Re: [Ksummit-discuss] [CORE TOPIC] More useful types in the linux kernel

[Julia Lawall <julia.lawall@lip6.fr>]

On Fri, 12 Aug 2016, Michael S. Tsirkin wrote:

> On Fri, Aug 12, 2016 at 08:29:20AM +0300, Alexey Dobriyan wrote:
> > On Fri, Aug 12, 2016 at 12:07:11AM -0400, Matthew Wilcox wrote:
> > > On Aug 11, 2016 9:02 PM, "Josh Triplett" <josh@joshtriplett.org> wrote:
> > > > On Thu, Aug 11, 2016 at 11:51:52PM -0400, Matthew Wilcox wrote:
> > > > > Can we introduce types for this? We have a number of different return
> > > type
> > > > > conventions in the kernel:
> > > > >
> > > > > bool
> > > > > errno_t (-4095 to 0 are valid)
> > > > > count_t (-4095 to INT_MAX)
> > > > > long_count_t (-4095 to LONG_MAX)
> > > > > ulong_count_t (-4095 to -4096)
> > > > > struct foo _err*
> > > > >
> > > > > I think this is good programmer documentation in addition to being
> > > > > potentially useful to smatch.
> > > >
> > > > I'd love to see an explicit type distinct from "int" for "potentially an
> > > > errno".  And if any code uses "potentially an errno *or* a non-errno
> > > > non-zero return value", that should ideally use a distinct type as well.
> > >
> > > I think the biggest problem is coming up with good names for the types. And
> > > the churn of introducing them, particularly converting function pointers
> > > and all occurrences.
> >
> > Names are easy part (errno_t is perfect actually). The problem is that
> > once error is cleared, variable doesn't change to regular type anymore:
> >
> > 	errno_t rv;
> >
> > 	rv = f();
> > 	if (rv < 0)
> > 		return rv;
> > 	int rv = rv;
> >
> > which agains boils down to a language with real type system.
>
> We could maybe do
>
>  	errno_t rv;
>
>  	rv = f();
>  	if (IS_ERR(rv))
>  		return rv;
>  	int r = CHECKED(rv);
>
>
> Tools could maybe verify that all paths to CHECKED
> are actually going through an IS_ERR test as well.

What does the return value of f look like?
Was it intentional to use IS_ERR, which checks for a pointer?  At least in
that case gcc would ensure that CHECKED is used.

julia