[LLVMdev] alloc_size metadata

[Hal Finkel]

On Fri, 25 May 2012 17:43:52 +0200
Duncan Sands <baldrick at free.fr> wrote:

> Hi John,
> 
> On 25/05/12 17:22, John Criswell wrote:
> > On 5/25/12 2:16 AM, Duncan Sands wrote:
> >> Hi John,
> >>
> >>>>> I'm implementing the alloc_size function attribute in clang.
> >>>> does anyone actually use this attribute? And if they do, can it
> >>>> really buy them anything? How about "implementing" it by
> >>>> ignoring it!
> >>>
> >> ...
> >>>
> >>> Currently, SAFECode has a pass which just recognizes certain
> >>> functions as allocators and knows how to interpret the arguments
> >>> to find the size. If we want SAFECode to work with another
> >>> allocator (like a program's custom allocator, the Objective-C
> >>> allocator, the Boehm garbage collector, etc), then that pass
> >>> needs to be modified to recognize it. Having to update this pass
> >>> for every allocator name and type is one of the few reasons why
> >>> SAFECode only works with C/C++ and not just any old language that
> >>> is compiled down to LLVM IR.
> >>
> >>
> >>> Nuno's proposed feature would allow programmers to communicate
> >>> the relevant information about allocators to tools like SAFECode
> >>> and ASan. I think it might also make some of the optimizations in
> >>> LLVM that require knowing about allocators work on non-C/C++ code.
> >>
> >> these are good points. The attribute and proposed implementation
> >> feel pretty clunky though, which is my main gripe.
> >
> > Hrm. I haven't formed an opinion on what the attributes should look
> > like. I think supporting the ones established by GCC would be
> > important for compatibility, and on the surface, they look
> > reasonable. Devising better ones for Clang is fine with me. What
> > about them feels klunky?
> 
> basically it feels like "I only know about C, here's something that
> pretends to be general but only handles C".  Consider a language with
> a string type that contains the string length as well as the
> characters.  It has a library function allocate_string(length).  How
> much does it allocate?  length+4 bytes. That can't be represented by
> alloc_size.  What's more, it may well store the length at the start,
> and return a pointer to just after the length: a pointer to the first
> character.  alloc_size can't represent "the allocated memory starts 4
> bytes before the return value" either.  In short, it feels like a
> hack for handling something that turns up in some particular C code
> that someone has, rather than a general solution to the general
> problem.

I think this is a good point, here's a suggestion:

Have the metadata name two functions, both assumed to have the same
signature as the tagged function, one which returns the offset of the
start of the allocated region and one which returns the length of the
allocated region. Alternatively, these functions could take the same
signature and additionally the returned pointer of the tagged
function, and then one function can return the start of the region and
the other the length.

For static analysis, we can attempt to inline these functions and then
use SCEV (dead code elimination will then get rid of the unused
results). For runtime checks, calls (which may also be inlined) can be
easily constructed.

> 
> >> Since LLVM already has utility functions for recognizing
> >> allocators (i.e. that know about malloc, realloc and -fno-builtin
> >> etc) can't SAFECode just make use of them?
> >
> > It probably could. It doesn't simply because SAFECode was written
> > before these features existed within LLVM.
> > :)
> >
> >> Then either (1) something like alloc_size is implemented, the LLVM
> >> utility learns about it, and SAFECode benefits automagically, or
> >> (2) the LLVM utility is taught about other allocators like Ada's,
> >> and SAFECode benefits automagically.
> >
> > I'm not sure what you mean by "LLVM utility," but I think we're
> > thinking along the same lines. Clang/LLVM implement the alloc_size
> > attributes, we change SAFECode to recognize it, and so when people
> > use it, SAFECode benefits automagically.
> >
> > Am I right that we're thinking the same thing, or did I completely
> > misunderstand you?
> 
> no, I'm thinking that SAFECode won't need to look at or worry about
> the attribute at all, because the LLVM methods will know about it and
> serve up the appropriate info.  Take a look at
> Analysis/MemoryBuiltins.h.  In spite of the names, things like
> extractMallocCall are dealing with "malloc like" functions, such as
> C++'s "new" as well as malloc.  Similarly for calloc.  So you could
> use those right now to extract "malloc" and "calloc" sizes.  If
> alloc_size is implemented, presumably these would just magically
> start to give you useful sizes for functions annotated with that
> attribute too.

Does the current code even handle calloc? I only see malloc and new.

 -Hal

> 
> Ciao, Duncan.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev



-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory