On Wed, Aug 25, 2021 at 12:01:42PM +0200, Christoph Lameter wrote:

On Thu, 19 Aug 2021, Daniel Micay wrote:

quoted

For example, it will know that kmalloc(n) returns either NULL or an
allocation of size n. A simple sample program with calloc in
userspace:

    #include <stdlib.h>
    #include <stdio.h>

    int main(void) {
        char *p = calloc(64, 1);
        if (!p) {
            return 1;
        }
        printf("%zu\n", __builtin_object_size(p, 1));
        return 0;
    }

It will also detect an out-of-bounds access via the allocation with
-fsanitize=object-size including with a runtime value as the index.

It's not as useful as it should be yet because __builtin_object_size
must return a compile-time constant. Clang has a new
__builtin_dynamic_object_size that's allowed to return a value that's
not a compile-time constant so it can work for kmalloc(n) where n is a
runtime value. It might not be quite ready for use yet but it should
be able to make it a lot more useful. GCC also seems open to adding it
too.

The other complication with kmalloc etc is that the slab allocators may
decided to allocate more bytes than needed because it does not support
that particular allocation size. Some functions check the allocated true
size and make use of that. See ksize().

Yup, this is known. For the current iteration, this doesn't pose a
problem since the compile-time checking has very limited scope.

-- 
Kees Cook