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?
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.
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