stdinc: Add overflow-checking add and multiply for size_t

This can be used to check whether untrusted sizes would cause overflow
when used to calculate how much memory is needed.

Signed-off-by: Simon McVittie <smcv@collabora.com>

include/SDL_stdinc.h

@@ -115,6 +115,12 @@ char *alloca();
 # endif
 #endif
 
+#ifdef SIZE_MAX
+# define SDL_SIZE_MAX SIZE_MAX
+#else
+# define SDL_SIZE_MAX ((size_t) -1)
+#endif
+
 /**
  * Check if the compiler supports a given builtin.
  * Supported by virtually all clang versions and recent gcc. Use this
@@ -728,6 +734,60 @@ SDL_FORCE_INLINE void *SDL_memcpy4(SDL_OUT_BYTECAP(dwords*4) void *dst, SDL_IN_B
     return SDL_memcpy(dst, src, dwords * 4);
 }
 
+/**
+ * If a * b would overflow, return -1. Otherwise store a * b via ret
+ * and return 0.
+ *
+ * \since This function is available since SDL 2.24.0.
+ */
+SDL_FORCE_INLINE int SDL_size_mul_overflow (size_t a,
+                                            size_t b,
+                                            size_t *ret)
+{
+    if (a != 0 && b > SDL_SIZE_MAX / a) {
+        return -1;
+    }
+    *ret = a * b;
+    return 0;
+}
+
+#if _SDL_HAS_BUILTIN(__builtin_mul_overflow)
+SDL_FORCE_INLINE int _SDL_size_mul_overflow_builtin (size_t a,
+                                                     size_t b,
+                                                     size_t *ret)
+{
+    return __builtin_mul_overflow(a, b, ret) == 0 ? 0 : -1;
+}
+#define SDL_size_mul_overflow(a, b, ret) (_SDL_size_mul_overflow_builtin(a, b, ret))
+#endif
+
+/**
+ * If a + b would overflow, return -1. Otherwise store a + b via ret
+ * and return 0.
+ *
+ * \since This function is available since SDL 2.24.0.
+ */
+SDL_FORCE_INLINE int SDL_size_add_overflow (size_t a,
+                                            size_t b,
+                                            size_t *ret)
+{
+    if (b > SDL_SIZE_MAX - a) {
+        return -1;
+    }
+    *ret = a + b;
+    return 0;
+}
+
+#if _SDL_HAS_BUILTIN(__builtin_add_overflow)
+SDL_FORCE_INLINE int _SDL_size_add_overflow_builtin (size_t a,
+                                                     size_t b,
+                                                     size_t *ret)
+{
+    return __builtin_add_overflow(a, b, ret) == 0 ? 0 : -1;
+}
+#define SDL_size_add_overflow(a, b, ret) (_SDL_size_add_overflow_builtin(a, b, ret))
+#endif
+
 /* Ends C function definitions when using C++ */
 #ifdef __cplusplus
 }