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

On Thu, 12 Sep 2013, Marek Polacek wrote:

> This patch adds the instrumentation of VLA bounds.  Basically, it just 
> checks that the size of a VLA is positive.  I.e., We also issue an error 
> if the size of the VLA is 0.  It catches e.g.

This is not an objection to this patch, but there are a few other bits of 
VLA bound instrumentation that could be done as well.  If the size has a 
wide-enough type to be bigger than the target's SIZE_MAX, and is indeed 
bigger than SIZE_MAX, that could be detected at runtime as well.  Or if 
the multiplication of array size and element size exceeds SIZE_MAX (this 
covers both elements of constant size, and elements that are themselves 
VLAs, but the former can be handled more efficiently by comparing against 
an appropriate constant rather than needing a runtime test for whether a 
multiplication in size_t overflows).

(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.)

Joseph S. Myers

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