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

[Kees Cook]

On Tue, Mar 31, 2026 at 10:09:33AM -0700, Justin Stitt wrote:
> Hi,
> 
> On Tue, Mar 31, 2026 at 10:02 AM Miguel Ojeda
> <miguel.ojeda.sandonis@xxxxxxxxx> wrote:
> >
> > On Tue, Mar 31, 2026 at 6:37 PM Kees Cook <kees@xxxxxxxxxx> wrote:
> > >
> > > +/*
> > > + * Optional: only supported by Clang with -Xclang -experimental-foverflow-behavior-types
> > > + * passed via CONFIG_OVERFLOW_BEHAVIOR_TYPES. When not available, define empty macros for
> > > + * the trap/wrap annotations.
> > > + *
> > > + * clang: https://clang.llvm.org/docs/OverflowBehaviorTypes.html
> > > + */
> > > +#if !__has_attribute(overflow_behavior) || !defined(OVERFLOW_BEHAVIOR_TYPES)
> > > +# define __ob_trap
> > > +# define __ob_wrap
> > > +#endif
> >
> > Should that have `CONFIG_*`? i.e.
> >
> >      !defined(CONFIG_OVERFLOW_BEHAVIOR_TYPES)
> >
> > In addition, since this depends on a `CONFIG_`, with the current setup
> > we would put them elsewhere instead of `compiler_attributes.h` until
> > they are promoted to be "unconditional" (i.e. without the compiler
> > flag):
> >
> >      * Any other "attributes" (i.e. those that depend on a configuration option,
> >      * on a compiler, on an architecture, on plugins, on other attributes...)
> >      * should be defined elsewhere (e.g. compiler_types.h or compiler-*.h).
> >      * The intention is to keep this file as simple as possible, as well as
> >      * compiler- and version-agnostic (e.g. avoiding GCC_VERSION checks).
> >
> > However, thinking about it, why is the config needed?
> >
> > i.e. if the compiler is not passed that flag, shouldn't the
> > `__has_attribute` simply return false?
> >
> > Also, I am a bit confused -- does the compiler flag automatically
> > recognize the names like `__ob_trap`? i.e. I see the docs mention
> > using the attribute,
> >
> >     typedef unsigned int __attribute__((overflow_behavior(trap))) safe_uint;
> >     typedef unsigned int __attribute__((overflow_behavior(wrap))) wrapping_uint;
> >
> > But then we don't actually use it?
> 
> __ob_trap and __ob_wrap are defined by the compiler.
> 
> There are some examples within the documentation additions of this patch.
> 
> Kees, is it possible to make it more clear about what we expect of
> kernel developers in terms of style? Should they use keyword
> spellings? attribute spellings? only use custom types?

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).

-- 
Kees Cook