From: Theo Buehler <tb@theobuehler.org>
Subject: Re: Fix rmdir -p /foo/bar
To: Christian Weisgerber <naddy@mips.inka.de>
Cc: tech@openbsd.org
Date: Mon, 12 May 2025 08:24:34 +0200

On Sun, May 11, 2025 at 08:35:37PM +0200, Christian Weisgerber wrote:
> Theo Buehler:
> 
> > > -		while (--p > path && *p == '/')
> > > +		while (--p >= path && *p == '/')
> > >  			continue;
> > 
> > I don't think this approach is kosher. p can now end up pointing before
> > path, which is UB.
> 
> Is it UB?  Those are all pointers, not arrays.

Yes, I'm pretty sure it is. Pointer arithmetic is only defined if the
initial pointer and the result point to the same "array" or the element
immediately past it. A pointer to an object that isn't an array is to be
interpreted as an array of size one.

Ref: C99 6.5.6 items 7 and 8. Same language in C23 6.5.6 itmes 8 and 9.

Whether compilers can actually benefit from this is a different question.

> Diff for the sake of completeness below, but...

That's too bad.