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

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Message-ID: <CAMP5Xgd-n4dgPsGN9KnD1c6WQ-CTAU1-3Z3mWgg=vqZQeru+UA@mail.gmail.com>
Date:	Wed, 4 Dec 2013 18:02:30 -0800
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Colin Cross <ccross@...roid.com>,
	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
	Ian Rogers <irogers@...gle.com>,
	Serban Constantinescu <serban.constantinescu@....com>,
	lkml <linux-kernel@...r.kernel.org>,
	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 4, 2013 at 2:02 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> 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"?
>

Yes libbinder will have to be changed to support calls between 32 bit
and 64 bit processes, so I don't see much value in a patchset that
only supports all 32 bit or all 64 bit processes. If user space is
fixed to use 64 bit pointers on a 64 bit system, then much of the code
added in this patchset becomes useless (and probably harmful as it
appears to prevent 32 bit processes from communicating with 64 bit
processes).

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

I don't think we need to support old 32 bit userspace framework code
on a 64 bit system. I think it is more important to not prevent mixed
mode systems.

-- 
Arve Hjønnevåg
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