Re: [PATCH 2/3] x86: tss: Eliminate fragile calculation of TSS segment limit

[Josh Triplett <josh@joshtriplett.org>]

On Wed, Oct 30, 2013 at 03:53:11PM -0700, H. Peter Anvin wrote:
> On 10/30/2013 03:22 PM, Kees Cook wrote:
> >>
> >> -       /*
> >> -        * sizeof(unsigned long) coming from an extra "long" at the end
> >> -        * of the iobitmap. See tss_struct definition in processor.h
> >> -        *
> >> -        * -1? seg base+limit should be pointing to the address of the
> >> -        * last valid byte
> > 
> > I think it might be better to keep at least a minimal comment near the
> > TSS_LIMIT declaration, just to explain the "-1" part, which is not
> > entirely obvious from just reading the code.
> > 
> 
> Agreed, although it doesn't need to be an unsigned long at all... the
> CPU will only ever access one extra byte past the end.

True, but the thing immediately following the iobitmap is a stack, which
needs aligning, so the array does need to contain a full additional
unsigned long even if the CPU only accesses a byte of it.  In any case,
that isn't the reason for the -1, just the reason for the
sizeof(unsigned long) mentioned in the comment above, which goes away
now that TSS_LIMIT uses the offset of the *following* field rather than
recalculating the size of the iobitmap.

- Josh Triplett