[PATCH] [comiler-rt/ubsan] getVtablePrefix must not sanity-check on Prefix->Offset > 0

[Stephan Bergmann]

ping^4

On 06/05/2015 07:06 PM, David Blaikie wrote:
> On Fri, Jun 5, 2015 at 10:02 AM, Alexey Samsonov <vonosmas at gmail.com
> <mailto:vonosmas at gmail.com>> wrote:
>
>     I referred to David Majnemer, who touched this code a while ago. But
>     thanks for suggesting help :)
>
>
> Ah, +Majnemer.
>
> - Dave
>
>
>     On Fri, Jun 5, 2015 at 8:26 AM, David Blaikie <dblaikie at gmail.com
>     <mailto:dblaikie at gmail.com>> wrote:
>
>
>
>         On Thu, Jun 4, 2015 at 5:52 PM, Alexey Samsonov
>         <vonosmas at gmail.com <mailto:vonosmas at gmail.com>> wrote:
>
>             Richard or David, do you want to look into this, or you'd
>             prefer to leave this for me?
>
>
>         I can't say I have much more context in vtable layout & C++ ABI
>         than you do, most likely. Happy to bounce some the ideas around
>         in person if that's helpful.
>
>         - David
>
>
>             On Tue, Jun 2, 2015 at 3:32 AM, Stephan Bergmann
>             <sbergman at redhat.com <mailto:sbergman at redhat.com>> wrote:
>
>                 ping^3
>
>
>                 On 12/16/2014 10:52 AM, Stephan Bergmann wrote:
>
>                     ping
>                     On 12/05/2014 09:33 AM, Stephan Bergmann wrote:
>
>                         ping
>                         On 08/12/2014 09:10 PM, Alexey Samsonov wrote:
>
>                             +Richard
>
>
>                             On Tue, Aug 12, 2014 at 3:51 AM, Stephan
>                             Bergmann <sbergman at redhat.com
>                             <mailto:sbergman at redhat.com>
>                             <mailto:sbergman at redhat.com
>                             <mailto:sbergman at redhat.com>>> wrote:
>
>                                  On 08/11/2014 10:19 PM, Alexey Samsonov
>                             wrote:
>
>                                      +Richard
>
>                                      Note, that you'd also have to
>                             update comment for
>                                      VtablePrefix::Offset field.
>
>
>                                  ah, right; updated patch
>
>                                  Stephan
>
>                                      On Mon, Aug 11, 2014 at 6:30 AM,
>                             Stephan Bergmann
>                                      <sbergman at redhat.com
>                             <mailto:sbergman at redhat.com>
>                             <mailto:sbergman at redhat.com
>                             <mailto:sbergman at redhat.com>>
>                                      <mailto:sbergman at redhat.com
>                             <mailto:sbergman at redhat.com>
>                             <mailto:sbergman at redhat.com
>                             <mailto:sbergman at redhat.com>>>>
>                             wrote:
>
>                                           At least with recent Clang
>                             trunk on Linux x86_64:
>
>                                               $ cat test.cc
>                                               #include <iostream>
>                                               struct A { virtual ~A() {} };
>                                               struct B: virtual A {};
>                                               struct C: virtual A { ~C()
>                             { std::cout << '\n'; } };
>                                               struct D: virtual B,
>                             virtual C {};
>                                               int main() { delete new D; }
>
>                                               $ clang++
>                             -fsanitize=undefined test.cc
>
>                                               $ ./a.out
>                                               <unknown>: runtime error:
>                             member call on address
>                                      0x000002a35010
>                                               which does not point to an
>                             object of type 'A'
>                                               0x000002a35010: note:
>                             object has invalid vptr
>                                                 00 00 00 00  58 0e 43 00
>                             00 00 00 00  30 0e 43 00 00
>                                      00 00 00
>                                                 00 00 00 00 00 00 00 00
>                             e1 0f 02 00
>
>                             ^~~~~~~~~~~~~~~~~~~~~~~
>                                                              invalid vptr
>
>
>                                           The problem is that
>                             getVtablePrefix
>                                      (lib/ubsan/ubsan_type_hash.cc)
>                                           rejects any VtablePrefix with
>                             Offset > 0 as "This can't
>                                      possibly be
>                                           a valid vtable" but, according
>                             to the Itanium ABI, "in some
>                                           construction virtual tables
>                             will some virtual base virtual
>                                      tables
>                                           have positive offsets."
>
>                                           The apparent fix is to remove
>                             the check, see the attached
>                                           getVtablePrefix.patch.
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