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I'll explain:

Let's say I'm interested in replacing the rand() function used by a certain application.

So I attach gdb to this process and make it load my custom shared library (which has a customized rand() function):

call (int) dlopen("path_to_library/asdf.so")

This would place the customized rand() function inside the process' memory. However, at this point the symbol rand will still point to the default rand() function. Is there a way to make gdb point the symbol to the new rand() function, forcing the process to use my version?

I must say I'm also not allowed to use the LD_PRELOAD (linux) nor DYLD_INSERT_LIBRARIES (mac os x) methods for this, because they allow code injection only in the beginning of the program execution.

The application that I would like to replace rand(), starts several threads and some of them start new processes, and I'm interested in injecting code on one of these new processes. As I mentioned above, GDB is great for this purpose because it allows code injection into a specific process.

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Are the subprocesses of interest just forked copies of the original, or do they also exec() new images? –  llasram Sep 14 '10 at 2:30
@llasram they are not forked copies. –  karlphillip Sep 14 '10 at 6:34
Another question -- do you have any gdb version constraints, or could you install and use an arbitrarily recent version? –  llasram Sep 15 '10 at 17:06
There are no gdb version constraints. –  karlphillip Sep 15 '10 at 22:02

10 Answers 10

up vote 9 down vote accepted

I followed this post and this presentation and came up with the following set of gdb commands for OSX with x86-64 executable, which can be loaded with -x option when attaching to the process:

set $s = dyld_stub_rand
set $p = ($s+6+*(int*)($s+2))
call (void*)dlsym((void*)dlopen("myrand.dylib"), "my_rand")
set *(void**)$p = my_rand

The magic is in set $p = ... command. dyld_stub_rand is a 6-byte jump instruction. Jump offset is at dyld_stub_rand+2 (4 bytes). This is a $rip-relative jump, so add offset to what $rip would be at this point (right after the instruction, dyld_stub_rand+6).

This points to a symbol table entry, which should be either real rand or dynamic linker routine to load it (if it was never called). It is then replaced by my_rand.

Sometimes gdb will pick up dyld_stub_rand from libSystem or another shared library, if that happens, unload them first with remove-symbol-file before running other commands.

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This question intrigued me, so I did a little research. What you are looking for is a 'dll injection'. You write a function to replace some library function, put it in a .so, and tell ld to preload your dll. I just tried it out and it worked great! I realize this doesn't really answer your question in relation to gdb, but I think it offers a viable workaround.

For a gdb-only solution, see my other solution.

// -*- compile-command: "gcc -Wall -ggdb -o test test.c"; -*-
// test.c

#include "stdio.h"
#include "stdlib.h"

int main(int argc, char** argv)
    //should print a fairly random number...
    printf("Super random number: %d\n", rand());

    return 0;

/ -*- compile-command: "gcc -Wall -fPIC -shared my_rand.c -o my_rand.so"; -*-

int rand(void)
    return 42;

compile both files, then run: LD_PRELOAD="./my_rand.so" ./test

Super random number: 42

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I should have added that I could not use LD_PRELOAD (linux) nor DYLD_INSERT_LIBRARIES (mac os x) to achieve this. The reason for this is that the program I'm trying to "hack" starts a dozen of threads that later loads a bunch of other processes, and my interest is one of these processes. So, the LD_PRELOAD method does not allow me to inject code in the process I'm actually interested, it only allows code to be injected in the process created by the initial execution of the program. That's why my question is specific for GDB, it allows me to attach my code on the right process! –  karlphillip Jul 17 '10 at 15:53
@karlphillip That does thicken the plot considerably! –  zdav Jul 18 '10 at 1:48
Why not just export LD_PRELOAD="./my_rand.so"; test? –  Michael Mior Sep 18 '10 at 0:49
As stated in the question, please keep answers restricted to the Mac OS X environment. –  karlphillip Sep 18 '10 at 1:06

I have a new solution, based on the new original constraints. (I am not deleting my first answer, as others may find it useful.)

I have been doing a bunch of research, and I think it would work with a bit more fiddling.

  1. In your .so rename your replacement rand function, e.g my_rand
  2. Compile everything and load up gdb
  3. Use info functions to find the address of rand in the symbol table
  4. Use dlopen then dlsym to load the function into memory and get its address

    call (int) dlopen("my_rand.so", 1) -> -val-

    call (unsigned int) dlsym(-val-, "my_rand") -> my_rand_addr

  5. -the tricky part- Find the hex code of a jumpq 0x*my_rand_addr* instruction
  6. Use set {int}*rand_addr* = *my_rand_addr* to change symbol table instruction
  7. Continue execution: now whenever rand is called, it will jump to my_rand instead

This is a bit complicated, and very round-about, but I'm pretty sure it would work. The only thing I haven't accomplished yet is creating the jumpq instruction code. Everything up until that point works fine.

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The problem with step 5 is that you won't be able to find any jumps to my_rand_addr since this code was just loaded and is not used by the application. But I tested the following stackoverflow.com/questions/3270281/… and it worked. –  karlphillip Jul 19 '10 at 0:01
@karlphillip You don't need to find the jump instruction in the code, you make it yourself. The format of the jumpq instruction is well specified, you just look it up and the find out what the hex value is. –  zdav Jul 20 '10 at 14:22

I'm not sure how to do this in a running program, but perhaps LD_PRELOAD will work for you. If you set this environment variable to a list of shared objects, the runtime loader will load the shared object early in the process and allow the functions in it to take precedence over others.

LD_PRELOAD=path_to_library/asdf.so path/to/prog 

You do have to do this before you start the process but you don't have to rebuild the program.

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Several of the answers here and the code injection article you linked to in your answer cover chunks of what I consider the optimal gdb-oriented solution, but none of them pull it all together or cover all the points. The code-expression of the solution is a bit long, so here's a summary of the important steps:

  1. Load the code to inject. Most of the answers posted here use what I consider the best approach -- call dlopen() in the inferior process to link in a shared library containing the injected code. In the article you linked to the author instead loaded a relocatable object file and hand-linked it against the inferior. This is quite frankly insane -- relocatable objects are not "ready-to-run" and include relocations even for internal references. And hand-linking is tedious and error-prone -- far simpler to let the real runtime dynamic linker do the work. This does mean getting libdl into the process in the first place, but there are many options for doing that.
  2. Create a detour. Most of the answers posted here so far have involved locating the PLT entry for the function of interest, using that to find the matching GOT entry, then modifying the GOT entry to point to your injected function. This is fine up to a point, but certain linker features -- e.g., use of dlsym -- can circumvent the GOT and provide direct access to the function of interest. The only way to be certain of intercepting all calls to a particular function is overwrite the initial instructions of that function's code in-memory to create a "detour" redirecting execution to your injected function.
  3. Create a trampoline (optional). Frequently when doing this sort of injection you'll want to call the original function whose invocation you are intercepting. The way to allow this with a function detour is to create a small code "trampoline" which includes the overwritten instructions of the original function then a jump to the remainder of the original. This can be complex, because any IP-relative instructions in the copied set need to be modified to account for their new addresses.
  4. Automate it all. These steps can be tedious, even if doing some of the simpler solutions posted in other answers. The best way to ensure that the steps are done correctly every time with variable parameters (injecting different functions, etc) is to automate their execution. Starting with the 7.0 series, gdb has included the ability to write new commands in Python. This support can be used to implement a turn-key solution for injecting and detouring code in/to the inferior process.

Here's an example. I have the same a and b executables as before and an inject2.so created from the following code:

#include <unistd.h>
#include <stdio.h>

int (*rand__)(void) = NULL;

    int result = rand__();
    printf("rand invoked! result = %d\n", result);
    return result % 47;

I can then place my Python detour command in detour.py and have the following gdb session:

(gdb) source detour.py
(gdb) exec-file a
(gdb) set follow-fork-mode child
(gdb) catch exec
Catchpoint 1 (exec)
(gdb) run
Starting program: /home/llasram/ws/detour/a 
a: 1933263113
a: 831502921
[New process 8500]
b: 918844931
process 8500 is executing new program: /home/llasram/ws/detour/b
[Switching to process 8500]

Catchpoint 1 (exec'd /home/llasram/ws/detour/b), 0x00007ffff7ddfaf0 in _start ()
(gdb) break main
Breakpoint 2 at 0x4005d0: file b.c, line 7.
(gdb) cont

Breakpoint 2, main (argc=1, argv=0x7fffffffdd68) at b.c:7
7       {
(gdb) detour libc.so.6:rand inject2.so:rand inject2.so:rand__
(gdb) cont
rand invoked! result = 392103444
b: 22

Program exited normally.

In the child process, I create a detour from the rand() function in libc.so.6 to the rand() function in inject2.so and store a pointer to a trampoline for the original rand() in the rand__ variable of inject2.so. And as expected, the injected code calls the original, displays the full result, and returns that result modulo 47.

Due to length, I'm just linking to a pastie containing the code for my detour command. This is a fairly superficial implementation (especially in terms of the trampoline generation), but it should work well in a large percentage of cases. I've tested it with gdb 7.2 (most recently released version) on Linux with both 32-bit and 64-bit executables. I haven't tested it on OS X, but any differences should be relatively minor.

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For executables you can easily find the address where the function pointer is stored by using objdump. For example:

objdump -R /bin/bash | grep write
00000000006db558 R_X86_64_JUMP_SLOT  fwrite
00000000006db5a0 R_X86_64_JUMP_SLOT  write

Therefore, 0x6db5a0 is the adress of the pointer for write. If you change it, calls to write will be redirected to your chosen function. Loading new libraries in gdb and getting function pointers has been covered in earlier posts. The executable and every library have their own pointers. Replacing affects only the module whose pointer was changed.

For libraries, you need to find the base address of the library and add it to the address given by objdump. In Linux, /proc/<pid>/maps gives it out. I don't know whether position-independent executables with address randomization would work. maps-information might be unavailable in such cases.

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As long as the function you want to replace is in a shared library, you can redirect calls to that function at runtime (during debugging) by poking at the PLT. Here is an article that might be helpful:

Shared library call redirection using ELF PLT infection

It's written from the standpoint of malware modifying a program, but a much easier procedure is adaptable to live use in the debugger. Basically you just need to find the function's entry in the PLT and overwrite the address with the address of the function you want to replace it with.

Googling for "PLT" along with terms like "ELF", "shared library", "dynamic linking", "PIC", etc. might find you more details on the subject.

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I don't think this works on Mac OS X, but thank you anyway! –  karlphillip Sep 16 '10 at 3:36
Does OSX use some strange form of dynamic linking? Almost everyone else uses ELF... –  R.. Sep 16 '10 at 3:38
Yeah, due its Mach and NeXTSTEP heritage, OS X uses Mach-O. –  llasram Sep 16 '10 at 10:42

You can still us LD_PRELOAD if you make the preloaded function understand the situations it's getting used in. Here is an example that will use the rand() as normal, except inside a forked process when it will always return 42. I use the dl routines to load the standard library's rand() function into a function pointer for use by the hijacked rand().

// -*- compile-command: "gcc -Wall -fPIC -shared my_rand.c -o my_rand.so -ldl"; -*-
#include <sys/types.h>
#include <unistd.h>

#include <dlfcn.h>

int pid = 0;
int (*real_rand)(void) = NULL;

void f(void) __attribute__ ((constructor));

void f(void) {
    pid = getpid();
    void* dl = dlopen("libc.so.6", RTLD_LAZY);
    if(dl) {
        real_rand = dlsym(dl, "rand");

int rand(void) 
    if(pid == getpid() && real_rand)
        return real_rand();
        return 42;

#include <dlfcn.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char** argv)

    if(fork()) {
        printf("original process rand: %d\n", rand());

    } else {
        printf("forked process rand: %d\n", rand());

    return 0;

jdizzle@pudding:~$ ./test
Super random number: 1804289383
original process rand: 846930886
forked process rand: 846930886

jdizzle@pudding:~$ LD_PRELOAD="/lib/ld-linux.so.2 ./my_rand.so" ./test
Super random number: 1804289383
original process rand: 846930886
forked process rand: 42
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Did you notice that I already had an answer describing the same technique stackoverflow.com/questions/3270281/… ? –  llasram Sep 18 '10 at 15:10
Your example both won't compile as posted and doesn't work. Mine is also simpler. –  jdizzle Sep 20 '10 at 19:24
Those are reasons to improve my answer or propose improvements, not create an entirely different answer and down-vote mine. –  llasram Sep 20 '10 at 19:49

I frequently use code injection as a method of mocking for automated testing of C code. If that's the sort of situation you're in -- if your use of GDB is simply because you're not interested in the parent processes, and not because you want to interactively select the processes which are of interest -- then you can still use LD_PRELOAD to achieve your solution. Your injected code just needs to determine whether it is in the parent or child processes. There are several ways you could do this, but on Linux, since your child processes exec(), the simplest is probably to look at the active executable image.

I produced two executables, one named a and the other b. Executable a prints the result of calling rand() twice, then fork()s and exec()s b twice. Executable b print the result of calling rand() once. I use LD_PRELOAD to inject the result of compiling the following code into the executables:

// -*- compile-command: "gcc -D_GNU_SOURCE=1 -Wall -std=gnu99 -O2 -pipe -fPIC -shared -o inject.so inject.c"; -*-
#include <sys/types.h>
#include <unistd.h>
#include <limits.h>
#include <stdio.h>
#include <dlfcn.h>

#define constructor __attribute__((__constructor__))

typedef int (*rand_t)(void);

typedef enum {
} state_t;

state_t state = UNKNOWN;
rand_t rand__ = NULL;

    pid_t pid = getpid();
    char linkpath[PATH_MAX] = { 0, };
    char exepath[PATH_MAX] = { 0, };
    ssize_t exesz = 0;

    snprintf(linkpath, PATH_MAX, "/proc/%d/exe", pid);
    exesz = readlink(linkpath, exepath, PATH_MAX);
    if (exesz < 0)
        return UNKNOWN;

    switch (exepath[exesz - 1]) {
    case 'a':
        return PARENT;
    case 'b':
        return CHILD;

    return UNKNOWN;

    if (state == CHILD)
        return 47;
    return rand__();

constructor static void
    rand__ = dlsym(RTLD_NEXT, "rand");
    state = determine_state();

The result of running a with and without injection:

$ ./a
a: 644034683
a: 2011954203
b: 375870504
b: 1222326746
$ LD_PRELOAD=$PWD/inject.so ./a
a: 1023059566
a: 986551064
b: 47
b: 47

I'll post a gdb-oriented solution later.

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Thank you! But as I previously stated on the question, I'm not allowed to use LD_PRELOAD. –  karlphillip Sep 15 '10 at 16:34
@karlphillip Oh! I thought you just didn't think LD_PRELOAD could provide the effect you were trying to achieve, or at least that's how I read your LD_PRELOAD-related comments. I'll post a (IMHO) clean gdb-based solution soon. –  llasram Sep 15 '10 at 17:05
You need to #define __USE_GNU before including dlfcn.h if you want to use RTLD_NEXT (at least in my version of dlfcn.h). Also, determine_state() is not run at fork() time, so the state will never change. –  jdizzle Sep 20 '10 at 19:25
@jdizzle Nice down-vote, jerk. My compile command included -D_GNU_SOURCE, which I'm pretty sure is what you meant. And if you'd bothered to read the entire answer you'd have noticed mine is built around the case of exec()-ing a new executable, not just fork()-ing. –  llasram Sep 20 '10 at 19:44
@llasram don't be such a whiner next time. –  jdizzle Sep 20 '10 at 23:45

I found this tutorial incredibly useful, and so far its the only way I managed to achieve what I was looking with GDB: Code Injection into Running Linux Application: http://www.codeproject.com/KB/DLL/code_injection.aspx

There is also a good Q&A on code injection for Mac here: http://www.mikeash.com/pyblog/friday-qa-2009-01-30-code-injection.html

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