Re: [PATCH for-4.16] freebsd/privcmd: fix MMAP_RESOURCE ioctl definition

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Nov 15, 2021 at 12:03:26PM +0000, Andrew Cooper wrote:
> On 15/11/2021 11:08, Roger Pau Monne wrote:
> > Current ioctl definition was wrong in both FreeBSD and Xen sources, as
> > the MMAP_RESOURCE ioctl needs to copy back the size of the resource
> > when passed a zero address and size. FreeBSD encodes in the definition
> > of the ioctl number whether parameters should be copied in (W) and/or
> > copied out (R). The current definition for MMAP_RESOURCE is lacking
> > the copy out part (R), and thus the call to query the size of a
> > resource would always return 0.
> >
> > This change will break the current ioctl interface, the tools can
> > however fall back to using the foreign memory interface in order to
> > map resources from guests.
> >
> > This was a shortcoming from when the hypercall and ioctl gained the
> > ability to query the size of the resources, as originally the
> > MMAP_RESOURCE ioctl didn't need to copy out any data.
> >
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> > Cc: Ian Jackson <iwj@xxxxxxxxxxxxxx>
> >
> > The change only affects FreeBSD, and it's only a change in a
> > definition of an ioctl, so it's unlikely to break existing code logic.
> > Without this change Xen tools won't be able to use the MMAP_RESOURCE
> > ioctl.
> 
> I guess you found this while trying to fix test-resource, in which case
> a further argument for the change is "the unit tests now pass on FreeBSD" ?

Indeed. It seems like this is the only instance of a resource size
query that we have implemented so far.

Thanks, Roger.