[cfe-dev] Converting pointers to fat pointers

[John Criswell]

On 5/14/12 8:11 PM, John McCall wrote:
> On May 14, 2012, at 5:59 PM, Sai Charan wrote:
>> I am looking at using LLVM/Clang to automatically convert pointer 
>> declarations to fat pointers & the corresponding dereferences to 
>> something appropriate. I am looking for guidance on doing this. Will 
>> an LLVM pass be better suited to this or would this be better handled 
>> using Clang. Any guidance on getting started would be helpful.
>
> It would be best handled by modifying Clang, both in semantic analysis 
> (to change the size of a pointer) and IR generation (to generate, 
> propagate, and consume your fat pointer values).  I'm afraid that 
> clang's IR generation widely assumes that pointers are represented as 
> a single llvm::Value, though, and you might be in for a lot of work.

Converting to fat pointers can also be done at the LLVM IR level and, in 
fact, there's a modern implementation of fat pointers at the LLVM IR 
level in the SAFECode project (http://sva.cs.illinois.edu).  The 
implementation is SoftBound from University of Pennsylvania, and it 
implements what is essentially a fat pointer approach that does not 
modify data structure layout.  You can read about SoftBound at 
http://www.cis.upenn.edu/acg/papers/pldi09_softbound.pdf.

One of the problems with implementing fat pointers within clang is that 
clang does not have the entire program, and so you cannot use whole 
program analysis to determine if parts of the program are aware of the 
data structure layout.  An LLVM IR analysis that is part of the 
link-time optimization framework can, and so a transform at the LLVM IR 
level could determine when it is safe to modify a data structure layout 
and when it is not.

All that said, if you're using a fat pointer method that doesn't modify 
data structure layout (SoftBound has this feature; Xu et. al.'s work at 
http://seclab.cs.sunysb.edu/seclab/pubs/fse04.pdf doesn't either, IIRC), 
implementing it in Clang would also work.

As an FYI, I'm advocating for a common infrastructure in LLVM for adding 
and optimizing memory safety run-time checks; the idea is to have common 
infrastructure that will work both for fat pointer approaches, object 
metadata approaches, and other approaches.  You can find my proposal at 
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20120507/142532.html.  
I'd welcome any feedback or comments you may have on it.

-- John T.

>
> John.
>
>
>
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