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Intercept C++ methods with LD_PRELOAD
// this file is an example of how to intercept a C++ method by using the
// LD_PRELOAD environment variable of the GNU dynamic linker.
//
// it works like this:
//
// 1) define a method that will have the same symbol as the intercepted
// method when compiled. For example, the method Foo::getValue()
// defined here has the mangled symbol "_ZNK3Foo8getValueEv".
// tools like nm, objdump or readelf can display the symbols of
// binaries.
//
// 2) if you want to call the original method from your new method,
// you can get a pointer to it with dlsym() and the mangled name.
//
// 3) compile your intercept code into its own shared library,
// for example with "g++ -g -Wall -shared -fPIC -ldl foo.cpp -o foo.so".
// "-ldl" should only be necessary if you use dlsym().
//
// 4) set LD_PRELOAD to the path of your new shared library and call the
// program in which you want to intercept something. in bash, you can
// do it like this: "LD_PRELOAD=./foo.so /usr/bin/some_program".
#include <string.h>
// necessary for RTLD_NEXT in dlfcn.h
#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
#include <dlfcn.h>
class Foo
{
public:
unsigned int getValue() const;
};
unsigned int Foo::getValue() const
{
// define the pointer-to-member type
typedef unsigned int (Foo::*methodType)() const;
static methodType origMethod = 0;
if (origMethod == 0)
{
// use the mangled method name here. RTLD_NEXT means something like
// "search this symbol in any libraries loaded after the current one".
void *tmpPtr = dlsym(RTLD_NEXT, "_ZNK3Foo8getValueEv");
// not even reinterpret_cast can convert between void* and a method ptr,
// so i'm doing the worst hack i've ever seen.
memcpy(&origMethod, &tmpPtr, sizeof(&tmpPtr));
}
// here we call the original method
unsigned int origValue = (this->*origMethod)();
return 1000 + origValue;
}
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