how_it_work.md

How it work

What is a route ?

In mallocator a route is the path which is taken to go from the main to the allocation (malloc, calloc etc..) this is the call stack. A route is an array of address that look like this:

0x1234 (main)
0x1235 (other_func)
0x1236 (malloc)

Each address is where the function is called, it allow us to differentiate two alloc in the same function.

How we get the route ?

This is pretty simple, unix have a function called backtrace that fetch the route to itself.

void *routes[20]

size_t number_of_pointers = backtrace(routes, 20);

We can get symbol name using dladdr and the clang flag -rdynamic that allow dladdr to find symbol's name

char const *get_func_name(void *addr)
{
Dl_info info;

if (dladdr(addr, &info) && info.dli_sname)
	return (info.dli_sname);
else
	return ("cannot get name");
}

How to replace malloc ?

You have to make a library that have a malloc function, that will be the hook. You can use __libc_malloc malloc to use the real malloc

extern  void *__libc_malloc(size_t size);

Mallocator's malloc look like this:

int g_malloc_hook_active = 1;

void *alloc_hook(size_t size)
{
	void *result;
	g_malloc_hook_active = 0;
	// here we can call malloc safefly
	if (some_condition)
		result = NULL;
	else
		result =.malloc(size);
	g_malloc_hook_active = 1;
}

void *malloc(size_t size)
{
	if (g_malloc_hook_active)
		return (alloc_hook(size));
	return (__libc_malloc(size));
}

Steps

• Fetch all routes by hooking allocs, and use backtrace
• Run and block every route one by one
• If the route have multiple iteration run a second time the route but make the route fail the second time it is called