local/include/SDL
Symbols found: ['puts']
Template substitutions not matched: ['INCLUDE_SNDFILE', 'UND_SYMBOLS', 'INCLUDE_SDL', 'INCLUDE_RAND', 'INCLUDE_OPENGL', 'INCLUDE_FREETYPE'] (6)
Wrote header file: 'src/dnload.h'
Looking for 'compiler' executable... 'g++6'
Looking for 'assembler' executable... '/usr/local/bin/as'
Looking for 'linker' executable... '/usr/local/bin/ld'
Looking for 'objcopy' executable... '/usr/local/bin/objcopy'
Looking for 'strip' executable... '/usr/local/bin/strip'
Autodetected libraries to link against: ['c']
Executing command: g++6 -S src/hello_world.cpp -o src/hello_world.S -std=c++11 -Os -ffast-math -fno-asynchronous-unwind-tables -fno-enforce-eh-specs -fno-exceptions -fno-implicit-templates -fno-rtti -fno-stack-protector -fno-threadsafe-statics -fno-use-cxa-atexit -fno-use-cxa-get-exception-ptr -fnothrow-opt -fomit-frame-pointer -funsafe-math-optimizations -fvisibility=hidden -march=pentium4 -Wall -mpreferred-stack-boundary=2 -I/usr/local/include -I/usr/local/include/freetype2/ -I/usr/local/include/SDL -fwhole-program
Checking for required UND symbols... ['__progname', 'environ']
Using shared library 'libc.so.7' instead of 'libc.so'.
Read 4 sections in 'src/hello_world.S': text, rodata, text, data
Sorted sections: 1 rodata, 1 data, 2 text
Alignment adjustment(data): 4 -> 1
Erasing function header from '_start': 5 lines
Erasing function footer after interrupt '0x3': 9 lines
Constructed fake .bss segement: 0 bytes, one PT_LOAD sufficient
Merging headers phdr_dynamic and hash at 4 bytes.
Merging headers hash and dynamic at 4 bytes.
Merging headers dynamic and symtab at 15 bytes.
Merging headers interp and strtab at 1 bytes.
Size of headers: 245 bytes
Wrote assembler source: 'src/hello_world.final.S'
Executing command: /usr/local/bin/as src/hello_world.final.S -o src/hello_world.final.o
Executing command: /usr/local/bin/ld --verbose
Wrote linker script 'src/hello_world.ld'.
Executing command: /usr/local/bin/ld --entry=0x2000000 src/hello_world.final.o -o src/hello_world.bin -T src/hello_world.ld
Executing command: /usr/local/bin/objcopy --output-target=binary src/hello_world.bin src/hello_world.unprocessed
Executing command: readelf -l src/hello_world.unprocessed
Executable size equals PT_LOAD size (411 bytes), no operation necessary.
Executing command: xz --format=lzma --lzma1=preset=9,lc=1,lp=0,nice=273,pb=0 --stdout src/hello_world.stripped
Wrote 'src/hello_world': 321 bytes

Actual program output should be:

> ./src/hello_world
Hello World!
./src/hello_world: line 1:  8835 Trace/BPT trap          ~

The error message seen here is normal, because the program does not exactly terminate normally. For details, see next chapter.

Including dnload into your project

First of all, all programs wanting to use the loader will have to include the generated header file:

#include "dnload.h"

This will internally include the relevant loader and some other header(s) present in the src/ subdirectory into the project. The user may of course include any other source files necessary, but all function calls should be done through the interface wrapped herein.

To understand what the script does, we will look at the main function:

#if defined(USE_LD)
int main()
#else
void _start()
#endif
{
  dnload();
  dnload_puts("Hello World!");
#if defined(USE_LD)
  return 0;
#else
  asm_exit();
#endif
}

If the macro USE_LD would be defined, all this would simply evaluate to a self-explanatory hello world program:

int main()
{
  puts("Hello World!")
  return 0;
}

If USE_LD is not defined, this would evaluate to a minified version instead:

void _start()
{
  dnload();
  dnload_puts("Hello World!");
  asm_exit();
}

Even if using main() normally, it is not the real entry point into the program. In practice, the linker will generate code that will perform initialization of global variables, environment, etc., and only afterwards pass the control to the main function created by the programmer. The actual entry point generated by the compiler/linker system is traditionally called _start. Since we're aiming for a small binary, we will not be using any automatically generated entry point code, and declare it ourselves.

The two first lines of main function comprise the actual program:

dnload();
dnload_puts("Hello World!");

This will perform of dynamic loading of all required symbols (here: only puts) and call the appropriate function pointer. After saying our hellos, the only thing left to do is to exit the program:

asm_exit();

Since we already abandoned the default main procedure, no-one is going to terminate our program. We could make it "return" but that would normally only pass control to the shell provided by the system, we now have nowhere to return to. Thus, we must make a system call [Ref11] to terminate the program ourselves. The code that will do this is written into the dnload header directly as inline assembler. On ia32 architecture, it will evaluate into the following single instruction:

int $3

Note that this is not the traditional exit system call. Instead, it's a trap [ref30] that will break into debugger or exit the program if a debugger is not available. It has the advantage of taking less space than moving $1 into eax and executing a normal int $128 system call. The return value for the program is irrelevant, as the decompressor shell script the program is eventually wrapped in would eventually mask it anyway.

Advanced examples

The src/ folder contains two other examples: quad.cpp and intro.cpp. The quad example will simply open a coder-colored window, whereas the intro example performs (extremely primitive) raycasting and outputs 8-bit music (from very short programs) for a couple of seconds. The intro example can also be compiled with CMake for a interactive program with a 'debug mode'. To compile this, run (you will need Boost, GLEW, libPNG, OpenGL and SDL):

> python dnload.py -E src/intro.cpp
> cmake .
> make clean all
> ./intro -d -w

This should open a window allowing mouse pan and WASD movement.

You can size-optimize the program with:

> python dnload.py -v src/intro.cpp -lGL -lSDL
> ./src/intro

Have fun!


The quest for minimal ELF binaries

This section of the documentation explores both the current and historical methods of reducing executable file size on *nix systems. If you are only interested in the current "best practice" operation of the script, you can skip to Current compression procedure.

Compiler flags

We can alter the compiler output to produce smaller binaries both by making it actually optimize for size and by altering the output to be more compressable. The command line options would fall into three categories:

  • Options that decrease size of generated code or constants.
  • Options that (either randomly or unintentionally) produce a smaller or better compressable binary.
  • Options that disable language features, making output smaller.

Despite Clang being all the rage nowadays, gcc seems to still produce binaries that size-optimize better. In particular, the script will attempt to use whichever g++ is currently the latest available on FreeBSD (g++6 at the time of writing).

Note: Any compiler may be specified with the -C or --compiler command line option, but compilation is currently only supported with gcc or clang. cl.exe may be used to generate the header in Windows.

Using g++, the flags of the first type (just smaller) would be:

  • -Os
  • -ffast-math
  • -fomit-frame-pointer
  • -fsingle-precision-constant
  • -fwhole-program

These are all self-explanatory. In general, mere -Os -ffast-math -fomit-frame-pointer seems to do an excellent job.

The following options are of the second type, seeming to consistently produce code that compresses better:

  • -march=<arch>: Some subsets of the instruction set seem to yield better results. As an example, on i386 architecture, after permutating through all available instruction sets with several different intros, Pentium 4 was usually the best choice.
  • -mpreferred-stack-boundary=<align>: Forces the compiler to attempt keeping the stack aligned at 2^<align> bytes. It seems it's advantageous to keep this at the smallest possible value for any given architecture.

Some flags of the third type, which disable fancy language features, are:

  • -fno-threadsafe-statics: By default some code is generated to ensure thread-safe initialization of local static variables. This code seems to get generated even if no statics actually exist.
  • -fno-asynchronous-unwind-tables: Disables generation of stack unwind information that could be used to debug stack contents at locations other than at the function call boundary.
  • -funsafe-math-optimizations: If you're doing math that needs exception checking in an intro context, you're probably doing it wrong.
  • -fno-exceptions: Self-evident.
  • -fno-rtti: Self-evident.

Note: One could ask why C++ to begin with if we're not using any of its features? The answer is, that it should never be detrimental. After manually disabling all the features that would increase code footprint, we can basically write C using a C++ compiler. In some cases the C++ syntax will be beneficial.

The self-dumping shell script

Scouring old releases, the first instance of a *nix 4k using a self-dumping shell script seems to be helsinki-spiegelberg by tsygä [ref12]. However, instead of dumping an executable binary, this entry actually unpacks to a source and compiles before execution.

There are some other variants afterwards, but the first use of the modern one-line filedump is found in Yellow Rose of Texas by Fit & Bandwagon [ref13]. Consequently, this is also the first "big" Linux 4k intro.

The concept of self-dumping shell script is to have the first n bytes of a file be plain text that will be executed by normal sh-compatible shells. The shell code will:

  • Extract the rest of the file using common *nix tools (the compressed data starts immediately after the script).
  • Write the extracted data as a file into system temporary folder.
  • Make the file executable.
  • Run it.
  • Remove the file after program exits, since that's the proper thing to do.

The header in Yellow Rose looks like this:

dd bs=1 skip=83<$0|gunzip>/tmp/T;cd /tmp;chmod +x T;__GL_FSAA_MODE=4 ./T;rm T;exit

There are multiple programs that crunch classic gzip files to as small sizes as possible, but modern *nix systems come with better compressors and better tricks. For example, using a trick from ts/TDA [ref27], redefining HOME allows using the tilde character for clever reduction:

HOME=/tmp/i;tail -n+2 $0|lzcat>~;chmod +x ~;~;rm ~;exit

This unpack header has gone to 57 bytes (newline for tail is needed at the end) from the 66 needed by Marq, but it's still unnecessarily large as it's trying to be nice. If we omit removing the extracted file and allow the compressed garble to flood the screen, we can do the same in 44 bytes:

HOME=/tmp/i;sed 1d $0|lzcat>~;chmod +x ~;~

One could assume that xz format would be more advantageous than the earlier lzma, but this seems not to be the case as lzma compression headers are smaller [ref28] [ref29].

We use the xz toolkit, however, as it allows free customization of the compression parameters. For actual compression parameters, which yields decent results, we use:

xz --format=lzma --lzma1=preset=9,lc=1,lp=0,pb=0

At least on our bruteforced tests, this seems to consistently yield the best results for binaries aiming to the 4k threshold.

For academic interest, the script can create its output using nothing besides the unpack header trick. To do this, use the command line flag -m vanilla (as in --method). This yields us the baseline whencefrom we begin our quest for smaller binaries:

'src/hello_world.stripped': 1396 bytes
'src/hello_world': 625 bytes
'src/intro.stripped': 3360 bytes
'src/intro': 1546 bytes

Note: Unless otherwise mentioned, all sizes, code excerpts and implementation details in this chapter refer to FreeBSD-ia32, which was the original supported platform.

The "stripped" binaries listed above are generated by just removing all non-needed sections from output binaries. These include .comment, .eh_frame, .eh_frame_hdr, .fini, .gnu.hash, .gnu.version, .jcr, .note, .note.ABI-tag and .note.tag. Using the compiler/linker -s flag seems to leave some of these in, so strip is called manually afterwards.

My symbol table is too big - dlopen/dlsym

Besides the modern unpack header, Yellow Rose also introduced the concept of using POSIX dlopen [ref14] and dlsym [ref15] to load the OpenGL functions it required.

This is relevant, because the symbol tables consume quite a lot of space. Taking a look into the FreeBSD system headers [ref17] we can examine the symbol structs (going into .dynsym):

typedef struct {
  Elf32_Word st_name;      /* String table index of name. */
  Elf32_Addr st_value;     /* Symbol value. */
  Elf32_Word st_size;      /* Size of associated object. */
  unsigned char st_info;   /* Type and binding information. */
  unsigned char st_other;  /* Reserved (not used). */
  Elf32_Half st_shndx;     /* Section index of symbol. */
} Elf32_Sym;

This is 16 bytes per symbol. In addition the linker will generate linkage tables (going into .plt), which seem to be at least 2 instructions per symbol (look at Prodedure Linkage Table in the ELF specification part 2 [ref16]). Global Offset Table adds 4 bytes each. In any case, we're talking roughly 30 bytes per symbol. Not counting the space consumed by the function names themselves.

Luckily, using the forementioned two symbols, we can perform dynamic loading ourselves. Yellow Rose loaded only the GL functions this way, but using a clever arrangement of text, we can embed the library information in a text block at practically no cost:

  • First string is a library name, terminated by zero.
  • Successive strings are function names.
  • Two successive zeroes revert to initial state, next string will again be a library name.
  • Third successive zero stops loading.

This produces the following loader (using intro.cpp example):

static const char g_dynstr[] = ""
"libGL.so\0"
"glBindProgramPipeline\0"
"glCreateShaderProgramv\0"
"glGenProgramPipelines\0"
"glProgramUniform3fv\0"
"glRects\0"
"glUseProgramStages\0"
"\0libSDL.so\0"
"SDL_GL_SwapBuffers\0"
"SDL_Init\0"
"SDL_OpenAudio\0"
"SDL_PauseAudio\0"
"SDL_PollEvent\0"
"SDL_Quit\0"
"SDL_SetVideoMode\0"
"SDL_ShowCursor\0"
"\0";

static void dnload(void)
{
  char *src = (char*)g_dynstr;
  void **dst = (void**)&g_symbol_table;
  do {
    void *handle = dlopen(src, RTLD_LAZY);
    for(;;)
    {
      while(*(src++));
      if(!*(src))
      {
        break;
      }
      *dst++ = dlsym(handle, src);
    }
  } while(*(++src));
}

The code snippet reveals a "fake" symbol table with the name g_symbol_table. It is simply a list of function pointers constructed from the prefixed function names found when preprocessing the source file earlier. Depending on the build mode, we will either select the regular function call or redirect to our table. For the hello world example, the code would look like this:

#if defined(USE_LD)
#define dnload_puts puts
#else
#define dnload_puts g_symbol_table.puts
static struct SymbolTableStruct
{
  int (*puts)(const char*);
} g_symbol_table;
#endif

To gain access to these functions, we need to link against the standard C library with -lc flag. On Linux, use -ldl instead. Additionally on FreeBSD, when using SDL, user must manually link against libthr. Since SDL is not explicitly linked against, it is not pulling the thread library as consequence. It seems threading is not properly initialized unless the dynamic linker gets to load libthr.

Recompiling with -m dlfcn, we get new sizes:

'hello_world.stripped': 1544 bytes
'hello_world': 735 bytes
'intro.stripped': 2996 bytes
'intro': 1515 bytes

Interestingly, this method does not really seem to be that advantageous. Small binaries actually get larger due to additional program logic in the function loading system. On any nontrivial program there is a slight tradeoff in favor of just using plain ld though.

Import by hash - scouring ELF headers

Even if the cost of function name only is rather little, it still adds up for a large block of data, especially as using dlopen and dlsym requires us to have symbol definitions and all the PLT/GOT information required for them in the binary.

Fortunately, there is a better way. In 2008, Pouet users parcelshit [ref3] and las/Mercury [ref4] published code to search for symbols in ELF32 shared objects by their hashed names. Some of the code linked in the discussion [ref19] seems to be unaccessible by now, but at least the original import-by-hash implementation [ref18] (used as proof-of-concept for our implementation) seems to still be available.

The technique essentially takes advantage of the DT_DEBUG element in the .dynamic section (when present) that will contain information about linked shared libraries. We simply need to gain access to it.

Programs using ELF will have access to their own headers directly at the program load address (0x8048000 by default). The ELF32 header is 52 bytes long, and is immediately followed by an array of program headers. Each program header looks like this (from FreeBSD system headers):

typedef struct {
  Elf32_Word  p_type;    /* Entry type. */
  Elf32_Off   p_offset;  /* File offset of contents. */
  Elf32_Addr  p_vaddr;   /* Virtual address in memory image. */
  Elf32_Addr  p_paddr;   /* Physical address (not used). */
  Elf32_Word  p_filesz;  /* Size of contents in file. */
  Elf32_Word  p_memsz;   /* Size of contents in memory. */
  Elf32_Word  p_flags;   /* Access permission flags. */
  Elf32_Word  p_align;   /* Alignment in memory and file. */
} Elf32_Phdr;

For examining the dynamically loaded elements, we need to look for a program header of type PT_DYNAMIC. Upon finding the header, the p_vaddr virtual address pointer will point directly to the dynamic section. The dynamic section, then further consists of an array of structures of type Elf32_Dyn, they look like this:

typedef struct {
  Elf32_Sword  d_tag;   /* Entry type. */
  union {
    Elf32_Word  d_val;  /* Integer value. */
    Elf32_Addr  d_ptr;  /* Address value. */
  } d_un;
} Elf32_Dyn;

When the dynamic linker passes control to your program, the Elf32_Dyn structure with the tag DT_DEBUG will have been filled with a pointer to the debug structure associated with the program. The debug structure looks like this:

struct r_debug {
  int             r_version;  /* not used */
  struct link_map *r_map;     /* list of loaded images */
  void            (*r_brk)(struct r_debug *, struct link_map *);
                              /* pointer to break point */
  enum {
    RT_CONSISTENT,            /* things are stable */
    RT_ADD,                   /* adding a shared library */
    RT_DELETE                 /* removing a shared library */
}                 r_state;
};

In here, the element r_map will contain a pointer to a linked list of structures representing shared objects, that is, libraries. They look like this:

typedef struct link_map {
  caddr_t          l_addr;            /* Base Address of library */
#ifdef __mips__
  caddr_t          l_offs;            /* Load Offset of library */
#endif
  const char       *l_name;           /* Absolute Path to Library */
  const void       *l_ld;             /* Pointer to .dynamic in memory */
  struct link_map  *l_next, *l_prev;  /* linked list of of mapped libs */
} Link_map;

The field l_ld in this structure is a pointer to the particular .dynamic section of this shared object. Reading the ELF specification [#References [16]], we also know that:

  • All symbols of the shared object are located in the section pointed by tag DT_SYMTAB.
  • All symbol names are located in the section pointed by tag DT_STRTAB.
  • Symbols names point to offsets from the start of DT_STRTAB section.
  • The total number of symbols in a shared object is the number of chains in the hash table of the shared object.

Using this information, we can simply go through the shared objects, one by one, then go through the symbols in these shared objects, one by one, only stopping when we find a name with a hash matching the hash we want. We can also use the symbol table format defined in the earlier section by storing these hashes at the same location the function pointer is going to get stored at. The only thing that remains to be done is to find a convenient hashing algorithm.

Fortunately, this has been done for us already [ref20]. There are many good candidates where collisions are improbable enough to be nonexistent. We simply pick the one that produces smallest code. Parcelshit's original code used the DJB hash, which is already good, but in my testing, using SDBM hash produced a smaller binary.

Combining all this, gives us the following proof-of-concept implementation:

#include <stdint.h>
#include <sys/link_elf.h> // <link.h> on Linux

#define ELF_BASE_ADDRESS 0x8048000

static uint32_t sdbm_hash(const uint8_t *op)
{
  uint32_t ret = 0;
  for(;;)
  {
    uint32_t cc = *op++;
    if(!cc)
    {
      return ret;
    }
    ret = ret * 65599 + cc;
  }
}

static const void* elf32_get_dynamic_address_by_tag(const void *dyn, Elf32_Sword tag)
{
  const Elf32_Dyn *dynamic = (Elf32_Dyn*)dyn;
  for(;;)
  {
    if(dynamic->d_tag == tag)
    {
      return (const void*)dynamic->d_un.d_ptr;
    }
    ++dynamic;
  }
}

static const void* elf32_get_library_dynamic_section(const struct link_map *lmap, Elf32_Sword op)
{
  const void *ret = elf32_get_dynamic_address_by_tag(lmap->l_ld, op);
  // Sometimes the value is an offset instead of a naked pointer.
  return (ret < (void*)lmap->l_addr) ? (uint8_t*)ret + (size_t)lmap->l_addr : ret;
}

static const struct link_map* elf32_get_link_map()
{
  // ELF header is in a fixed location in memory.
  // First program header is located directly afterwards.
  const Elf32_Ehdr *ehdr = (const Elf32_Ehdr*)ELF_BASE_ADDRESS;
  const Elf32_Phdr *phdr = (const Elf32_Phdr*)((size_t)ehdr + (size_t)ehdr->e_phoff);
  // Find the dynamic header by traversing the phdr array.
  for(; (phdr->p_type != PT_DYNAMIC); ++phdr) { }
  // Find the debug entry in the dynamic header array.
  {
    const struct r_debug *debug = (const struct r_debug*)elf32_get_dynamic_address_by_tag((const void*)phdr->p_vaddr, DT_DEBUG);
    return debug->r_map;
  }
}

static void* dnload_find_symbol(uint32_t hash)
{
  const struct link_map* lmap = elf32_get_link_map();
  for(;;)
  {
    /* Find symbol from link map. We need the string table and a corresponding symbol table. */
    const char* strtab = (const char*)elf32_get_library_dynamic_section(lmap, DT_STRTAB);
    const Elf32_Sym* symtab = (const Elf32_Sym*)elf32_get_library_dynamic_section(lmap, DT_SYMTAB);
    const uint32_t* hashtable = (const uint32_t*)elf32_get_library_dynamic_section(lmap, DT_HASH);
    unsigned numchains = hashtable[1]; /* Number of symbols. */
    unsigned ii;
    for(ii = 0; (ii < numchains); ++ii)
    {
      const Elf32_Sym* sym = &symtab[ii];
      const char *name = &strtab[sym->st_name];
      if(sdbm_hash((const uint8_t*)name) == hash)
      {
        return (void*)((const uint8_t*)sym->st_value + (size_t)lmap->l_addr);
      }
    }
    lmap = lmap->l_next;
  }
}

static void dnload(void)
{
  unsigned ii;
  for(ii = 0; (24 > ii); ++ii)
  {
    void **iter = ((void**)&g_symbol_table) + ii;
    *iter = dnload_find_symbol(*(uint32_t*)iter);
  }
}

Compiling with -m hash and linking against normal libraries (as opposed to what was needed with the dlfcn method) gives us new sizes:

'hello_world.stripped': 1116 bytes
'hello_world': 567 bytes
'intro.stripped': 2240 bytes
'intro': 1265 bytes

Significantly better.

The command line option -m hash additionally does some low-hanging optimizations such as removing all known unneeded symbols (_end, _edata and __bss_start) and combining .rodata and .text into one section to free 40 bytes that would be consumed by a section header. This kind of hacking is, however, ultimately uninteresting as the only real way to further reduce the size is to construct all the ELF headers manually.

Crafting headers manually

There are earlier examples, on manually writing Linux ELF32 headers byte [ref21] by byte [ref22]. These are easily executable even on FreeBSD 32-bit Linux emulation. The also come complete with source code, so one would assume switching between operating systems would be as easy as changing 8th byte of the ELF header into ELFOSABI_FREEBSD (i.e. 9) and recompiling with nasm. Unfortunately, things are not quite so easy, and binaries constructed thus will just crash.

However, even if the examples themselves are not usable, they prove that manual hacks are least possible. What we need to do is, have some kind of access into the process of dynamic loading itself, and see what we can do to decrease space. This is all done for us already, quoting manpages of ld-elf.so:

The ``ld-elf.so.1`` utility is a self-contained shared object providing run- time support for loading and link-editing shared objects into a process' address space. It is also commonly known as the dynamic linker.

Full operating system sources have already been provided in /usr/src. Recompiling the dynamic linker is as easy as going there and building (but not installing) world by issuing:

cd /usr/src && make buildworld

This constructs our dynamic linker in /usr/obj/usr/src/libexec/rtld-elf/ld-elf.so.1, and gives us access to the source in /usr/src/libexec/rtld-elf/.

The authors have been kind enough to literally point out where the hacking should begin. rtld.c line 334:

/*
 * On entry, the dynamic linker itself has not been relocated yet.
 * Be very careful not to reference any global data until after
 * init_rtld has returned.  It is OK to reference file-scope statics
 * and string constants, and to call static and global functions.
 */

What can I say? Thank you!

We obviously do not want to replace our own rtld with a crappy, hacked version so it's better to use the custom version directly. Reading A Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux [ref2] and Executable and Linkable Format (ELF) [ref16], we can construct the minimal program to do this and start the investigation:

ehdr:
  .byte 0x7f                        # e_ident[EI_MAG0], magic value 0x7F
  .ascii "ELF"                      # e_ident[EI_MAG1] to e_indent[EI_MAG3], magic value "ELF"
  .byte 0x1                         # e_ident[EI_CLASS], ELFCLASS32 = 1
  .byte 0x1                         # e_ident[EI_DATA], ELFDATA2LSB = 1
  .byte 0x1                         # e_ident[EI_VERSION], EV_CURRENT = 1
  .byte 0x9                         # e_ident[EI_OSABI], ELFOSABI_LINUX = 3, ELFOSABI_FREEBSD = 9
  .zero 8                           # e_indent[EI_MAG9 to EI_MAG15], unused
  .short 0x2                        # e_type, ET_EXEC = 2
  .short 0x3                        # e_machine, EM_386 = 3
  .long 0x1                         # e_version, EV_CURRENT = 1
  .long _start                      # e_entry, execution starting point
  .long ehdr_end - ehdr             # e_phoff, offset from start to program headers
  .long 0x0                         # e_shoff, start of section headers
  .long 0x0                         # e_flags, unused
  .short ehdr_end - ehdr            # e_ehsize, Elf32_Ehdr size
  .short phdr_load_end - phdr_load  # e_phentsize, Elf32_Phdr size
  .short 0x2                        # e_phnum, Elf32_Phdr count, PT_LOAD, PT_INTERP = 2
  .short 0x0                        # e_shentsize, Elf32_Shdr size
  .short 0x0                        # e_shnum, Elf32_Shdr count
  .short 0x0                        # e_shstrndx, index of section containing string table of section header names
ehdr_end:

phdr_load:
  .long 0x1         # p_type, PT_LOAD = 1
  .long 0x0         # p_offset, offset of program start
  .long 0x8048000   # p_vaddr, program virtual address
  .long 0x0         # p_paddr, unused
  .long end - ehdr  # p_filesz, program size on disk
  .long end - ehdr  # p_memsz, program size in memory
  .long 0x7         # p_flags, rwx = 7
  .long 0x1000      # p_align, usually 0x1000
phdr_load_end:

phdr_interp:
  .long 0x3                  # p_type, PT_INTERP = 3
  .long interp - ehdr        # p_offset, offset of block
  .long interp               # p_vaddr, address of block
  .long 0x0                  # p_paddr, unused
  .long interp_end - interp  # p_filesz, block size on disk
  .long interp_end - interp  # p_memsz, block size in memory
  .long 0x0                  # p_flags, ignored
  .long 0x1                  # p_align, 1 for strtab
phdr_interp_end:

interp:
  .asciz "/usr/obj/usr/src/libexec/rtld-elf/ld-elf.so.1"
interp_end:

_start:
  pushl $42
  pushl $0
  movl $1, %eax
  int $128
end:

You can copypaste the earlier into, say, header.S and run:

as -o header.o header.S && ld --oformat=binary -o header header.o && ./header

This should produce:

ld: warning: cannot find entry symbol _start; defaulting to 0000000008048000
Segmentation fault

Disappointing. But add something, anything, into your custom rtld sources, recompile rtld, and try again. At the time of writing this document, mine says:

> as -o header.o header.S && ld --oformat=binary --entry=0x8048000 -o header header.o && ./header
Going to process program header.
Going to read interp.
.interp read: /usr/obj/usr/src/libexec/rtld-elf/ld-elf.so.1
obj->dynamic = 0x0
Segmentation fault

So that's why it crashes.

From this on, it's all just manual work, instrumenting the dynamic linker and seeing what can be done to minimize size. As this is an ongoing process, we will simply itemize the current findings.

Current compression procedure

The maximum compression mode uses certain techniques to decrease binary size, described below.

Section headers and sections

These seem to be not needed. At all.

ELF uses the section information for introspection. Names of sections are contained in a specific section, readelf will expect fixed section names to decrypt symbol tables, etc.

However, when push comes to shove, the program will still be executed solely based on instructions in program headers and the .dynamic information. Section headers are 40 bytes a piece. We will have none of that.

Fake .bss section

Traditionally programs put uninitialized but statically reserved memory in a section named .bss or Block Started by Symbol that will be handled by the linker. We naturally do not have anything of the like. However, the program header PT_LOAD provides a possibility to set the size on disk (p_filesz) as different from size in memory (p_memsz).

Setting these into different sizes will simply cause more memory to be allocated. We parse the variable definitions directly from assembler code and construct virtual locations and pointers 'after' the end-of-file in the following manner (taken from quad.cpp example):

end:
.balign 4
aligned_end:
.equ bss_start, aligned_end + 0
.equ __progname, bss_start + 0
.equ environ, bss_start + 4
.equ g_attribute_quad_a, bss_start + 8
.equ g_program_quad, bss_start + 12
.equ _ZL14g_audio_buffer, bss_start + 16
.equ bss_end, bss_start + 144016

The file size will use end while the memory size will use bss_end.

Note: This will fail with a bus error if the size of fake .bss segment nears 128 megabytes. If this happens, the solution is to do the exact same thing ld would do - construct another PT_LOAD segment at the next memory page without the execute flag set and assign all uninitialized memory there instead.

Entry into and exit from _start

As described in Example program earlier, we take care from entry and exit from the _start procedure ourselves. The compiler will happily push registers at the beginning and subtract the stack at the end of the function. On i386, this looks like the following (from hello_world.cpp example):

_start:
  pushl %ebp
  pushl %edi
  pushl %esi
  pushl %ebx
...
<_start content here>
...
  movl $1,%eax
  int $128
# 0 "" 2
#NO_APP
  popl %eax
  addl $12, %esp
  popl %ebx
  popl %esi
  popl %edi
  popl %ebp
  ret

None of the push operations at the beginning or any operations after the system call are necessary. They may be discarded.

Alignment

Compilers seem to generate alignment directives rather arbitrarily. This is necessary on some architectures, but unnecessarily high alignments are irrelevant.

All alignment takes space on disk. To prevent this, all alignment directives greater than the register width (32 or 64-bit) in the generated assembler source are converted to explicit .balign directives for 4 or 8 bytes.

Note: It turns out that on alignment can sometimes be completely removed by effectively aligning one byte. This is not possible on any architectures where instruction pointer is assumed to be aligned to instruction size, but works at least on amd64 and on ia32.

Merging headers

As Brian Raiter already noted in A Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux [ref2], it does not really seem to matter if the ELF headers overlap. The structures will be assigned addresses exactly based on the offsets given, and they can freely overlap with other structs in memory.

The process of interleaving the structs is automated. This produces, for example, the following:

Merging headers phdr_dynamic and hash at 4 bytes.
Merging headers hash and dynamic at 4 bytes.
Merging headers dynamic and symtab at 15 bytes.
Merging headers interp and strtab at 1 bytes.

In this particular case, the merging takes advantage of:

  • .interp section is aligned at one-byte boundary. This alignment is the last value of PT_INTERP phdr. Hash table containing only the symbols required by libc starts with number 1 (one chain).
  • .dynamic section can start with many numbers. We put the PT_NEEDED library requirements at the top so it can correctly interleave with the last value of the hash table.
  • .dynamic section ends with the PT_NULL terminator. Symbol table starts with the null symbol. Size of null terminator is 8 bytes plus varying amount (depends on byte order) of remaining null bytes from the earlier DT_DEBUG dynamic structure. These can be partially interleaved.
  • .interp section ends with terminating zero for the interpreter string, and .strtab always starts with a zero as per specification.

Note: Interleaving with DT_DEBUG is dangerous, as the structure will be filled on runtime as seen in Import by hash - scouring ELF headers. In practice, it seems to not cause problems currently.

Entry point

Default ELF entry point is fixed to address 0x8048000. This can be changed to a better compressable address.

The only issue here is that specifying --entry to ld does not actually change the entry point (it probably would if ld would also construct the headers). We need to modify linker scripts. This is done the same way as in Import by hash - scouring ELF headers - make the linker export the full linker script and change the SEGMENT_START directives into a better constant (0x2000000 at the time of writing).

Minimal DT_HASH

If symbols are present in the object, a hash table must be present to allow the dynamic linker to look for symbols. Either a GNU hash table or the traditional SYSV ELF hash table could be used. GNU hash table however uses 16 bytes for the mere headers, and does not look all that promising. SYSV hash tables seem to be relatively small. They look like this (all values are unsigned 32-bit integers):

  • Number of buckets.
  • Number of chains.
  • List of indices, size equal to number of buckets.
  • List of indices, size equal to number of chains.

In here, the number of buckets serves as the basis of the hashing. Symbol equates into a hash value, that is modulated by number of buckets. Value from given bucket points to an index to start iterating the chain array from. The FreeBSD rtld.c implementation looks like this:

for (symnum = obj->buckets[req->hash % obj->nbuckets];
     symnum != STN_UNDEF; symnum = obj->chains[symnum]) {
  if (symnum >= obj->nchains)
    return (ESRCH); /* Bad object */

  if (matched_symbol(req, obj, &matchres, symnum)) {
    req->sym_out = matchres.sym_out;
    req->defobj_out = obj;
    return (0);
  }
}

The obvious solution is to only have one bucket and point it at the end of the chain array, then have the chain array count down from this index, going through the symbols one by one. It is definitely ineffective, but that hardly matters. This results as a generation algorithm as follows:

  • Write one integer, number of buckets (1).
  • Write one integer, number of symbols plus one.
  • Write one integer, pointing at the last symbol index, adding one for the obligatory empty symbol (STN_UNDEF is 0).
  • Write a zero, for padding.
  • Write an increasing list of integers, starting from 0, ending at index of last symbol minus one (last symbol index was already at the only bucket we had).

The total cost of adding symbols would thus be 8 (for dynamic structure referencing DT_HASH) + (4 + numsymbols) * 4 (for hash itself) + (1 + numsymbols) * 16 (for symbol structs plus one empty symbol struct) + strlen(symnames) bytes.

Note: In FreeBSD, where libc requires symbols environ and __progname this would be 99 bytes exactly.

Location of r_debug

When not constructing headers manually, the r_debug debugger structure containing the link map to iterate over linked shared libraries must be found by examining the program headers, starting right from the entry point.

In his similar project [ref32], minas/Calodox uses the manually constructed headers to directly know the location into which the dynamic linker will write this address.

After adding a label into the header assembler code, accessing the link map is thus reduced from:

static const struct link_map* elf_get_link_map()
{
  const Elf32_Ehdr *ehdr = (const Elf32_Ehdr*)ELF_BASE_ADDRESS;
  const Elf32_Phdr *phdr = (const Elf32_Phdr*)((size_t)ehdr + (size_t)ehdr->e_phoff);
  for(; (phdr->p_type != PT_DYNAMIC); ++phdr) { }
  {
    const struct r_debug *debug = (const struct r_debug*)elf_get_dynamic_address_by_tag((const void*)phdr->p_vaddr, DT_DEBUG);
    return debug->r_map;
  }
}

Into:

extern const struct r_debug *dynamic_r_debug;
#define elf_get_link_map() dynamic_r_debug

Ordering of DT_STRTAB and DT_SYMTAB

Logically, before iterating through the symbols in a library, their total amount would be interpreted from that library's hash table. This only takes a bit of space on FreeBSD where (easily interpretable) SYSV hash tables seem to be present in every library. On Linux, some libraries only contain GNU hash tables the parsing of which significantly increases code footprint [ref25].

Luckily, minas/calodox noticed that DT_STRTAB and DT_SYMTAB seem to have two interesting relations:

  • Linkers seem to always put DT_STRTAB directly before DT_SYMTAB in the dynamic section.
  • Conversely, symbol table seems to always immediately precede string table in program memory.

Additionally:

  • The only program that does not obey these rules is our own binary, which is the first entry in the ELF debug listing.

Using this information, the symbol scourer part of the loader can be reduced into something like this:

for(;;)
{
  // First entry is this object itself, safe to advance first.
  lmap = lmap->l_next;
  {
    // Take advantage of DT_STRTAB and DT_SYMTAB orientation in memory.
    const Elf32_Dyn *dynamic = elf_get_dynamic_element_by_tag(lmap->l_ld, DT_STRTAB);
    const char* strtab = (const char*)elf_transform_dynamic_address(lmap, (const void*)(dynamic->d_un.d_ptr));
    const Elf32_Sym *sym = (const dnload_elf_sym_t*)elf_transform_dynamic_address(lmap, (const void*)((dynamic + 1)->d_un.d_ptr));
    for(; ((void*)sym < (void*)strtab); ++sym)
    {
      const char *name = strtab + sym->st_name;
      if(sdbm_hash((const uint8_t*)name) == hash)
      {
        return (void*)((const uint8_t*)sym->st_value + (size_t)lmap->l_addr);
      }
    }
  }
}

To see the difference compared to actually interpreting the hash, compile using --safe-symtab command line option.

Final sizes

Compiling with all the tricks listed above (using -m maximum or just omitting the option) gives us:

'hello_world.stripped': 411 bytes
'hello_world': 321 bytes
'intro.stripped': 1510 bytes
'intro': 1024 bytes

Note: Sizes subject to change.


Platform-specific details

Some platforms allow additional optimizations and/or need specific things to be taken into account when creating the binary.

Auto-generated functions (arm32l)

What we consider to be regular instuctions for the cpu might be missing for for some architectures. The most obvious example of this is the absence of integer division on 32-bit ARM.

If using integer division on ARM, the compiler will actually generate a call to an internal function that implements the division, then include it to the program at the linking phase. Since it's impossible to rely on this when generating minimal binaries, we have to supply implementations for these functions ourselves.

Suppose, using unsigned integer division, the compiler would generate link in a function with the name __aeabi_uidivmod. We know that the eabi calling convention passes input variables in r0 and r1 and passes the output variables in r0 and r1 also. Using this information, we can implement our own unsigned division:

unsigned __aeabi_uidivmod(unsigned num, unsigned den)
{
  unsigned shift = 1;
  unsigned quotient = 0;

  for(;;)
  {
    unsigned next = den << 1;
    if(next > num)
    {
      break;
    }
    den = next;
    shift <<= 1;
  }

  while(shift > 0)
  {
    if(den <= num)
    {
      num -= den;
      quotient += shift;
    }
    den >>= 1;
    shift >>= 1;
  }
  volatile register int r1 asm("r1") = num;
  asm("" : /**/ : "r"(r1) : /**/); // output: remainder
  return quotient; // r0
}

Same mechanic can be used for other functions such as (memset) or signed division (__aeabi_idivmod), which can actually use unsigned division as a sub-call to save space.

Empty DT_SYMTAB (Linux)

Linux libc does not require the user program to define environ and __progname. I was initially just leaving the hash and symtab segments blank. A blank symtab consists of just one empty (NULL) symbol, which already saves quite a lot of space.

However, as Amand Tihon [ref23] points in his own similar project [ref24], on Linux the whole of symbol table can be omitted. This is done by having the DT_SYMTAB dynamic structure entry point to address value 0 and by omitting DT_HASH completely. All in all, this means that size-optimized binaries on Linux are 99 bytes (Minimal DT_HASH) smaller than on FreeBSD. Interleaving of headers takes away some of this advantage, in practice it seems to be about 30 compressed bytes.


GLSL minification

TODO: Write this chapter. Explain renaming, local frequency analysis, token simplification, etc.


Acknowledgements

This script would not have been possible without the prior work done by various other parties. Especially the following entities deserve kudos for their efforts:

  • Marq/Fit [ref1] for the original unpack header and dlopen/dlsym implementation.
  • Brian Raiter for A Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux [ref2] and the insight of interleaving headers.
  • parcelshit [ref3] and las/Mercury [ref4] for the original ELF32 import-by hash algorithm.
  • Hymy [ref5] and Ye Olde Laptops Posse [ref6] for earlier forays into manual ELF32 header construction.
  • Amand Tihon [ref23] for BOLD - The Byte Optimized Linker [ref24] and noticing that DT_SYMTAB can be empty.
  • ts/TDA [ref26] for INT 3 exit, HOME= -shell script trick, and probably something else.
  • minas/calodox [ref31] for elfling [ref32] and various symbol scouring tricks.

And:

  • viznut/PWP [ref7] for the series Experimental music from very short C programs [ref8], a snipped of which is used in one of the examples.

The list might be missing some parties. Please notify me of any errors or omissions, so that people will get the credit they deserve.

Legalese

All contained code is licensed under the new BSD license [ref9].

Note that this license only pertains to the code of the script(s) themselves. There are no restrictions imposed on the end products of the script(s) just like there are no restrictions imposed on a binary built with a compiler.

To be honest, even that doesn't really mean anything. Just do whatever you want, but if you improve on the mechanisms, I would prefer to incorporate the improvements.

FAQ

No-one runs 32-bit FreeBSD anymore, especially if it's only for curiosities like this. Why bother?

Even on a 64-bit system, you should be able to execute the result file if the compatibility layer is set up correctly. The easiest way to do it is to just install a 32-bit jail [ref10] and point LD_32_LIBRARY_PATH environment variable to the /usr/local/lib of that jail. This has the added benefit of enabling full 32-bit compatibility and easy cross-compiling.

There are probably easy ways to do the same on Linux, but they are out of the scope of this document.

What does USE_LD stand for?

The name USE_LD is legacy, which has preserved unchanged from earlier Faemiyah prods. You may change the definition with the -d or --definition command line argument when invoking the script.

Do I need to use _start?

When manually creating the program headers, the symbol would not necessarily need to be named _start - it could be anything, and the name will be stripped out anyway. However, this is a known convention.

What are environ and __progname?

You you looked into the generated header, you might have seen something like this:

#if defined(__FreeBSD__)
#if defined(__clang__)
void *environ;
void *__progname;
#else
void *environ __attribute__((externally_visible));
void *__progname __attribute__((externally_visible));
#endif
#endif

These symbols might seem nonsensical, as they are not used anywhere in the program or the generated code. Taking a look into the standard C library (i.e. /lib/libc.so.7) will clarify their purpose:

> readelf -a /lib/libc.so.7
...
Symbol table '.dynsym' contains 3079 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
     2: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND __progname
     3: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND environ
...

We do not need these symbols, but libc expects them to be present in the binary. In practice, the dynamic linking procedure will fail if the program symbol table does not contain them.

Note: These symbols seem to be not needed on Linux.

Why is there an attribute externally_visible specified for some symbols?

The suffix __attribute__((externally_visible)) is present in some symbols defined, most notable in _start. This is due to Gnu C Compiler semantics.

When compiling for binary size, it is necessary to use the -fwhole-program flag to make g++ discard all irrelevant code. Unfortunately, unless the compiler finds something it needs, this will actually cause it to discard _everything_ within the source file as there is no main() present to start building a dependency graph from.

This attribute explicitly marks functions as symbols to be externally visible, so the dependency graph build shall include them.

Note: Clang does not seem to either require or support this attribute.

<x> does not work / crashes / is not supported?!?

Please:

  • Send E-mail to <trilkk ATSYMBOL iki.fi>. Please include as much debug information and logs as possible.
  • Contact Trilkk @ freenode/ircnet/quakenet. This is better and faster than E-mail.
  • Fix it yourself and make a patch/PR.

This software has a very narrow usecase, but it's not that unlikely there will at least be an attempt for a fix.

TODO

  • Explain GLSL minification in documentation.
  • Add support for a separate output directory for temporary files.
  • Examine dlopen/dlsym failures in linux-ia32 / linux-amd64 on modern distros?
  • Extend cross-compilation support to other operating systems and architectures as opposed to 32-bit / 64-bit switch only.
  • Only SDL/OpenGL supported right now. Should probably also support GLFW.
  • Perhaps there are more efficient ways to interleave the header structs? Perhaps this can be permutated?
  • Perhaps .data segment contents can be sorted and rearranged for better compression?

References

[ref1]http://www.pouet.net/user.php?who=4078 Marq/Fit in Pouet]
[ref2](1, 2, 3) http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html A Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux
[ref3](1, 2) http://www.pouet.net/user.php?who=9348 parcelshit in Pouet
[ref4](1, 2) http://www.pouet.net/user.php?who=4548 las/Mercury in Pouet
[ref5]http://www.pouet.net/groups.php?which=11106 Hymy in Pouet
[ref6]http://www.pouet.net/groups.php?which=11436 Ye Olde Laptops Posse in Pouet
[ref7]http://www.pouet.net/user.php?who=2547 viznut/PWP in Pouet
[ref8]https://www.youtube.com/watch?v=tCRPUv8V22o Music from very short programs - the 3rd iteration in Youtube
[ref9]http://opensource.org/licenses/BSD-3-Clause New BSD license
[ref10]http://www.freebsd.org/doc/handbook/jails.html Chapter 15. Jails in FreeBSD manual
[ref11]http://www.freebsd.org/doc/en/books/developers-handbook/x86-system-calls.html Chapter 11.3. System Calls in FreeBSD manual
[ref12]http://www.pouet.net/prod.php?which=5021 helsinki-spiegelberg by tsygä in Pouet
[ref13]http://www.pouet.net/prod.php?which=10562 Yellow Rose of Texas by Fit & Bandwagon in Pouet
[ref14]http://pubs.opengroup.org/onlinepubs/009695399/functions/dlopen.html dlopen specification at The Open Group Base Specifications
[ref15]http://pubs.opengroup.org/onlinepubs/009695399/functions/dlsym.html dlsym specification at The Open Group Base Specifications
[ref16](1, 2) http://www.skyfree.org/linux/references/ELF_Format.pdf Executable and Linkable Format (ELF) specification
[ref17]http://svnweb.freebsd.org/base/stable/10/sys/sys/ FreeBSD system headers
[ref18]http://pastebin.com/f479f8beb parcelshit's original import-by-hash implementation
[ref19]http://www.pouet.net/topic.php?which=5392&page=4 Haxxoring the ELF format for 1k/4k stuff -thread on Pouet
[ref20]http://programmers.stackexchange.com/questions/49550/which-hashing-algorithm-is-best-for-uniqueness-and-speed Which hashing algorithm is best for uniqueness and speed? - Ian Boyd's answer at Programmers Stack Exchange
[ref21]http://www.pouet.net/prod.php?which=61753 Inconsistency by Hymy in Pouet
[ref22]http://www.pouet.net/prod.php?which=59264 tutorial2 by Ye Olde Laptops Posse in Pouet
[ref23](1, 2) http://www.alrj.org/ Amand Tihon
[ref24](1, 2) http://www.alrj.org/projects/bold/ BOLD - The Byte Optimized Linker
[ref25]https://blogs.oracle.com/ali/entry/gnu_hash_elf_sections GNU Hash ELF Sections by Ali Bahrami
[ref26]http://www.pouet.net/user.php?who=58551 ts/TDA in Pouet
[ref27]http://www.pouet.net/topic.php?which=10038&page=2#c482906 File-dumping example by ts/TDA
[ref28]http://svn.python.org/projects/external/xz-5.0.3/doc/lzma-file-format.txt LZMA file format
[ref29]http://tukaani.org/xz/xz-file-format-1.0.4.txt XZ file format
[ref30]http://x86.renejeschke.de/html/file_module_x86_id_142.html INT 3 instruction at René Jeschke's x86 instruction set reference mirror
[ref31]http://www.pouet.net/user.php?who=6102 minas/calodox in Pouet
[ref32](1, 2) https://github.com/google/elfling elfling