Re: [PATCH 3/5] compiler_attributes: Add overflow_behavior macros __ob_trap and __ob_wrap

[Kees Cook <kees@kernel.org>]

On Wed, Apr 01, 2026 at 10:30:53PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 01, 2026 at 01:21:17PM -0700, Kees Cook wrote:
> > On Wed, Apr 01, 2026 at 11:08:15AM +0200, Peter Zijlstra wrote:
> > > On Tue, Mar 31, 2026 at 12:52:10PM -0700, Kees Cook wrote:
> > > 
> > > > I think for this series, __ob_trap/__ob_wrap is what should be used.
> > > > 
> > > > And for other folks, the background here is that we originally wanted
> > > > to use macros for "__trap" and "__wrap", but the powerpc C compiler
> > > > (both Clang and GCC) have a builtin macro named "__trap" already. So
> > > > I switched to just using the Clang-native type qualifier. We can use
> > > > the attribute style too, but there was a lot of confusion during the
> > > > Clang development phases where people kept forgetting this was a type
> > > > qualifier, not an attribute (i.e. the attribute is an internal alias
> > > > for the qualifier, and the qualifier is a new type).
> > > 
> > > Since you mention qualifiers...
> > > 
> > > What is the result of __typeof_unqual__(int __ob_trap) ?
> > 
> > Hmm, it seems like "const" doesn't get peeled off. That can be fixed, if
> > that's needed?
> > 
> > 'typeof_unqual(int)' (aka 'int')
> > 'typeof_unqual(__ob_trap int)' (aka '__ob_trap int')
> > 'typeof_unqual(const int)' (aka 'int')
> > 'typeof_unqual(__ob_trap const int)' (aka '__ob_trap const int')
> 
> So how can something be called a qualifier if unqual doesn't strip it?
> 
> (We might already have had this discussion, but I can't find the answer
> in the LLVM documentation page and didn't search our previous
> correspondence on this).

I'll let Justin answer this correctly, but I suspect I'm using the wrong
name for things. I think I should have said "keyword"? The mechanism
produces a distinct type, so it's not actually a "qualifier", but the
intent is that they are distinct types (this came from a long long
discussion with the LLVM devs, etc), but that they have implicit casts
to the non-obt scalars, but Justin knows more on this.

-- 
Kees Cook