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

[Hal Finkel via llvm-dev]

----- Original Message ----- 

> From: "Chandler Carruth" <chandlerc at google.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>, "Philip Reames"
> <listmail at philipreames.com>, "Duncan P. N. Exon Smith"
> <dexonsmith at apple.com>, "Sanjoy Das"
> <sanjoy at playingwithpointers.com>
> Sent: Wednesday, February 24, 2016 11:41:59 PM
> Subject: Re: [llvm-dev] Possible soundness issue with
> available_externally (split from "RFC: Add guard intrinsics")

> 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.

Sure; which is way I said that we should not perform those optimizations (instead of saying that we just need to make sure that the same choice will be made everywhere - as you say, LTO aside, we can't do that).

> 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?

But it is not all optimizations that are the problem. Rather, it seems like a select few (e.g. things involving collapsing allowed non-determinism in atomics), and losing those optimizations seems better than generally losing function-attribute deduction.

 -Hal

> > 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
> 
> > >
> 

> > > > _______________________________________________
> 
> > >
> 
> > > > LLVM Developers mailing list
> 
> > >
> 
> > > > llvm-dev at lists.llvm.org
> 
> > >
> 
> > > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> > >
> 

> > > _______________________________________________
> 
> > > LLVM Developers mailing list
> 
> > > llvm-dev at lists.llvm.org
> 
> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 

> > --
> 

> > --
> 
> > Hal Finkel
> 
> > Assistant Computational Scientist
> 
> > Leadership Computing Facility
> 
> > Argonne National Laboratory
> 

-- 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory