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

["Michael S. Tsirkin" <mst@redhat.com>]

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.


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