dlsym causing segfault when dynamically loading shm_open - c++

Question

I am trying to figure out to load the function shm_open dynamically at runtime and have hit a wall.
My current code is as such:
CMakeLists.txt
project(dlsysm_stack_overflow_example)
cmake_minimum_required(VERSION 3.16)
add_executable(${PROJECT_NAME}
main.cpp
custom_mman.cpp
)
target_link_libraries(${PROJECT_NAME}
dl
)
custom_mman.cpp
#include <iostream>
#define _GNU_SOURCE
#include <dlfcn.h>
extern "C"
{
int shm_open(const char* name, int oflag, mode_t mode)
{
using ShmOpenFuncPtr_t = int (*)(const char*, int, mode_t);
static ShmOpenFuncPtr_t real_shm_open = nullptr;
std::cout << "custom shm_open\n";
if (nullptr == real_shm_open)
{
real_shm_open = reinterpret_cast<ShmOpenFuncPtr_t>(dlsym(RTLD_NEXT, "shm_open"));
char* error = dlerror();
if (error != nullptr)
{
std::cout << "could not load shm_open at runtime! error: " << error << "\n";
return -1;
}
}
return real_shm_open(name, oflag, mode);
}
}
main.cpp
#include <sys/mman.h>
#include <fcntl.h>
#include <iostream>
int main(int argc, char** argv)
{
int return_value = shm_open("/shm_name", O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
std::cout << "return_value: " << return_value << "\n";
return 0;
}
At the moment, because I give the executable the custom_mann.cpp source file, the linker finds my definition of shm_open before the standard library definition and calls that instead. My intention is then to do some logic before calling the real shm_open function.
However, the call to dlsym is returning a null pointer so when real_shm_open is called, a segfault occurs. Strangely, dlerror does not set the error code so the function doesn't return early as I would expect if dlsym is not returning the proper pointer.
Any ideas why this could be happening is much appreciated?

Answer 1

shm_open is supplied by librt.so. Per the Linux shm_open() man page (bolding mine):
NAME
shm_open, shm_unlink - create/open or unlink POSIX shared memory
objects
SYNOPSIS
#include <sys/mman.h>
#include <sys/stat.h> /* For mode constants */
#include <fcntl.h> /* For O_* constants */
int shm_open(const char *name, int oflag, mode_t mode);
int shm_unlink(const char *name);
Link with -lrt.
You need to add rt to your target_link_libraries.
You should be checking the value returned from dlsym() directly for errors. Per the dlsym man page:
RETURN VALUE
On success, these functions return the address associated with
symbol. On failure, they return NULL; the cause of the error can
be diagnosed using dlerror(3).
Additionally, your code is not thread-safe:
static ShmOpenFuncPtr_t real_shm_open = nullptr;
std::cout << "custom shm_open\n";
if (nullptr == real_shm_open)
{
real_shm_open = reinterpret_cast<ShmOpenFuncPtr_t>(dlsym(RTLD_NEXT, "shm_open"));
char* error = dlerror();
if (error != nullptr)
{
std::cout << "could not load shm_open at runtime! error: " << error << "\n";
return -1;
}
}
Two or more threads can be executing the code in parallel, resulting in a corrupted real_shm_open that is neither NULL nor correct.