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

Tue Nov 29 08:01:16 PST 2016

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 (the section on the -fno-semantic-interposition flag) 
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.airs.com/blog/archives/307 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19520 

Thanks again, 
Hal 

P.S. For some previous discussion on this, see below... 

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

> From: "Hal Finkel via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "James Y Knight" <jyknight at google.com>
> Cc: "llvm-dev" <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")

> ----- Original Message -----

> > From: "James Y Knight" <jyknight at google.com>
> 
> > To: "Hal Finkel" <hfinkel at anl.gov>
> 
> > Cc: "Sanjoy Das" <sanjoy at playingwithpointers.com>, "llvm-dev"
> > <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 > wrote:
> 

> > > > From: "James Y Knight via llvm-dev" < llvm-dev at lists.llvm.org >
> > > 
> > 
> 
> > > > To: "Sanjoy Das" < sanjoy at playingwithpointers.com >
> > > 
> > 
> 
> > > > Cc: "llvm-dev" < 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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