Change safe_{sub,add,mul} so they perform the operation only once

kernel/src/memory/Heap.cpp

@@ -5,6 +5,7 @@
 #include "memory/KernelVM.h"
 #include "memory/MemoryManager.h"
 #include <luna/Alignment.h>
+#include <luna/SafeArithmetic.h>
 #include <luna/String.h>
 #include <luna/SystemError.h>
 
@@ -332,8 +333,7 @@ Result<void*> krealloc(void* ptr, usize size)
 
 Result<void*> kcalloc(usize nmemb, usize size)
 {
-    // FIXME: Check for overflows.
-    const usize realsize = nmemb * size;
+    const usize realsize = TRY(safe_mul(nmemb, size));
     void* const ptr = TRY(kmalloc(realsize));
     return memset(ptr, 0, realsize);
 }

luna/include/luna/SafeArithmetic.h

@@ -0,0 +1,44 @@
+#pragma once
+#include <luna/Result.h>
+
+template <typename T> constexpr bool add_will_overflow(T a, T b)
+{
+    return __builtin_add_overflow_p(a, b, (T)0);
+}
+
+template <typename T> constexpr bool sub_will_overflow(T a, T b)
+{
+    return __builtin_sub_overflow_p(a, b, (T)0);
+}
+
+template <typename T> constexpr bool mul_will_overflow(T a, T b)
+{
+    return __builtin_mul_overflow_p(a, b, (T)0);
+}
+
+template <typename T> Result<T> safe_add(T a, T b)
+{
+    T result;
+
+    if (__builtin_add_overflow(a, b, &result)) return err(EOVERFLOW);
+
+    return result;
+}
+
+template <typename T> Result<T> safe_sub(T a, T b)
+{
+    T result;
+
+    if (__builtin_sub_overflow(a, b, &result)) return err(EOVERFLOW);
+
+    return result;
+}
+
+template <typename T> Result<T> safe_mul(T a, T b)
+{
+    T result;
+
+    if (__builtin_mul_overflow(a, b, &result)) return err(EOVERFLOW);
+
+    return result;
+}