alloc-util: introduce MALLOC_SIZEOF_SAFE() helper

It's a wrapper around malloc_usable_size() that is supposed to be
compatible with _FORTIFY_SOURCES=1, by taking the
__builtin_object_size() data into account, the same way as the
_FORTIFY_SOURCES=1 logic does.

Fixes: #19203

src/basic/alloc-util.h

@@ -163,3 +163,15 @@ void* greedy_realloc0(void **p, size_t *allocated, size_t need, size_t size);
 #else
 #  define msan_unpoison(r, s)
 #endif
+
+/* This returns the number of usable bytes in a malloc()ed region as per malloc_usable_size(), in a way that
+ * is compatible with _FORTIFY_SOURCES. If _FORTIFY_SOURCES is used many memory operations will take the
+ * object size as returned by __builtin_object_size() into account. Hence, let's return the smaller size of
+ * malloc_usable_size() and __builtin_object_size() here, so that we definitely operate in safe territory by
+ * both the compiler's and libc's standards. Note that __builtin_object_size() evaluates to SIZE_MAX if the
+ * size cannot be determined, hence the MIN() expression should be safe with dynamically sized memory,
+ * too. Moreover, when NULL is passed malloc_usable_size() is documented to return zero, and
+ * __builtin_object_size() returns SIZE_MAX too, hence we also return a sensible value of 0 in this corner
+ * case. */
+#define MALLOC_SIZEOF_SAFE(x) \
+        MIN(malloc_usable_size(x), __builtin_object_size(x, 0))