operator new[] overflow (PR 19351)

Gabriel Dos Reis gdr@integrable-solutions.net
Sat Dec 4 16:09:00 GMT 2010


On Sat, Dec 4, 2010 at 7:22 AM, Florian Weimer <fw@deneb.enyo.de> wrote:
> * Joe Buck:
>
>> It's wasted code if the multiply instruction detects the overflow.
>> It's true that the cost is small (maybe just one extra instruction
>> and the same number of tests, maybe one more on architectures where you
>> have to load a large constant), but it is slightly worse code than what
>> Chris Lattner showed.
>
> It's possible to improve slightly on the LLVM code by using the
> overflow flag (at least on i386/amd64), as explained in this blog
> post:
>
> <http://blogs.msdn.com/b/michael_howard/archive/2005/12/06/500629.aspx>
>
> My patch emits a run-time division if a VLA is used in an allocator.
> But that's a semi-deprecated GCC extension, so I don't think we need
> to care.

Personally, the VLA issue is not one I would care much about.
If it can be done without much cost, fine.  Otherwise, I would
not tie the checking of the standard construct to it.

>
>> Still, it's certainly an improvement on the current
>> situation and the cost is negligible compared to the call to the
>> allocator.  Since it's a security issue, some form of the patch should
>> go in.
>
> Well, should I resubmit, with the fix for the problem building
> size_t(-1)?

I think that would help.

-- Gaby



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