[llvm-dev] RFC: Add an "interposible" linkage type (and implement -fsemantic-interposition)

Wed Nov 30 20:23:22 PST 2016

> On Nov 30, 2016, at 11:28 AM, James Y Knight via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> You present a good case for not using "protected" visibility in ELF, despite it being exactly what it is supposed to mean.
> 
> From what http://www.airs.com/blog/archives/307 <http://www.airs.com/blog/archives/307> says, it sounds like the correctness issue with protected visibility is because LLVM is doing it wrong -- not an intrinsic property of protected visibility in ELF, or even ELF/x86. The blog says that the compiler should be referencing through the GOT for the addressof operation on a function within the same shared library, thus keeping the address consistent within, and without. But it's not. We should at least fix that, I think?
> 
> But, the more serious problem to me seems to be the issue with the dynamic link loader performing horribly. That performance penalty certainly means that defaulting to protected visibility would be a very bad plan for normal use-cases. So, yes, I concur that we need something new to indicate that our "default" is effectively "we'll treat this as if it were protected, but we'll keep it a secret from the linker, ssssh...", and also allow for the "usual" ELF default visibility behavior.
> 
> You don't mention in your proposal anything about changing the existing behavior when the current-default -fno-semantic-interposition is in effect. But, I think we do need to change it. LLVM ought to be consistent with itself -- that part of my original argument remains just as valid. That is: if we're going to assume attributes of functions with visible definitions, based on their definitions, we should also be generating a direct call to the function symbol, and not go through the PLT. 
> 
> Because we can't use protected visibility, we are unable to tell the linker to make the same optimization when linking multiple objects, which is sad, but oh well...
> 
> Although, actually, now I'm wondering, shouldn't we also be able to mark a declaration as definitely in the shared library? Something like:
> declare protected_but_dont_tell_the_linker i32 @f()

I usually do this by marking the declaration hidden (protected should be enough, but we don’t have this on MachO).

We haven’t hooked everything in LTO yet, but the idea is to add this information on declaration automatically (the new API has a flag for the linker to tell us when the symbol is defined in the current DSO).

— 
Mehdi

> to indicate to LLVM that @f must be defined by another object inside the same shared library -- but not in this file -- and that calls should be made directly to the symbol, not through the PLT.
> 
> That is, we have:
> - The default for llvm functions is that if the definition is visible, it is assumed to be the one that will be used (not interposable), and optimizations can assume properties of the function, and it can be called directly without going through the PLT. (But addressof still needs to be careful).
> - If the definition is NOT visible, the default is to assume the symbol MIGHT be external to the shared object, and needs to go through PLT.
> 
> Hal's proposal adds the "interposable" linkage type, which makes sense on definitions only (not declarations), and indicates that despite seeing a definition, you can't assume the local version will be used. We should also have a way to do the opposite for declarations, somehow.
> 
> ...(after reading more of the thread)....oh looky, that's EXACTLY what rafael's said in his patch in D20217, too. :)
> 
> On Tue, Nov 29, 2016 at 11:01 AM, Hal Finkel <hfinkel at anl.gov <mailto:hfinkel at anl.gov>> wrote:
> Hi everyone,
> 
> Clang/LLVM's support for ELF interposition is in a confusing state, and I propose making a few (hopefully simple) adjustments in order to bring our model into a self-consistent state.
> 
> The problem: On ELF systems, global symbols can be interposed. This means, for example, that calls to global functions in some (shared) library defined in that same library might end up being redirected to an implementation in some other library (or in the main executable). The most common reason for this is the use of LD_PRELOAD, but there are plenty of other ways to trigger interposition as well. As a result, it is technically inconsistent to inline any global function or do inter-procedural analysis on them because the implementation might be replaced by code with completely different behavior at runtme (or link time). Clang has never supported this (i.e. we do treat these functions as being eligible for inlining and perform IPA on them). GCC, on the other hand, has traditionally respected the possibility of ELF interposition and refrained from doing these things (at least when compiling with -fPIC).
> 
> I believe that Clang/LLVM's current behavior is the most-useful behavior and we should keep the current behavior (at least as a default). I do understand, however, that there are valid use cases for ELF interposition and places where we should allow it (e.g. when compiling certain system libraries). GCC recently added a flag -fsemantic-interposition/-fno-semantic-interposition, where using -fno-semantic-interposition provides Clang/LLVM's behavior of assuming that ELF interposition will not be used.
> 
> It has been suggested that, to be self consistent, LLVM should emit global symbols with protected ELF visibility in cases where we've assumed that ELF interposition won't happen. ELF protected visibility does seem to have exactly that meaning: A protected global symbol is externally visible but cannot be interposed. Unfortunately, as I understand it, on some major platforms (e.g. x86), protected-visibility symbols have a major flaw: Non-uniqueness of function pointers (i.e. the function pointer obtained to a function outside of the defining library might be different from the pointer obtained within the defining library). As a result, making this change might be practically prohibited (even if it makes sense in theory).
> 
> Proposal:
> 
>  1. Add a new linkage type, interposible, which is like external except that isInterposableLinkage will return true (thus preventing inlining, IPA, etc.). This is similar to weak linkage, in a sense, except that such symbols are never discarded and are not marked as weak for linking, etc.
> 
>  2. Add -fsemantic-interposition/-fno-semantic-interposition to Clang. Default to -fno-semantic-interposition, but when -fsemantic-interposition is used, use interposible linkage for all functions where external linkage might otherwise have been used.
> 
> Thoughts?
> 
> Some useful links:
> http://hubicka.blogspot.com/2015/04/GCC5-IPA-LTO-news.html <http://hubicka.blogspot.com/2015/04/GCC5-IPA-LTO-news.html> (the section on the -fno-semantic-interposition flag)
> https://gcc.gnu.org/ml/gcc-patches/2014-05/msg01671.html <https://gcc.gnu.org/ml/gcc-patches/2014-05/msg01671.html>
> 
> On some issues with ELF protected-visibility symbols:
> http://www.macieira.org/blog/2012/01/sorry-state-of-dynamic-libraries-on-linux/ <http://www.macieira.org/blog/2012/01/sorry-state-of-dynamic-libraries-on-linux/>
> http://www.airs.com/blog/archives/307 <http://www.airs.com/blog/archives/307>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19520 <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19520>
> 
> Thanks again,
> Hal
> 
> P.S. For some previous discussion on this, see below...
> 
> From: "Hal Finkel via llvm-dev" <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> To: "James Y Knight" <jyknight at google.com <mailto:jyknight at google.com>>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> Sent: Monday, February 29, 2016 9:50:15 AM
> Subject: Re: [llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")
> 
> 
> From: "James Y Knight" <jyknight at google.com <mailto:jyknight at google.com>>
> To: "Hal Finkel" <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>
> Cc: "Sanjoy Das" <sanjoy at playingwithpointers.com <mailto:sanjoy at playingwithpointers.com>>, "llvm-dev" <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> Sent: Monday, February 29, 2016 9:31:24 AM
> Subject: Re: [llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")
> 
> On Feb 26, 2016 8:50 PM, "Hal Finkel" <hfinkel at anl.gov <mailto:hfinkel at anl.gov>> wrote:
> 
> From: "James Y Knight via llvm-dev" <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> To: "Sanjoy Das" <sanjoy at playingwithpointers.com <mailto:sanjoy at playingwithpointers.com>>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> Sent: Thursday, February 25, 2016 1:41:43 PM
> Subject: Re: [llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")
> 
> While we're talking about this, I'd just mention again that the same issue arises for *normal* functions too, when linked into a shared library:
>    int foo() { return 1; }
>    int bar() { return foo(); }
> 
> Now, compare:
>   clang -fPIC -O1 -S -o - test.c
>   gcc -fPIC -O1 -S -o - test.c
> 
> GCC will refuse to inline foo into bar, or use any information about foo in compiling bar, because foo is exported in the dynamic symbol table, and thus replaceable via symbol interposition.
> 
> Clang assumes that you won't do that, or that you don't care what happens if you do. It will happily inline. And, in absense of inlining (e.g. if foo is too long to inline), clang will deduce function attributes about foo and rely on those in bar -- despite that the call goes through the PLT and could in fact be an entirely different unrelated implementation (or, for that matter, a differently-optimized version of the same implementation).
> 
> Is that *really* okay?
> 
> I'm comfortable with saying that symbol interposition falls outside of the model we have for the targeted system (at least by default), and thus, this is okay. We also don't model the possibility of someone hex-editing the binary ;)
> 
> I'm not really okay with it; the current behavior feels unprincipled.
> 
> We have a visibility attribute which can be used to control this: On ELF systems, "default" visibililty allows interposition (unlike on Darwin) -- that is, it explicitly ALLOWS for replacing the symbol's definition. The policy of "You can't replace the definition of the symbol, but it is globally visible" is exactly what the "protected" visibility mode is for.
> 
> If we want to say that you can't interpose by default on ELF targets, that would be a choice. Then, we should make the default symbol visibility "protected" instead of "default". But, continuing to generate calls through the PLT -- which is only needed because the symbols might be replaced -- while simultaneously making optimizations that are broken if they actually ARE replaced, seems kinda bogus.
> 
> This makes sense, and I think you understand my concern here: Most programmers don't understand these issues, nor do they ever expect to use dynamic interposition. They do expect, however, that the compiler has good IPA and will use the information it is provided effectively. I'd be happy to make the default visibility protected, allowing us to continue optimizing well, and provide a principled behavior otherwise. Given, as you point out, this is the default on Darwin, is there experience from Darwin porting, or any other factors, that would indicate this would be a hardship?
> 
> Thanks again,
> Hal
> 
> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 
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> 
> 
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
> 
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