Reports

Yep malloc being called before main was indeed the problem. Thanks @IgorTandetnik and @Mike for the detailed clarification.

To fix the issue I'm currently doing this. Its dirty but seems to work. I'll have to clean this later.

1.h

#define end 0x10007fff7fff
#define start 0x02008fff7000
#define MAPFLAGS (MAP_PRIVATE | MAP_FIXED | MAP_ANON | MAP_NORESERVE)

extern bool ismapped;
void internal_map(void);

1.cpp

#include <cstdint>
#include <sys/mman.h>
#include "1.h"

void internal_map() {
    if (!ismapped) {
        void *ret;
        uintptr_t size = end - start + 1;
        ret = mmap((void*) start, size, PROT_READ | PROT_WRITE, MAPFLAGS, -1, 0);
        (MAP_FAILED == ret? ismapped = false: ismapped = true);
    }
    else
        ismapped = true;
}

extern "C" {
void map()
{
    if (ismapped)
        return;
    internal_map();
}
}

2.cpp

#include <cstdio>
#include <dlfcn.h>
#include "1.h"

extern "C" void map(void);
bool ismapped = false;
void* (*libc_malloc)(size_t size) = NULL;


extern "C" void* malloc(size_t usize)
{
    void *ret;
    if (!libc_malloc)
        libc_malloc = (void*(*)(size_t)) dlsym(RTLD_NEXT, "malloc");

    ret = libc_malloc(usize);

    if (!ismapped) {
        internal_map();
    }
    else {
        void* ptr = (void*) 0x8003fffb000;
        *(char*)ptr = 0xab;
    }
    return ret;
}

Basically, checking if the mapping is successful or not each time malloc is called.

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