How to restrict user from using unallocated memory in the malloc implementation at run time in C?

Question

I'm trying to implement ownmalloc() by calling original malloc() only once at beginning of the code.

Using this memory returned by original malloc(), each ownmalloc() call from user program uses this memory for allocation.

There are 2 lists maintained allocated and unallocated.

ownfree() function removes the allocated memory and adds back to unallocated blocks, orders them and makes the contiguous address blocks as a single block

I have two questions:

How can I stop user from using unauthorized memory at run time, e.g. user asked 10 blocks and using 13 blocks to write the data.
ownmalloc() calls a function(say x) which in turn calls original malloc() and takes block of memory which will be used by ownmalloc() multiple times. How can I make sure that ownmalloc() calls this function x() only once (during the first call) and at later calls, function call to x() shouldn't be done.

I tried it with this approach, but its not working in c

typedef struct mem{
    int sz;
    struct mem *next;
}node;

node *block;
static bool init=false;

void* x(){
    //call to original malloc
}

void* ownmalloc(size){
    if(!init){
        block=x();
        init=true;
    }
}

Whenever I try to make call to this ownmalloc() x() function is being called every time. How do I make it call only once?

In C, it's impossible to stop programs from using memory other than what they're supposed to use. This is why memory corruption bugs in C are so common. — Joseph Sible-Reinstate Monica, May 12 '19 at 14:44
Any other alternative or atleast can i display a warning kind of thing using any library — asterix, May 12 '19 at 14:45
Use a tool such as AddressSanitizer, which has been included by default in gcc since version 4.8. See https://stackoverflow.com/questions/37970758/how-to-use-addresssanitizer-in-gcc for instructions on use — user3147395, May 12 '19 at 14:50
Sure, there are lots and lots of absolutely fantastic languages around. — n. m. could be an AI, May 12 '19 at 16:29
Is there anything which I can implement in my code which will take care of unauthorized access of memory — asterix, May 12 '19 at 19:25

score 0 · Answer 1 · answered May 12 '19 at 14:55

Try get the input from the user into a static allocated data structure and check it's size.

if the size is smaller than your allocated memory assign the input and otherwise give an error. its missing a big point of dynamic memory allocation but i think it should work.

How to restrict user from using unallocated memory in the malloc implementation at run time in C?

1 Answers1