Using the GNU Compiler Collection (GCC): Pointer Bounds Checker builtins

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### 6.55 Pointer Bounds Checker Built-in Functions

GCC provides a set of built-in functions to control Pointer Bounds Checker instrumentation. Note that all Pointer Bounds Checker builtins can be used even if you compile with Pointer Bounds Checker off (`-fno-check-pointer-bounds`). The behavior may differ in such case as documented below.

Built-in Function: void * __builtin___bnd_set_ptr_bounds (const void *q, size_t size)

  This built-in function returns a new pointer with the value of q, and associate it with the bounds [q, q+size-1]. With Pointer Bounds Checker off, the built-in function just returns the first argument.

      extern void *__wrap_malloc (size_t n)
      {
        void *p = (void *)__real_malloc (n);
        if (!p) return __builtin___bnd_null_ptr_bounds (p);
        return __builtin___bnd_set_ptr_bounds (p, n);
      }

Built-in Function: void * __builtin___bnd_narrow_ptr_bounds (const void *p, const void *q, size_t size)

  This built-in function returns a new pointer with the value of p and associates it with the narrowed bounds formed by the intersection of bounds associated with q and the bounds [p, p + size - 1]. With Pointer Bounds Checker off, the built-in function just returns the first argument.

      void init_objects (object *objs, size_t size)
      {
        size_t i;
        /* Initialize objects one-by-one passing pointers with bounds of
           an object, not the full array of objects.  */
        for (i = 0; i < size; i++)
          init_object (__builtin___bnd_narrow_ptr_bounds (objs + i, objs,
                                                          sizeof(object)));
      }

Built-in Function: void * __builtin___bnd_copy_ptr_bounds (const void *q, const void *r)

  This built-in function returns a new pointer with the value of q, and associates it with the bounds already associated with pointer r. With Pointer Bounds Checker off, the built-in function just returns the first argument.

      /* Here is a way to get pointer to object's field but
         still with the full object's bounds.  */
      int *field_ptr = __builtin___bnd_copy_ptr_bounds (&objptr->int_field,
                                                        objptr);

Built-in Function: void * __builtin___bnd_init_ptr_bounds (const void *q)

  This built-in function returns a new pointer with the value of q, and associates it with INIT (allowing full memory access) bounds. With Pointer Bounds Checker off, the built-in function just returns the first argument.

Built-in Function: void * __builtin___bnd_null_ptr_bounds (const void *q)

  This built-in function returns a new pointer with the value of q, and associates it with NULL (allowing no memory access) bounds. With Pointer Bounds Checker off, the built-in function just returns the first argument.

Built-in Function: void __builtin___bnd_store_ptr_bounds (const void **ptr_addr, const void *ptr_val)

  This built-in function stores the bounds associated with pointer ptr_val and location ptr_addr into Bounds Table. This can be useful to propagate bounds from legacy code without touching the associated pointer’s memory when pointers are copied as integers. With Pointer Bounds Checker off, the built-in function call is ignored.

Built-in Function: void __builtin___bnd_chk_ptr_lbounds (const void *q)

  This built-in function checks if the pointer q is within the lower bound of its associated bounds. With Pointer Bounds Checker off, the built-in function call is ignored.

      extern void *__wrap_memset (void *dst, int c, size_t len)
      {
        if (len > 0)
          {
            __builtin___bnd_chk_ptr_lbounds (dst);
            __builtin___bnd_chk_ptr_ubounds ((char *)dst + len - 1);
            __real_memset (dst, c, len);
          }
        return dst;
      }

Built-in Function: void __builtin___bnd_chk_ptr_ubounds (const void *q)

  This built-in function checks if the pointer q is within the upper bound of its associated bounds. With Pointer Bounds Checker off, the built-in function call is ignored.

Built-in Function: void __builtin___bnd_chk_ptr_bounds (const void *q, size_t size)

  This built-in function checks if [q, q + size - 1] is within the lower and upper bounds associated with q. With Pointer Bounds Checker off, the built-in function call is ignored.

      extern void *__wrap_memcpy (void *dst, const void *src, size_t n)
      {
        if (n > 0)
          {
            __bnd_chk_ptr_bounds (dst, n);
            __bnd_chk_ptr_bounds (src, n);
            __real_memcpy (dst, src, n);
          }
        return dst;
      }

Built-in Function: const void * __builtin___bnd_get_ptr_lbound (const void *q)

  This built-in function returns the lower bound associated with the pointer q, as a pointer value. This is useful for debugging using `printf`. With Pointer Bounds Checker off, the built-in function returns 0.

      void *lb = __builtin___bnd_get_ptr_lbound (q);
      void *ub = __builtin___bnd_get_ptr_ubound (q);
      printf ("q = %p  lb(q) = %p  ub(q) = %p", q, lb, ub);

Built-in Function: const void * __builtin___bnd_get_ptr_ubound (const void *q)

  This built-in function returns the upper bound (which is a pointer) associated with the pointer q. With Pointer Bounds Checker off, the built-in function returns -1.

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