linux-kernel - Re: [PATCH v2 09/17] mm: Add unsafe_follow

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Date:   Sun, 11 Oct 2020 08:27:41 +0200
From:   Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Tomasz Figa <tfiga@...omium.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Kees Cook <keescook@...omium.org>,
        KVM list <kvm@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        LKML <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jérôme Glisse <jglisse@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>, Hans Verkuil <hverkuil@...all.nl>,
        "Lad, Prabhakar" <prabhakar.csengg@...il.com>
Subject: Re: [PATCH v2 09/17] mm: Add unsafe_follow_pfn

Em Sat, 10 Oct 2020 23:50:27 +0200
Daniel Vetter <daniel.vetter@...ll.ch> escreveu:

> On Sat, Oct 10, 2020 at 11:36 PM Laurent Pinchart
> <laurent.pinchart@...asonboard.com> wrote:
> >

> > > We probably still have a few legacy drivers using videobuf (non-2),
> > > but IMHO those should be safe to put behind some disabled-by-default
> > > Kconfig symbol or even completely drop, as the legacy framework has
> > > been deprecated for many years already.  
> >
> > There's 8 drivers left, and they support a very large number of devices.
> > I expect unhappy users distros stop shipping them. On the other hand,
> > videobuf has been deprecated for a loooooooong time, so there has been
> > plenty of time to convert the remaining drivers to videobuf2. If nobody
> > can do it, then we'll have to drop support for these devices given the
> > security issues.  
> 
> Again, the issue here is _only_ with follow_pfn. For videobuf1 this
> means videbuf-dma-contig.c userptr support is broken. Unlike videobuf2
> it means it's broken for all usage (not just zero-copy userptr),
> because videbuf-dma-contig.c lacks the pin_user_pages path.

Well, follow_pfn() is used only by videbuf-dma-contig.c. If this is 
the only part of VB1 that will have userptr broken, then there's
just one driver that might be affected: davinci.

Yet, taking a deeper look:

	$ git grep include drivers/media/platform/davinci/|grep -i videobuf
	drivers/media/platform/davinci/vpif_capture.h:#include <media/videobuf2-dma-contig.h>
	drivers/media/platform/davinci/vpif_display.h:#include <media/videobuf2-dma-contig.h>

It sounds to me that it was already converted to VB2, but some VB1
symbols were not converted at its Kconfig.

It sounds to me that there are other drivers with some VB1 left overs
at Kconfig, as those are the only ones using VB1 those days:

	$ for i in $(git grep media/videobuf drivers |grep -v videobuf2 |grep -v v4l2-core|cut -d: -f1); do dirname $i; done|sort|uniq
	drivers/media/pci/bt8xx
	drivers/media/pci/cx18
	drivers/media/platform
	drivers/media/usb/tm6000
	drivers/media/usb/zr364xx
	drivers/staging/media/atomisp/pci

> But that
> would be easy to add if this poses a  problem I think - we just need
> to carry over the pin_user_pages_fast logic from videbuf2, no driver
> changes required. But of course I don't think we should do that before
> someone reports the regression, since videobuf1 userptr is doubly
> deprecated :-)

I think otherwise. Keeping a broken component at the Kernel is 
a bad idea. 

Yet, from my quick search above, it sounds to me that it is time for 
us to retire the VB1 DMA contig support as a hole, as there's no client 
for it anymore.

I'll work on some patches cleaning up the VB1 left overs at
Kconfig and removing videbuf-dma-contig.c for good, if there's
no hidden dependency on it.


Thanks,
Mauro