linux-kernel - Re: [PATCH v1 9/9] staging: android: binder: Add binder compat layer

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Message-ID: <20131204220247.GA22031@kroah.com>
Date:	Wed, 4 Dec 2013 14:02:47 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Colin Cross <ccross@...roid.com>
Cc:	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
	Ian Rogers <irogers@...gle.com>,
	Serban Constantinescu <serban.constantinescu@....com>,
	lkml <linux-kernel@...r.kernel.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	John Stultz <john.stultz@...aro.org>,
	David Butcher <Dave.Butcher@....com>, romlem@...roid.com
Subject: Re: [PATCH v1 9/9] staging: android: binder: Add binder compat layer

On Wed, Dec 04, 2013 at 01:55:34PM -0800, Colin Cross wrote:
> On Wed, Dec 4, 2013 at 1:43 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> > On Wed, Dec 04, 2013 at 12:46:42PM -0800, Colin Cross wrote:
> >> On Wed, Dec 4, 2013 at 10:35 AM, Greg KH <gregkh@...uxfoundation.org> wrote:
> >> <snip>
> >>
> >> > And finally, is this all really needed?  Why not just fix the structures
> >> > to be "correct", and then fix userspace to use the correct structures as
> >> > well, thereby not needing a compat layer at all?
> >>
> >> Some of the binder ioctls take userspace pointers.  Are you suggesting
> >> storing those pointers in a __u64 to avoid having to have a
> >> compat_ioctl?
> >
> > Yes, that's the best way to solve the issue, right?
> 
> It's the least code, but in exchange you lose all the type safety and
> warnings when copying in and out of the pointers, as well as sparse
> checking on the __user attribute.

Not if you make the cast right at the beginning, when you first "touch"
the data, but yes, it does take some of the type saftey away, at the
expense of simpler code to mess up :)

> That doesn't seem like a good tradeoff to me.  In addition it requires
> modifying the existing heavily used 32 bit api, which means a
> mostly-equivalent compat layer added in libbinder to support old
> kernels.

Wait, I thought that libbinder would have to be changed anyway here, to
handle 64bit kernels (in both 32 and 64bit userspace).  Since you are
already changing it, why not just "do it correctly"?

Or does this patch series mean that no userspace code is changed?  Is
that a "requirement" here?

thanks,

greg k-h
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