system() function not called from LD_PRELOAD'ed library - c++

Question

I am trying to use LD_PRELOAD on linux to wrap calls to system function to add some preprocessing to the argument. Here's my system.cpp:
#define _GNU_SOURCE
#include <dlfcn.h>
#include <string>
#include <iostream>
typedef int (*orig_system_type)(const char *command);
int system(const char *command)
{
std::string new_cmd = std::string("set -f;") + command;
// next line is for debuggin only
std::cout << new_cmd << std::endl;
orig_system_type orig_system;
orig_system = (orig_system_type)dlsym(RTLD_NEXT,"system");
return orig_system(new_cmd.c_str());
}
I build it with
g++ -shared -fPIC -ldl -o libsystem.so system.cpp
which produces the .so object. I then run my program with
$ LD_PRELOAD=/path/to/libsystem.so ./myprogram
I do not get any errors - but seemingly my system function is not called. Running with LD_DEBUG=libs, I can see that my .so is being loaded, however my system function is not being called and the one from the standard library is called instead.
What do I need to change in code/build to get it to work?

Answer 1

You need
extern "C" int system ...
because it is called by a C function. The C++ version has its name mangled so it is not recognizable.

Answer 2

You might also want to consider saving the "orig_system" pointer so that you avoid calling dlsym every time. You can do this in a constructor/init function, so you would have something like
extern "C" {
typedef int (*orig_system_type)(const char *command);
static orig_system_type orig_system;
static void myInit() __attribute__((constructor));
void myInit()
{
orig_system = (orig_system_type)dlsym(RTLD_NEXT,"system");
}
int system(const char *command)
{
std::string new_cmd = std::string("set -f;") + command;
// next line is for debuggin only
std::cout << new_cmd << std::endl;
return orig_system(new_cmd.c_str());
}
}
(this code isn't tested, but I have used this technique in the past).
An alternative would be to use GNU ld's --wrap option.
If you compile your shared lib with
-Wl,--wrap system
then in your code you write
extern "C" {
void* __real_system(const char* command);
void* __wrap_system(const char* command)
{
std::string new_cmd = std::string("set -f;") + command;
// next line is for debuggin only
std::cout << new_cmd << std::endl;
return __real_system(new_cmd.c_str());
}
}
(Note that I've never used this).

Answer 3

The code should work perfectly fine. Assuming the driver program is something like this:
#include <cstdlib>
int main() {
system("find -name *.cpp");
}
Then env LD_PRELOAD=$PWD/libsystem.so ./a.out gives me this output:
set -f;find -name *.cpp
./system.cpp
./test.cpp
Which shows that not only is your debug statement appearing, but that glob is disabled for that command.