Breakage in "track dev_mapping in more robust and flexible way"
thomas at shipmail.org
Mon Oct 29 01:39:09 PDT 2012
On 10/26/2012 03:14 PM, Ilija Hadzic wrote:
> On Fri, 26 Oct 2012, Thomas Hellstrom wrote:
>> On 10/25/2012 11:27 PM, Ilija Hadzic wrote:
>>> Can you give the attached patch a whirl and let me know if it fixes
>>> the problem?
>>> As I indicated in my previous note, vmwgfx should be the only
>>> affected driver because it looks at dev_mapping in the open hook
>>> (others do it when they create an object, so they are not affected).
>>> I'll probably revise it (and I'll have some general questions about
>>> drm_open syscall) before officially send the patch, but I wanted to
>>> get something quickly to you to check if it fixes your problem. I
>>> hope that your vmwgfx test environment is such that you can
>>> reproduce the original
>>> -- Ilija
>> Yes, it appears like this patch fixes the problem. It'd be good to
>> have it in 3.7 (drm-fixes) with a cc to stable.
> OK great. Thanks for testing. Before I send out an "official" patch, I
> have a few questions for those who have been around longer and can
> possibly reflect better than me on the history of drm_open syscall.
> Currently, before touching dev->dev_mapping field we grab dev->struct
> mutex. This has been introduced by Dave Airlie a long time ago in
> a2c0a97b784f837300f7b0869c82ab712c600952. I tried to preserve that in
> all patches where I touched dev_open, but looking at the code I don't
> think the mutex is necessary. Namely, drm_open is only set in
> drm_open, and concurrent openers are protected with drm_global_mutex.
> Other places (drivers) where dev->dev_mapping is accessed is read-only
> and dev_mapping is written at first open when there are no file
> descriptors around to issue any other call. Also, it doesn't look to
> me that any driver locks dev->struct_mutex before accessing
> dev->dev_mapping anyway. So I am thinking of dropping the mutex
> completely, but I would like to hear a second thought.
Without having looked a the code, with your current changes
dev->dev_mapping should be immutable and initialized before any
it, and as such would need no mutex, so dropping the protection of
dev->dev_mapping from that point of view should be fine. I think people
sooner or later want to get rid of drm_global_mutex, though, but at that
point we probably want another mutex that protects open-time
initialization of immutable members only, so from my point of view this
is OK, but you might want to double-check with Dave.
> The other issue, I noticed is that of the drm_setup() call fails, the
> open_count counter would remain incremented and I think we need to
> restore it back (or if I am missing something, would someone please
> enlighten me). This was also in the kernel all this time (and I have
> not noticed until now), so I "smuggled" that fix in the patch that I
> sent you. However, wonder if I should cut the separate patch for
> open_count fix.
> Actually, I think that I should cut three patches: one to drop the
> mutex, one to fix the open_count and one to fix your problem with
> dev_mapping and that probably all three should CC stable. Before I do
> that, I'd like to hear opinions of others.
I think you should, However stable doesn't want fixes for theoretical
stuff that have never been triggered in real life, so the patch to drop
mutex protection doesn't belong there. That's a patch for drm-next, so
people get a decent chance to see if it breaks something. The
dev_mapping thing opens up a quite severe security issue and should got
into drm-fixes with Cc to stable as soon as ever possible. The
open_count stuff should go into drm-fixes, possibly cc'd to stable.
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