[cfe-commits] r168124 - /cfe/trunk/lib/AST/MicrosoftMangle.cpp

[Kostya Serebryany]

+samsonov

Alexey, will we catch this with asan use-after-scope?

On Fri, Nov 16, 2012 at 10:45 AM, Matthieu Monrocq <
matthieu.monrocq at gmail.com> wrote:

>
>
> On Fri, Nov 16, 2012 at 2:58 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>> Can we warn about this?
>>
>>
> Unfortunately no. I don't think so. It would require inspecting the body
> of `cast`...
>
> Argyrios implemented a whole class of warnings around this theme a while
> ago now, and as we drilled into it (and I believe you helped) we just
> decided that looking through functions was too difficult. As such: `T
> const& foo(T const& t) { return t; }` completely dulls out binding to
> temporary warnings => the function itself is correct, but its use may not.
>
>
> However it seems to be that the problem is caused by the implementation of
> `cast` is wrong.
>
> template <class X, class Y>
> inline typename cast_retty<X, Y>::ret_type cast(const Y &Val) {
>   assert(isa<X>(Val) && "cast<Ty>() argument of incompatible type!");
>   return cast_convert_val<X, Y,
>                           typename
> simplify_type<Y>::SimpleType>::doit(Val);
> }
>
> We accept a `const&`, allowing bind to temporary, but then the return type
> can be bound to X& which means the `const` qualifier was dropped on the
> floor. This is pretty nasty.
>
> The culprit is actually just above:
>
> template<class To, class FromTy> struct cast_convert_val<To,FromTy,FromTy>
> {
>   // This _is_ a simple type, just cast it.
>   static typename cast_retty<To, FromTy>::ret_type doit(const FromTy &Val)
> {
>     typename cast_retty<To, FromTy>::ret_type Res2
>      = (typename cast_retty<To, FromTy>::ret_type)*const_cast*
> <FromTy&>(Val);
>     return Res2;
>   }
> };
>
>
> However, even overloading on const-ness would not make bind-to-const
> issues disappear. The only thing I can thing of is:
> => allow `cast` (and al) only on non-const reference
> => allow on non-const and const pointers
>
> which is the only way to circumvent bind-to-temporary situations I know of.
>
> I am not sure it's worth it.
>
>
> For detection I see only two possibilities:
> => Interprocedural (*) static analysis: when analyzing the function, note
> that its return may be an alias of the parameter; when analyzing the use,
> note that the statement is only valid if the return is not an alias of the
> temporary; cross-reference the two and warn about the incompatibility
> => Dynamic instrumentation: I wonder if *Asan* could catch this (maybe it
> does already). With the proper instrumentation the destructor of the
> temporary could trigger poisoning of the memory so that the next use cause
> an assertion.
>
> (*) Interprocedural in the general case, here it would not be.
>
> -- Matthieu
>
>
>> On Thu, Nov 15, 2012 at 5:14 PM, Matt Beaumont-Gay <matthewbg at google.com>
>> wrote:
>> > Author: matthewbg
>> > Date: Thu Nov 15 19:14:52 2012
>> > New Revision: 168124
>> >
>> > URL: http://llvm.org/viewvc/llvm-project?rev=168124&view=rev
>> > Log:
>> > Fix PR14321, a crash when Clang is built with GCC 4.7 at -O1 or greater.
>> >
>> > GCC 4.7 reuses stack slots fairly aggressively, which exposes more
>> temporary
>> > lifetime bugs.
>> >
>> > No new test, this was caught by the existing
>> CodeGenCXX/mangle-ms-templates.cpp.
>> >
>> > Modified:
>> >     cfe/trunk/lib/AST/MicrosoftMangle.cpp
>> >
>> > Modified: cfe/trunk/lib/AST/MicrosoftMangle.cpp
>> > URL:
>> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/MicrosoftMangle.cpp?rev=168124&r1=168123&r2=168124&view=diff
>> >
>> ==============================================================================
>> > --- cfe/trunk/lib/AST/MicrosoftMangle.cpp (original)
>> > +++ cfe/trunk/lib/AST/MicrosoftMangle.cpp Thu Nov 15 19:14:52 2012
>> > @@ -373,7 +373,7 @@
>> >        dyn_cast<ClassTemplateSpecializationDecl>(ND)) {
>> >      TypeSourceInfo *TSI = Spec->getTypeAsWritten();
>> >      if (TSI) {
>> > -      TemplateSpecializationTypeLoc &TSTL =
>> > +      TemplateSpecializationTypeLoc TSTL =
>> >          cast<TemplateSpecializationTypeLoc>(TSI->getTypeLoc());
>> >        TemplateArgumentListInfo LI(TSTL.getLAngleLoc(),
>> TSTL.getRAngleLoc());
>> >        for (unsigned i = 0, e = TSTL.getNumArgs(); i != e; ++i)
>> >
>> >
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