linux-kernel - Re: [PATCH] virt: Add vboxguest driver for Virtual Box Guest integration

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Date:   Wed, 4 Oct 2017 16:34:25 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Christoph Hellwig <hch@...radead.org>,
        Michael Thayer <michael.thayer@...cle.com>,
        "Knut St . Osmundsen" <knut.osmundsen@...cle.com>,
        Larry Finger <Larry.Finger@...inger.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] virt: Add vboxguest driver for Virtual Box Guest
 integration

On Wed, Oct 04, 2017 at 12:40:54PM +0200, Hans de Goede wrote:
> Hi,
> 
> On 04-10-17 12:30, Greg Kroah-Hartman wrote:
> > On Wed, Oct 04, 2017 at 12:23:41PM +0200, Arnd Bergmann wrote:
> > > On Wed, Oct 4, 2017 at 12:11 PM, Greg Kroah-Hartman
> > > <gregkh@...uxfoundation.org> wrote:
> > > > On Wed, Oct 04, 2017 at 11:32:23AM +0200, Hans de Goede wrote:
> > > > > Hi,
> > > > > 
> > > > > On 03-10-17 13:41, Hans de Goede wrote:
> > > > > 
> > > > > <snip>
> > > > > 
> > > > > > > > +#define CHECK_IOCTL_IN(req)                             \
> > > > > > > > +do {                                         \
> > > > > > > > +    if ((req)->Hdr.cbIn != (sizeof((req)->Hdr) + sizeof((req)->u.In)) || \
> > > > > > > > +        (req)->Hdr.cbOut != sizeof((req)->Hdr))                 \
> > > > > > > > +        return -EINVAL;                             \
> > > > > > > > +} while (0)
> > > > > > > 
> > > > > > > Make these things functions instead of macros.
> > > > > > 
> > > > > > Turning these into functions is a good idea I will do so for v2.
> > > > > 
> > > > > Correction, I forgot that the passed in "req" macro
> > > > > argument has a different type with all the calls, so
> > > > > these cannot be changed into functions because they
> > > > > rely on sizeof on the specific type to do the size
> > > > > checks.
> > > > 
> > > > Don't we already have built-in checks for these types of things?  Surely
> > > > we don't require each ioctl user in the kernel to do this by
> > > > themselves...
> > > 
> > > No other driver uses this kind of header for the ioctl structures,
> > > usually we just rely on the ioctl command number to encode the
> > > size, or we copy a fixed length.
> > 
> > Then why can't we do the same thing here as well?
> 
> VirtualBox uses the same ioctl interface for the guest-additions userspace
> parts on all supported platforms and not all platforms support encoding
> the size in the ioctl number, so all the ioctl data structs have a header
> with the in and out sizes in there, these macros check that header.
> 
> As mentioned during the RFC discussions changing the ioctl interface
> is going to be troublesome to do because we want to be ABI compatible
> with the kernel module shipped with the guest additions.
> 
> Having 2 separate guest additions builds, one for running with the out
> of tree driver (which eventually should go away I hope, but certainly not
> soon) and another build for the mainline version of the vboxguest driver
> is not supportable.
> 
> I'm confident that if we find issues during the review process, which
> have security implications, or where behavior is not clearly specified,
> that we can get VirtualBox upstream to fix the API for that, but outright
> re-designing the API is not really an option I believe.
> 
> As for these specific macros to check the ioctl data struct header,
> if these are considered a problem I can simply write out the code in
> the few places where this macro is called.

Ok, that makes a bit more sense, let's see what your next round of
patches look like, we can take it from there :)

thanks,

greg k-h