Re: [PATCH v4 2/2] linux/bits.h: simplify GENMASK_INPUT_CHECK()

[Vincent Mailhol]

On Mon. 18 Nov. 2024 at 04:45, David Laight <David.Laight@xxxxxxxxxx> wrote:
> From: David Laight
> > Sent: 17 November 2024 17:25
> >
> > From: Vincent Mailhol
> > > Sent: 13 November 2024 17:19
> > >
> > > In GENMASK_INPUT_CHECK(),
> > >
> > >   __builtin_choose_expr(__is_constexpr((l) > (h)), (l) > (h), 0)
> > >
> > > is the exact expansion of:
> > >
> > >   const_true((l) > (h))
> > >
> > > Apply const_true() to simplify GENMASK_INPUT_CHECK().
> >
> > Wouldn't statically_true() give better coverage ?

Yes, it would.

> > I wouldn't have though that GENMASK() got used anywhere where a constant
> > integer expression was needed.

It is used in many places, including some inline functions such as bitmap_set():

  https://elixir.bootlin.com/linux/v6.11/source/include/linux/bitmap.h#L469

where the input is not an integer constant expression (thus preventing
the use of statically_true()).

> If it is, maybe add a GENMASK_CONST() that uses BUILD_BUG_ON_ZERO_MSG()
> (recently proposed) and so validates that the values are constants.
> And then use statically_true() in the normal case to pick up more errors.

The issue if you introduce GENMASK_CONST() is that there is no gain.

The only advantage of const_true(x) is that it works on cases where
statically_true(x) would cause a compilation error. But for
statically_true(x) to cause a compilation error, x has to be a non
constant expression. And if x is non constant, then const_true(x)
returns false.

Regardless, considering the number of times where GENMASK() is used
with integer literals, I do not think it would be worth it to replace
all of these with GENMASK_CONST() tree wide.

Trying to go in your direction, we already have two genmasks:

   - GENMASK(): which uses GENMASK_INPUT_CHECK()

  - __GENMASK(): no check, used in uapi

What would make more sense to me would be to:

  1. replace const_true() by statically_true() in GENMASK_INPUT_CHECK()

  2. replace all the instances where GENMASK() breaks compilation with
     __GENMASK()

But this idea was already proposed in the past and was rejected here:

  https://lore.kernel.org/lkml/YJm5Dpo+RspbAtye@rikard/

> (Or just remove the check because coders really aren't that stupid!)

I think that this check exists in the first place *because* such a
mistake was made in the past.

> The interesting cases are the ones using variables.
> And they'd need run-time checks of some form.

Then, instead of introducing GENMASK_CONST(), maybe introduce
GENMASK_NON_CONST()? But then, the number of instances in which
GENMASK() is used with something other than literal integers is pretty
rare. So probably not worth it.


Yours sincerely,
Vincent Mailhol