linux-kernel - Re: [PATCH v9 2/3] kernel.h: add to_user

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Date:	Thu, 17 Mar 2016 16:50:14 -0400
From:	Rob Clark <robdclark@...il.com>
To:	Joe Perches <joe@...ches.com>
Cc:	Gustavo Padovan <gustavo@...ovan.org>,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devel@...verdev.osuosl.org,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Daniel Stone <daniels@...labora.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Riley Andrews <riandrews@...roid.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Greg Hackmann <ghackmann@...gle.com>,
	John Harrison <John.C.Harrison@...el.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Airlie <airlied@...ux.ie>,
	Daniel Vetter <daniel.vetter@...el.com>,
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v9 2/3] kernel.h: add to_user_ptr()

On Thu, Mar 17, 2016 at 4:40 PM, Joe Perches <joe@...ches.com> wrote:
> On Thu, 2016-03-17 at 16:33 -0400, Rob Clark wrote:
>> On Thu, Mar 17, 2016 at 4:22 PM, Joe Perches <joe@...ches.com> wrote:
>> > On Thu, 2016-03-17 at 15:43 -0300, Gustavo Padovan wrote:
>> > > 2016-03-17 Gustavo Padovan <gustavo.padovan@...labora.co.uk>:
>> > > > 2016-03-17 Joe Perches <joe@...ches.com>:
>> > > > > On Thu, 2016-03-17 at 14:30 -0300, Gustavo Padovan wrote:
>> > > > > > This function had copies in 3 different files. Unify them in
>> > > > > > kernel.h.
>> > > > > This is only used by gpu/drm.
>> > > > >
>> > > > > I think this is a poor name for a generic function
>> > > > > that would be in kernel.h.
>> > > > >
>> > > > > Isn't there an include file in linux/drm that's
>> > > > > appropriate for this.  Maybe drmP.h
>> > > > >
>> > > > > Maybe prefix this function name with drm_ too.
>> > > > No, the next patch adds a user to drivers/staging (which will be moved
>> > > > to drivers/dma-buf) soon. Maybe move to a different header in
>> > > > include/linux/? not sure which one.
>> > > >
>> > > > >
>> > > > >
>> > > > > Also, there's this that might conflict:
>> > > > >
>> > > > > arch/powerpc/kernel/signal_32.c:#define to_user_ptr(p)          ptr_to_compat(p)
>> > > > > arch/powerpc/kernel/signal_32.c:#define to_user_ptr(p)          ((unsigned long)(p))
>> > > > Right, I'll figure out how to replace these two too.
>> > > The powerpc to_user_ptr has a different meaning from the one I'm adding
>> > > in this patch. I propose we just rename powerpc's to_user_ptr to
>> > > __to_user_ptr and leave the rest as is.
>> > I think that's not a good idea, and you should really check
>> > this concept with the powerpc folk (added to to:s and cc:ed)
>> >
>> > If it were really added, then the function meaning is incorrect.
>> >
>> > This is taking a u64, casting that to (unsigned long/uint_ptr_t),
>> > then converting that to a user pointer.
>> >
>> > Does that naming and use make sense on x86-32 or arm32?
>> >
>> fwiw Gustavo's version of to_user_ptr() is in use on arm32 and arm64..
>> Not entirely sure what doesn't make sense about it
>
> It's a name that seems like it should be a straightforward
> cast of a kernel pointer to a __user pointer like:
>
> static inline void __user *to_user_ptr(void *p)
> {
>         return (void __user *)p;
> }

ahh, ok.  I guess I was used to using it in the context of ioctl
structs..  in that context u64 -> (void __user *) made more sense.

Maybe uapi_to_ptr()?  (ok, not super-creative.. maybe someone has a better idea)

BR,
-R

> As a static function in a single file, it's not
> great, but OK, fine, it's static.
>
> As a global function in kernel.h, it's misleading.
>
>