[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")

Chandler Carruth via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 24 21:41:59 PST 2016 

On Wed, Feb 24, 2016 at 9:35 PM Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
>
> > From: "Chandler Carruth via llvm-dev" <llvm-dev at lists.llvm.org>
> > To: "Philip Reames" <listmail at philipreames.com>, "Duncan P. N. Exon
> > Smith" <dexonsmith at apple.com>, "Sanjoy Das"
> > <sanjoy at playingwithpointers.com>
> > Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
> > Sent: Wednesday, February 24, 2016 10:29:23 PM
> > Subject: Re: [llvm-dev] Possible soundness issue with
> > available_externally (split from "RFC: Add guard intrinsics")
>
> > Yea, I'm pretty sad about all of this. I'm also not seeing a lot of
> > awesome paths forward.
>
> > Here is the least bad strategy I can come up with. Curious if folks
> > think this is sufficient:
>
> I may not completely understand the problem, but this seems like overkill.
> The underlying restriction is that, if the compiler makes a
> non-determinism-collapsing choice when optimizing a function, it must make
> the same choice for all definitions of that function (undefined behavior
> excluded).


This isn't enough, because some definition in some other module may *not be
optimized at all*, and yet may get selected at link time.

Put another way, it must *prove* that the same choice will *always* be made
for all definitions. This is akin to proving that the optimizer is run over
all translation units for C++ linkonce_odr functions, which you can't do.

The result would be failing to optimize the bodies of linkonce_odr
functions in any way which was externally detectable such as this. I think
that would be *much* worse than losing the ability to do function attribute
deduction for such functions?

Thus, with an externally_available function, the CSE in Sanjoy's original
> example should be forbidden. Richard's example again demonstrates this
> principle, although in this case the non-determinism is in the choice of a
> globally-visible implementation technique rather than non-determinism from
> memory-subsystem reordering.
>
> There is a complication, which you imply in your proposal, that such
> optimizations need to be forbidden not just in the externally_available
> functions themselves, but in any local function transitively called by one.
> This, however, we can take care of with an (easily-deduced) attribute.
>
> In short, it is not clear to me that the number of problematic
> optimizations is large (seems likely restricted to things involving atomics
> in practice), and while I understand the auditing difficulties here, we
> should just restrict these in appropriate contexts instead of trying to
> restrict all information flow into or out of comdats.
>
>  -Hal
>
> > 1) Stop deducing function attributes within comdats by examining the
> > bodies of the functions (so that we remain free to transform the
> > bodies of functions).
> > 2) Teach frontends to emit (even at O0!!!) trivially deduced function
> > attributes for comdats so that we continue to catch easy cases.
> > 3) Ensure and specify that we never hoist code *into* a comdat group
> > in which it would not have been executed previously. I don't know of
> > anything in LLVM that does this today, but it would become an
> > important invariant.
> > 4) Work a lot harder to do internalizing and removing of this
> > restriction.
>
> > Pretty horrible. But I think it is correct.
>
> > As a slight modification to #1 and #2, we could have a very carefully
> > crafted deduction rule where we only deduce function attributes for
> > functions prior to any modification of their function bodies. Such
> > attributes should be conservatively correct because we would never
> > lift new code into the function bodies. This would at least allow us
> > to do bottom-up deduction to catch interprocedural cases. But it
> > would become incredibly subtle that this is only valid prior to
> > *any* transformations of the comdat-containing functions.
>
> > I'm starting to think this subtle rule might be worth it. But I'm
> > frankly terrified by the implications.
>
> > On Wed, Feb 24, 2016 at 8:13 PM Philip Reames via llvm-dev <
> > llvm-dev at lists.llvm.org > wrote:
>
> > > On 02/24/2016 08:10 PM, Duncan P. N. Exon Smith via llvm-dev wrote:
> >
> > > >> On 2016-Feb-24, at 19:46, Sanjoy Das <
> > > >> sanjoy at playingwithpointers.com > wrote:
> >
> > > >>
> >
> > > >> On Wed, Feb 24, 2016 at 7:38 PM, Chandler Carruth <
> > > >> chandlerc at google.com > wrote:
> >
> > > >>> On Wed, Feb 24, 2016 at 7:34 PM Duncan P. N. Exon Smith
> >
> > > >>> < dexonsmith at apple.com > wrote:
> >
> > > >>>>
> >
> > > >>>>> On 2016-Feb-24, at 19:17, Chandler Carruth <
> > > >>>>> chandlerc at google.com > wrote:
> >
> > > >>>>>
> >
> > > >>>>> On Wed, Feb 24, 2016 at 7:10 PM Sanjoy Das via llvm-dev
> >
> > > >>>>> < llvm-dev at lists.llvm.org > wrote:
> >
> > > >>>>> On Wed, Feb 24, 2016 at 6:51 PM, Duncan P. N. Exon Smith
> >
> > > >>>>> < dexonsmith at apple.com > wrote:
> >
> > > >>>>>>> If we do not inline @foo(), and instead re-link the call
> > > >>>>>>> site
> > > >>>>>>> in
> >
> > > >>>>>>> @main
> >
> > > >>>>>>> to some non-optimized copy (or differently optimized copy)
> > > >>>>>>> of
> > > >>>>>>> @foo,
> >
> > > >>>>>>> then it is possible for the program to have the behavior
> > > >>>>>>> {print("Y");
> >
> > > >>>>>>> print ("X")}, which was disallowed in the earlier program.
> >
> > > >>>>>>>
> >
> > > >>>>>>> In other words, opt refined the semantics of @foo() (i.e.
> > > >>>>>>> reduced the
> >
> > > >>>>>>> set of behaviors it may have) in ways that would make later
> >
> > > >>>>>>> optimizations invalid if we de-refine the implementation of
> > > >>>>>>> @foo().
> >
> > > >>>>>> I'm probably missing something obvious here. How could the
> > > >>>>>> result of
> >
> > > >>>>>> `%t0 != %t1` be different at optimization time in one file
> > > >>>>>> than from
> >
> > > >>>>>> runtime in the "real" implementation? Doesn't this make the
> > > >>>>>> CSE
> >
> > > >>>>>> invalid?
> >
> > > >>>>> `%t0` and `%t1` are "allowed" to "always be the same", i.e.
> > > >>>>> an
> >
> > > >>>>> implementation of @foo that always feeds in the same
> >
> > > >>>>> value for `%t0` and `%t1` is a valid implementation (which is
> > > >>>>> why the
> >
> > > >>>>> CSE was valid); but it is not the *only* valid
> > > >>>>> implementation.
> > > >>>>> If I
> >
> > > >>>>> don't CSE the two load instructions (also a valid thing to
> > > >>>>> do),
> > > >>>>> and
> >
> > > >>>>> this is a second thread writing to `%par`, then the two
> > > >>>>> values
> > > >>>>> loaded
> >
> > > >>>>> can be different, and you could end up printing `"X"` in
> > > >>>>> `@foo`.
> >
> > > >>>>>
> >
> > > >>>>> Did that make sense?
> >
> > > >>>> Yes. To be sure I understand the scope: this is only a problem
> > > >>>> for
> >
> > > >>>> atomics, correct? (Because multi-threaded behaviour with other
> > > >>>> globals
> >
> > > >>>> is UB?)
> >
> > > >>>>
> >
> > > >>>>>> Does linkonce_odr linkage have the same problem?
> >
> > > >>>>>> - If so, do you want to change it too?
> >
> > > >>>>>> - Else, why not?
> >
> > > >>>>> Going by the specification in the LangRef, I'd say it depends
> > > >>>>> on how
> >
> > > >>>>> you define "definitive". If you're allowed to replace the
> > > >>>>> body
> > > >>>>> of a
> >
> > > >>>>> function with a differently optimized body, then the above
> > > >>>>> problem
> >
> > > >>>>> exists.
> >
> > > >>>>>
> >
> > > >>>>> I believe that is the case, and I strongly believe the
> > > >>>>> problem
> > > >>>>> you
> >
> > > >>>>> outline exists for linkonce_odr exactly as it does for
> > > >>>>> available_externally.
> >
> > > >>>>>
> >
> > > >>>>> Which is what makes this scary: every C++ inline function
> > > >>>>> today
> > > >>>>> can
> >
> > > >>>>> trigger this.
> >
> > > >>>> Every C/C++ inline or template function. But only the ones
> > > >>>> that
> > > >>>> use
> >
> > > >>>> atomics, right?
> >
> > > >>>
> >
> > > >>> Well, with *this* example...
> >
> > > >> Atomic are one source of non-determinism that compilers can
> > > >> reason
> >
> > > >> about. I don't know if the following snippet is well defined or
> > > >> not,
> >
> > > >> but you could have similar issues with
> >
> > > >>
> >
> > > >>
> >
> > > >> void foo() {
> >
> > > >> int *p = malloc(sizeof(int));
> >
> > > >> if (*p < 10) print("X");
> >
> > > >> }
> >
> > > >>
> >
> > > >> or (again, I don't know if this is actually well defined)
> >
> > > >>
> >
> > > >> void foo() {
> >
> > > >> int t; // it is probably reasonable to fold compares with
> >
> > > >> ptrtoint(alloca) to undef
> >
> > > >> if ((intptr_t)(&t) < 10) print("X");
> >
> > > >> }
> >
> > > >>
> >
> > > > The first one at least is UB, but as Richard pointed out the
> > > > scope
> >
> > > > is certainly broader than atomics (it's not even just
> > > > well-defined
> >
> > > > non-deterministism).
> >
> > > >
> >
> > > > I'm kind of terrified by the implications.
> >
> > > Me too. :(
> >
> > > >
> >
> > > >> -- Sanjoy
> >
> > > >>
> >
> > > >>>>
> >
> > > >>>> Not that I'm sure that will end up being a helpful
> > > >>>> distinction.
> >
> > > >>>
> >
> > > >>> Right. See Richard's comment. I think that sums up the real
> > > >>> issue
> > > >>> here. =/
> >
> > > > _______________________________________________
> >
> > > > LLVM Developers mailing list
> >
> > > > llvm-dev at lists.llvm.org
> >
> > > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
> > > _______________________________________________
> >
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> >
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> >
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> >
>
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>
> --
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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