[Bug sanitizer/67258] "invalid vptr" false positive from ubsan for virtual inheritance

[trippels at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67258

Markus Trippelsdorf <trippels at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-09-09
                 CC|                            |trippels at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
This was fixed upstream by:

commit 1d2477faafda9ad2cc19927b3c31efd22747f013
Author: Alexey Samsonov <vonos...@gmail.com>
Date:   Wed Aug 5 19:35:46 2015 +0000

    [UBSan] Fix UBSan-vptr false positive.

    Offset from vptr to the start of most-derived object can actually
    be positive in some virtual base class vtables.

    Patch by Stephan Bergmann!

    git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@244101
91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/ubsan/ubsan_type_hash_itanium.cc
b/lib/ubsan/ubsan_type_hash_itanium.cc
index 5cd46df16a33..b84e88d4c71d 100644
--- a/lib/ubsan/ubsan_type_hash_itanium.cc
+++ b/lib/ubsan/ubsan_type_hash_itanium.cc
@@ -185,8 +185,8 @@ namespace {

 struct VtablePrefix {
   /// The offset from the vptr to the start of the most-derived object.
-  /// This should never be greater than zero, and will usually be exactly
-  /// zero.
+  /// This will only be greater than zero in some virtual base class vtables
+  /// used during object con-/destruction, and will usually be exactly zero.
   sptr Offset;
   /// The type_info object describing the most-derived class type.
   std::type_info *TypeInfo;
@@ -196,7 +196,7 @@ VtablePrefix *getVtablePrefix(void *Vtable) {
   if (!Vptr)
     return 0;
   VtablePrefix *Prefix = Vptr - 1;
-  if (Prefix->Offset > 0 || !Prefix->TypeInfo)
+  if (!Prefix->TypeInfo)
     // This can't possibly be a valid vtable.
     return 0;
   return Prefix;
diff --git a/test/ubsan/TestCases/TypeCheck/vptr-virtual-base-construction.cpp
b/test/ubsan/TestCases/TypeCheck/vptr-virtual-base-construction.cpp
new file mode 100644
index 000000000000..dc27d9f39ce3
--- /dev/null
+++ b/test/ubsan/TestCases/TypeCheck/vptr-virtual-base-construction.cpp
@@ -0,0 +1,13 @@
+// RUN: %clangxx -frtti -fsanitize=vptr -fno-sanitize-recover=vptr %s -o %t
+// RUN: %run %t
+
+// REQUIRES: cxxabi
+
+int volatile n;
+
+struct A { virtual ~A() {} };
+struct B: virtual A {};
+struct C: virtual A { ~C() { n = 0; } };
+struct D: virtual B, virtual C {};
+
+int main() { delete new D; }

Analogous gcc patch would be:

diff --git a/libsanitizer/ubsan/ubsan_type_hash.cc
b/libsanitizer/ubsan/ubsan_type_hash.cc
index d01009426db0..495ec5386cf3 100644
--- a/libsanitizer/ubsan/ubsan_type_hash.cc
+++ b/libsanitizer/ubsan/ubsan_type_hash.cc
@@ -197,7 +197,7 @@ VtablePrefix *getVtablePrefix(void *Object) {
   if (!*VptrPtr)
     return 0;
   VtablePrefix *Prefix = *VptrPtr - 1;
-  if (Prefix->Offset > 0 || !Prefix->TypeInfo)
+  if (!Prefix->TypeInfo)
     // This can't possibly be a valid vtable.
     return 0;
   return Prefix;