[Linux] LD_PRELOAD injection without patching Wine

[kouteiheika]

If you are hacking on Linux chances are you are using LD_PRELOAD method described here. It's a very powerful, and easy, way to hook into another process. Unfortunately, when it comes to hooking into Wine *and* glXSwapBuffers (aka. endscene hook) it requires patching its source code. Not anymore! (If you don't care about my brief-yet-mostly-useless explanation and you're here to just copy and paste the code then scroll down to the bottom.)

To understand why Wine needs patching we need to take a peek into its source code:

    #define SONAME_LIBGL "libGL.so"
    (...snip...)
    opengl_handle = wine_dlopen(SONAME_LIBGL, RTLD_NOW|RTLD_GLOBAL, buffer, sizeof(buffer));
    (...snip...)
    pglXGetProcAddressARB = wine_dlsym(opengl_handle, "glXGetProcAddressARB", NULL, 0);
    (...snip...)
    pglXSwapBuffers = pglXGetProcAddressARB("glXSwapBuffers");

Basically, Wine directly opens libGL.so and directly requests glXGetProcAddress from it and then uses that to load glXSwapBuffers; to force Wine into using our version of glXSwapBuffers defined in our LD_PRELOAD library we had to patch this "opengl_handle" to RTLD_DEFAULT, which basically means "I don't care from where it comes from, just give me the damn pointer".

Let's take a closer look at wine_dlsym():

void *wine_dlsym( void *handle, const char *symbol, char *error, size_t errorsize )
{
    (...snip...)
    ret = dlsym( handle, symbol );
    (...error handling...)
    (...snip...)
}

We can see that this is just a simple wrapper over dlsym with some error handling throw in the mix. Now, an idea - we could get away without patching Wine if only we could trick dlsym() to return our version of glXGetProcAddress. How? By hooking into dlsym() with LD_PRELOAD!

static void * (*dlsym_real)( void * handle, const char * symbol ) = nullptr;

void * dlsym( void * handle, const char * symbol )
{
    if( dlsym_real == nullptr )
        dlsym_real = ( decltype(dlsym_real) )dlsym( RTLD_NEXT, "dlsym" );
        
    if( !strcmp( symbol, "glXGetProcAddressARB" ) )
        return ( void * )glXGetProcAddressARB_fake;
        
    return dlsym_real( handle, symbol );
}

There is just one little problem - this obviously won't work. Our dlsym() will just call dlsym(), and that will call dlsym() again and again. What we need is a function that does what dlsym() does but isn't dlsym(). A quick look at "man dlsym" turned out this:

   Glibc extensions: dladdr() and dlvsym()
       Glibc adds two functions not described by POSIX, with prototypes

       (...snip...)

       void *dlvsym(void *handle, char *symbol, char *version);

       (...snip...)

       The function dlvsym(), provided by glibc since version 2.1, does the same as dlsym() but takes a version string as an additional argument.

Precisely what we need. Now we just need to figure out what the hell we're supposed to put in the version string; fortunately, objdump comes to the rescue:

kou@rin:~$ objdump -T /lib32/libdl.so.2 

/lib32/libdl.so.2:     file format elf32-i386

DYNAMIC SYMBOL TABLE:
(...snip...)
00001940 g    DF .text  00000067 (GLIBC_2.0)  dlopen
00001430 g    DF .text  000000a8  GLIBC_2.3.3 dladdr1
00000000 g    DO *ABS*  00000000  GLIBC_PRIVATE GLIBC_PRIVATE
00000d50 g    DF .text  00000094  GLIBC_2.0   dlsym
00000000 g    DO *ABS*  00000000  GLIBC_2.0   GLIBC_2.0
00000000 g    DO *ABS*  00000000  GLIBC_2.1   GLIBC_2.1
00000cd0 g    DF .text  0000003f  GLIBC_2.0   dlclose
(...snip...)

We have out version string - "GLIBC_2.0", so we can rewrite that code as:

/* This definition is required to get dlvsym(). */
#define _GNU_SOURCE
#include <dlfcn.h>

static void * (*dlsym_real)( void * handle, const char * symbol ) = nullptr;

void * dlsym( void * handle, const char * symbol )
{
    if( dlsym_real == nullptr )
        dlsym_real = ( decltype(dlsym_real) )dlvsym( RTLD_DEFAULT, "dlsym", "GLIBC_2.0" );
        
    if( !strcmp( symbol, "glXGetProcAddressARB" ) )
        return ( void * )glXGetProcAddressARB_fake;
        
    return dlsym_real( handle, symbol );
}

And this is how you LD_PRELOAD without patching Wine.

[flo8464]

An easy and clean solution, thank you!

[SKU]

Neat, thanks a lot.

[argh44z]

Precisely what we need. Now we just need to figure out what the hell we're supposed to put in the version string; fortunately, objdump comes to the rescue:

Nice work. You can also set the env variable LD_DEBUG=all before launching wine to get the version number:

      2444: symbol=dlsym;  lookup in file=/usr/lib/fglrx/libGL.so.1 [0]
      2444: symbol=dlsym;  lookup in file=/usr/local/bin/wineserver [0]
      2444: symbol=dlsym;  lookup in file=/home/sb/hook/libsas.so [0]
      2444: symbol=dlsym;  lookup in file=/usr/local/bin/../lib/libwine.so.1 [0]
      2444: symbol=dlsym;  lookup in file=/lib/tls/i686/cmov/libc.so.6 [0]
      2444: symbol=dlsym;  lookup in file=/lib/tls/i686/cmov/libdl.so.2 [0]
      2444: binding file /usr/lib/fglrx/libGL.so.1 [0] to /lib/tls/i686/cmov/libdl.so.2 [0]: normal symbol `dlsym' [GLIBC_2.0]

and

 2447: symbol=wine_dlsym;  lookup in file=/usr/local/bin/wine [0]
      2447: symbol=wine_dlsym;  lookup in file=/home/sb/hook/libsas.so [0]
      2447: symbol=wine_dlsym;  lookup in file=/usr/local/bin/../lib/libwine.so.1 [0]
      2447: binding file /usr/local/bin/../lib/libwine.so.1 [0] to /usr/local/bin/../lib/libwine.so.1 [0]: normal symbol `wine_dlsym' [WINE_1.0]
      2447: symbol=dlsym;  lookup in file=/usr/local/bin/wine [0]
      2447: symbol=dlsym;  lookup in file=/home/sb/hook/libsas.so [0]
      2447: symbol=dlsym;  lookup in file=/usr/local/bin/../lib/libwine.so.1 [0]
      2447: symbol=dlsym;  lookup in file=/lib/tls/i686/cmov/libpthread.so.0 [0]
      2447: symbol=dlsym;  lookup in file=/lib/tls/i686/cmov/libc.so.6 [0]
      2447: symbol=dlsym;  lookup in file=/lib/tls/i686/cmov/libdl.so.2 [0]
      2447: binding file /usr/local/bin/../lib/libwine.so.1 [0] to /lib/tls/i686/cmov/libdl.so.2 [0]: normal symbol `dlsym' [GLIBC_2.0]
      2447: symbol=_dl_sym;  lookup in file=/usr/local/bin/wine [0]
      2447: symbol=_dl_sym;  lookup in file=/home/sb/hook/libsas.so [0]
      2447: symbol=_dl_sym;  lookup in file=/usr/local/bin/../lib/libwine.so.1 [0]
      2447: symbol=_dl_sym;  lookup in file=/lib/tls/i686/cmov/libpthread.so.0 [0]
      2447: symbol=_dl_sym;  lookup in file=/lib/tls/i686/cmov/libc.so.6 [0]
      2447: binding file /lib/tls/i686/cmov/libdl.so.2 [0] to /lib/tls/i686/cmov/libc.so.6 [0]: normal symbol `_dl_sym' [GLIBC_PRIVATE]

[Sednogmah]

Hey kouteiheika!
Very clever solution!

I just returned after a looong break and updated my injection method to hook dlsym().
Also linked this thread in my old Linux glx hook thread:
http://www.ownedcore.com/forums/worl...d_preload.html

Also relevant (check the latest answers for useful info):
http://www.ownedcore.com/forums/worl...n32-mingw.html

P.S.: Any Linux botters still around?
Feel free to PM me
I'm working on a pattern editor for function pointers. (Linux / Qt).