[llvm-dev] [RFC] Design of a TBAA sanitizer

[Kostya Serebryany via llvm-dev]

On Tue, Apr 11, 2017 at 3:14 PM, Sanjoy Das <sanjoy at playingwithpointers.com>
wrote:

> Hi Kostya,
>
> On April 11, 2017 at 2:39:44 PM, Kostya Serebryany (kcc at google.com) wrote:
> > > ptr0 = malloc();
> > > free(ptr0);
> > > ptr1 = malloc();
> > >
> > > ptr0 and ptr1 will be NoAlias despite overlapping (there is actually a
> > > real soundness issue here in LLVM's semantics, but I don't want to
> > > digress). You can also recreate the pattern with realloc.
> > >
> >
> > In both of your examples there is no place in the program where both P0
> and
> > P1 are live simultaneously,
> > i.e. no analysis path is expected to query MayAlias(AccessToP0,
> > AccessToP1). No?
>
> I may be misunderstanding what you meant, but I don't see why not.
>
> Say you had (all values are SSA values):
>
> %p0 = malloc()
> store i32 0, i32* %p0  // S0
> free(%p0)
> %p1 = malloc()
> store i32 1, i32* %p1  // S1
>
> and some pass wanted to sink S0 to after S1.  So it starts checking
> "from the bottom", as
>
>   Alias(S0, S1) = NoAlias
>   Alias(S0, malloc()) = NoAlias
>   Alias(S0, free(%p0)) = MayAlias
>
> etc.  The last MayAlias will prevent it from doing the sink, but I
> don't see why it can't ask the Alias(S0, S1) question.
>

Yea, that's a bit trickier.
But we can at least add checks for pairs of pointer accesses that the
analysis claims to be ok to reorder.



>
> > > The same problem exists with constant addresses. LLVM states that
> > > constant locations are noalias with themselves, and you again have the
> > > "noalias does not imply pointer inequality" problem.
> >
> > That won't even have to be special cased, because if we emit a check
> > ConstPtr != ConstPtr,
> > such a check will be trivially optimized away.
>
> But won't it be constant folded to trigger the sanitizer crash /
> warning?  That is, since LLVM will state the ConstPtr NoAlias
> ConstPtr, you'll emit the check:
>
> if (overlap(ConstPtr, Sz, ConstPtr, Sz))
>   abort();
>
> which will get constant folded to
>
> if (true) abort();
>

ah, yes, you are right, then this will have to be special-cased.


>
> If you meant that the implementation of overlap will differ based on
> whether the pointers are constant pointers or not, I'm not sure if
> that will work, since the fact that the values whose aliasness (I
> think I invented a new word :P ) you're checking could have been
> arbitrarily obscured (AA could have looked through PHIs and selects
> etc.) which will prevent you from rediscovering that the values were
> constant pointers in some cases.
>
> -- Sanjoy
>
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