Re: [PATCH][ubsan] Add VLA bound instrumentation

[Marek Polacek <polacek@redhat.com>]

On Thu, Sep 12, 2013 at 04:05:48PM +0000, Joseph S. Myers wrote:
> On Thu, 12 Sep 2013, Joseph S. Myers wrote:
> 
> > (Actually, I believe sizes (in bytes) greater than target PTRDIFF_MAX, not 
> > just SIZE_MAX, should be caught, because pointer subtraction cannot work 
> > reliably with larger objects.  So it's not just when the size or 
> > multiplication overflow size_t, but when they overflow ptrdiff_t.)
> 
> And, to add a bit more to the list of possible ubsan features (is this 
> todo list maintained anywhere?), even if the size is such that operations 
> on the array are in principle defined, it's possible that adjusting the 
> stack pointer by too much may take it into other areas of memory and so 
> cause stack overflow that doesn't get detected by the kernel.  So maybe 
> ubsan should imply -fstack-check or similar.
> 
> Everything about VLA checking - checks on the size being positive and on 
> it not being larger than PTRDIFF_MAX, and on avoiding stack overflow - 
> applies equally to alloca: calls to alloca should also be instrumented.  
> (But I think alloca (0) is valid.)

Problem here is that libubsan doesn't contain appropriate routines for
this VLA/alloca extended checking, it really can only issue "variable
length array bound evaluates to non-positive value", nothing else.

So perhaps reach out to some clang mailing list and try to implement
it first in the libubsan...

	Marek