KASSERT(val != 0) not triggering in linux_errno.c reading outside of array

[Konstantin Belousov]

On Tue, Nov 03, 2020 at 01:17:51PM +0300, xtouqh at hotmail.com wrote:
> Hans Petter Selasky wrote:
> > Hi,
> > 
> > Should:
> > 
> >          for (i = 1; i < sizeof(linux_errtbl); i++) {
> > 
> > Be:
> > 
> >          for (i = 1; i < sizeof(linux_errtbl)/sizeof(linux_errtbl[0]);
> > i++) {
> > 
> > 
> > Or:
> > 
> >          for (i = 1; i < (int)nitems(linux_errtbl); i++) {
> 
> That's right, and I added the same comment in the review.  My question is
> different though -- with the issue present, KASSERT() should have triggered
> (there are 0 values with incorrect indexes, added printf() confirms that)
> exposing the bug, but it does not -- WHY? -- I just want to understand
> what's going on.

I think this is a poster child for the current undefined behaviour treatment
by compilers.  You are accessing beyond array last element, and compiler can
prove it, so it allowed to do anything.