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

[Hal Finkel via llvm-dev]

----- 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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