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

[Stephan Bergmann]

ping1^5

On 06/19/2015 02:02 PM, Stephan Bergmann wrote:
> 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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