add cell/optional/box uaf example

Author: cmazakas

libsafecxx/test/broken/cell_box_uaf.cxx

@@ -0,0 +1,63 @@
+#feature on safety
+
+#include <std2.h>
+
+using namespace std2;
+
+struct S/(a)
+{
+  cell<optional<S/a const^/a>> x_;
+  box<int> p_;
+
+  S(box<int> p) safe
+    : x_(optional<S/a const^/a>(.none))
+    , p_(rel p)
+  {
+  }
+
+  ~S() safe {
+    match(x_.get()) {
+      .some(r) => println(*r->p_);
+      .none => void();
+    };
+  }
+};
+
+int main() safe
+{
+  {
+    S s1(box<int>(1234));
+    S s2(box<int>(4321));
+    s1.x_.set(.some(^const s2));
+    s2.x_.set(.some(^const s1));
+  }
+}
+
+// outputs:
+// ❯ valgrind ./cell_box_uaf
+// ==352616== Memcheck, a memory error detector
+// ==352616== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
+// ==352616== Using Valgrind-3.22.0 and LibVEX; rerun with -h for copyright info
+// ==352616== Command: ./cell_box_uaf
+// ==352616==
+// 1234
+// ==352616== Invalid read of size 4
+// ==352616==    at 0x4013E2: S::~S() (in /home/exbigboss/cpp/circle-root/safe-cpp/cell_box_uaf)
+// ==352616==    by 0x401294: main (in /home/exbigboss/cpp/circle-root/safe-cpp/cell_box_uaf)
+// ==352616==  Address 0x4e2c0d0 is 0 bytes inside a block of size 4 free'd
+// ==352616==    at 0x484A164: operator delete(void*) (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
+// ==352616==    by 0x4015B5: std2::box<int>::~box() (in /home/exbigboss/cpp/circle-root/safe-cpp/cell_box_uaf)
+// ==352616==    by 0x4013FE: S::~S() (in /home/exbigboss/cpp/circle-root/safe-cpp/cell_box_uaf)
+// ==352616==    by 0x40128A: main (in /home/exbigboss/cpp/circle-root/safe-cpp/cell_box_uaf)
+// ==352616==  Block was alloc'd at
+// ==352616==    at 0x48479E8: operator new(unsigned long, std::nothrow_t const&) (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
+// ==352616==    by 0x4012D6: std2::box<int>::box(int) (in /home/exbigboss/cpp/circle-root/safe-cpp/cell_box_uaf)
+// ==352616==    by 0x401223: main (in /home/exbigboss/cpp/circle-root/safe-cpp/cell_box_uaf)
+// ==352616==
+// 4321
+// ==352616==
+// ==352616== HEAP SUMMARY:
+// ==352616==     in use at exit: 0 bytes in 0 blocks
+// ==352616==   total heap usage: 4 allocs, 4 frees, 74,760 bytes allocated
+// ==352616==
+// ==352616== All heap blocks were freed -- no leaks are possible