multiple LD_REPLOAD sharing variables - c++

Question

My purpose is quite simple: when I start the mongoose server, the server will initialize a variable which is defined by me. To do this, I hooked __libc_start_main. Then when the server receives a request, it will print out that initialized variable. To do this, I hooked recv. Below is my code.
#include <string>
#include <stdio.h>
#include <dlfcn.h>
#include <stdlib.h>
#include "common-structure.h"
# define dprintf(fmt...)
data datainfo; //defined in common-structure.h
typedef int (*main_type)(int, char**, char**);
struct arg_type
{
char **argv;
int (*main_func) (int, char **, char **);
};
main_type saved_init_func = NULL;
void tern_init_func(int argc, char **argv, char **env){
dprintf("%04d: __tern_init_func() called.\n", (int) pthread_self());
if(saved_init_func)
saved_init_func(argc, argv, env);
datainfo.age = 10;
}
typedef void (*fini_type)(void*);
fini_type saved_fini_func = NULL;
extern "C" int my_main(int argc, char **pt, char **aa)
{
int ret;
arg_type *args = (arg_type*)pt;
dprintf("%04d: __libc_start_main() called.\n", (int) pthread_self());
ret = args->main_func(argc, args->argv, aa);
return ret;
}
extern "C" int __libc_start_main(
void *func_ptr,
int argc,
char* argv[],
void (*init_func)(void),
void (*fini_func)(void),
void (*rtld_fini_func)(void),
void *stack_end)
{
typedef void (*fnptr_type)(void);
typedef int (*orig_func_type)(void *, int, char *[], fnptr_type,
fnptr_type, fnptr_type, void*);
orig_func_type orig_func;
arg_type args;
void * handle;
int ret;
// Get lib path.
Dl_info dli;
dladdr((void *)dlsym, &dli);
std::string libPath = dli.dli_fname;
libPath = dli.dli_fname;
size_t lastSlash = libPath.find_last_of("/");
libPath = libPath.substr(0, lastSlash);
libPath += "/libc.so.6";
libPath = "/lib/x86_64-linux-gnu/libc.so.6";
if(!(handle=dlopen(libPath.c_str(), RTLD_LAZY))) {
puts("dlopen error");
abort();
}
orig_func = (orig_func_type) dlsym(handle, "__libc_start_main");
if(dlerror()) {
puts("dlerror");
abort();
}
dlclose(handle);
dprintf("%04d: __libc_start_main is hooked.\n", (int) pthread_self());
args.argv = argv;
args.main_func = (main_type)func_ptr;
saved_init_func = (main_type)init_func;
saved_fini_func = (fini_type)rtld_fini_func;
ret = orig_func((void*)my_main, argc, (char**)(&args),
(fnptr_type)tern_init_func, (fnptr_type)fini_func,
rtld_fini_func, stack_end);
return ret;
}
//hook recv
extern "C" ssize_t recv(int sockfd, void *buf, size_t len, int flags)
{
ssize_t (*orig_recv)(int sockfd, void *buf, size_t len, int flags);
orig_recv = dlsym(RTLD_NEXT, "recv");
orig_recv(sockfd, buf, len, flags);
printf("age is %d\n", datainfo.age);
}
However, when I makefile, I get the error: invalid conversion from ‘void*’ to ‘ssize_t (*)(int, void*, size_t, int) coming from dlsym(RTLD_NEXT, "recv");. My another question is can I achieve my goal in this way? If not, what is the correct way?

Answer 1

C++ is much more strongly typed than C, you need to explicitly cast void * to the correct type.
For example:
extern "C" ssize_t recv(int sockfd, void *buf, size_t len, int flags)
{
using orig_recv_t = ssize_t (*)(int, void *, size_t, int);
// Or for pre C++11 compilers: typedef ssize_t (*orig_recv_t)(int, void *, size_t, int);
orig_recv_t orig_recv;
orig_recv = reinterpret_cast<orig_recv_t>(dlsym(RTLD_NEXT, "recv"));
orig_recv(sockfd, buf, len, flags);
printf("age is %d\n", datainfo.age);
}