Function memguard_alloc

freebsd/vm/memguard.c:289–361

* Allocate a single object of specified size with specified flags * (either M_WAITOK or M_NOWAIT). */

 * (either M_WAITOK or M_NOWAIT).
 */
void *
memguard_alloc(unsigned long req_size, int flags)
{
	vm_offset_t addr, origaddr;
	u_long size_p, size_v;
	int do_guard, error, rv;

	size_p = round_page(req_size);
	if (size_p == 0)
		return (NULL);

	/*
	 * To ensure there are holes on both sides of the allocation,
	 * request 2 extra pages of KVA.  Save the value of memguard_options
	 * so that we use a consistent value throughout this function.
	 */
	size_v = size_p;
	do_guard = (memguard_options & MG_GUARD_AROUND) != 0;
	if (do_guard)
		size_v += 2 * PAGE_SIZE;

	/*
	 * When we pass our memory limit, reject sub-page allocations.
	 * Page-size and larger allocations will use the same amount
	 * of physical memory whether we allocate or hand off to
	 * malloc_large(), so keep those.
	 */
	if (vmem_size(memguard_arena, VMEM_ALLOC) >= memguard_physlimit &&
	    req_size < PAGE_SIZE) {
		addr = (vm_offset_t)NULL;
		memguard_fail_pgs++;
		goto out;
	}

	/*
	 * Attempt to avoid address reuse for as long as possible, to increase
	 * the likelihood of catching a use-after-free.
	 */
	error = vmem_alloc(memguard_arena, size_v, M_NEXTFIT | M_NOWAIT,
	    &origaddr);
	if (error != 0) {
		memguard_fail_kva++;
		addr = (vm_offset_t)NULL;
		goto out;
	}
	addr = origaddr;
	if (do_guard)
		addr += PAGE_SIZE;
	rv = kmem_back(kernel_object, addr, size_p, flags);
	if (rv != KERN_SUCCESS) {
		vmem_xfree(memguard_arena, origaddr, size_v);
		memguard_fail_pgs++;
		addr = (vm_offset_t)NULL;
		goto out;
	}
	*v2sizep(trunc_page(addr)) = req_size;
	*v2sizev(trunc_page(addr)) = size_v;
	memguard_succ++;