C++ Memory Tracking - Bad to have the allocation information before the real alloc?

Hi everybody!
Is it bad to have the memory allocation information before the real allocation?
Is it a not recommended and bad way to achieve memory tracking?
Is it better to keep the information of the allocation in a hash table somewhere else?
Basically is it bad to do: malloc(SizeOfInfo + SizeAlloc)
Is it going to change the behavior when the memory tracking is enabled?
Is it changing the memory alignment and other things?
Thanks a lot!