Re: [PATCH] tools/libxc: use uint32_t for pirq in xc_domain_irq_permission

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 07/07/2021 14:21, Julien Grall wrote:
> On 07/07/2021 14:14, Jan Beulich wrote:
> > On 07.07.2021 14:59, Julien Grall wrote:
> > > On 07/07/2021 13:54, Jan Beulich wrote:
> > > > On 07.07.2021 14:51, Julien Grall wrote:
> > > > > On 07/07/2021 02:02, Igor Druzhinin wrote:
> > > > > > Current unit8_t for pirq argument in this interface is too restrictive
> > > > > > causing failures on modern hardware with lots of GSIs. That extends down to
> > > > > > XEN_DOMCTL_irq_permission ABI structure where it needs to be fixed up
> > > > > > as well. Internal Xen structures appear to be fine. Existing users of
> > > > > > the interface in tree (libxl, ocaml and python bindings) are already using
> > > > > > int for pirq representation that should be wide enough.
> > > > > By "int", I am assuming you imply "signed int", is that correct?
Yes, just "int" in the meaning "signed int" - I can clarify that in the 
description.

> > > > > If so, should the function xc_domain_irq_permission() interface take an
> > > > > int in parameter and check it is not negative?
> > > > Please let's not make things worse than they are, the more that
> > > Well, what I am trying to prevent is surprise where the caller
> > > mistakenly pass a negative value that will be interpreted as a positive
> > > value...
> > This happens all the time when converting from signed to unsigned
> > perhaps just internally.
> I am not sure what's your point... Yes there are place in Xen that switch 
> between signed and unsigned. We likely have some (latent) problem because of 
> that...
Callers of libxc interface shouldn't have been using signed int at all.
They just happen to do it at least in-tree - that's what I found and mentioned
in the description. At the same time "int" type is for now wide enough so there
is no immediate rush to fix them up.

That gets a little bit tricky with bindings - they themselves expose pirq
as int. So a negative value could be passed by the caller and, given other
similar interace functions like xc_physdev_map_pirq() are using "int pirq"
to signal an error as negative value, that could be misinterpreted by lower
levels.

We can add extra checks in bindings to avoid passing all negative values to
libxc level. Would this be good enough?

> > > Such issues are beyong annoying to debug...
> > No worse than any other out-of-bounds value, I would say.
> >
> > >   > ./CODING_STYLE is unambiguous in cases like this one.
> > >
> > > Hmmm... The coding style mention the fixed size but nothing about the
> > > signedness of the type...
> > Oh, sorry, yes. The adjustment for this even pre-dates the two
> > patches to ./CODING_STYLE that I've on record as pending for
> > nearly two years.
> >
> > > The alternative suggestion is to keep a unsigned type but check the bit
> > > 31 is not set.
> > Why? Why not bit 30 or bit 27? There's nothing special about
> > bit 31 in an unsigned number.
> Bit 31 is the signed bit for signed number. The check would make sure that:
>   1) The value will fit other hypercall (the PIRQ is described as int in a few 
> of the structure)
>   2) Catch potentially caller that would use the number that could potentially 
> be interpreted as negative by other part of the hypervisor.
>
> That said, I can live with the implicit signed -> unsigned convertion, however 
> the commit message should at least be clarified because it is misleading.
Could you specify which statement exactly is misleading (or needs clariying)
in the commit message?

Igor