linux-kernel - Re: [PATCH 1/4] module: implement module_inhibit

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Date:	Tue, 25 Sep 2007 17:25:17 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Jonathan Corbet <corbet@....net>, ebiederm@...ssion.com,
	cornelia.huck@...ibm.com, greg@...ah.com,
	stern@...land.harvard.edu, kay.sievers@...y.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] module: implement module_inhibit_unload()

Rusty Russell wrote:
> On Tue, 2007-09-25 at 12:36 +0900, Tejun Heo wrote:
>> Rusty Russell wrote:
>>> As stated you cannot protect arbitrary code this way, as you are trying
>>> to do.  I do not think you've broken any of the current code, but I
>>> cannot tell.  You're certainly going to surprise unsuspecting future
>>> authors.
>> Can you elaborate a bit?  Why can't it protect the code?
> 
> Because you don't know what that code does.  After all, it's assumed
> that module code doesn't get called after exit and you're deliberately
> violating that assumption.

What I meant by protecting 'code' was the 'code' itself.  Those pages
containing instructions that cpu executes.  It of course can't protect
against all the things they do.

>>> Can you really not figure out the module owner of the sysfs entry to inc
>>> its use count during this procedure?  (__module_get()).
>> I can but I don't think it's worth the effort.  It will involve passing
>> @owner parameter down through kobject to sysfs but the path is pretty
>> obscure and thus difficult to test.
> 
> Have you tested that *this* path works?  Let's take your first change as
> an example:
> 
> +       mutex_lock(&gdev->reg_mutex);
> +       __ccwgroup_remove_symlinks(gdev);
> +       device_unregister(dev);
> +       mutex_unlock(&gdev->reg_mutex);
> 
> Now, are you sure that calling cleanup_ccwgroup just after
> device_unregister() works?
> 
> static void __exit
> cleanup_ccwgroup (void)
> {
> 	bus_unregister (&ccwgroup_bus_type);
> }

It should.  After ->exit() is called, there can't be any object left
behind.  If a module is hosting objects which can't be destroyed from
->exit(), its module ref count shouldn't be zero.  So, either 1.
refcount != 0 or 2. ->exit() can destroy all objects.  As Cornelia
explains, for ccwgroup, it's #1.  Note that unload inhibition doesn't
change anything about this.

>> I think it's too much work for the
>> users of the API and it will be easy to pass the wrong @owner and go
>> unnoticed.
> 
> But your shortcut insists that all module authors be aware that
> functions can be running after exit() is called.  That's a recipe for
> instability and disaster.

No, it doesn't change that at all.  All unload inhibition does is
postponing removal of code (and data too of course) section a bit so
that a module can host code which issues unloading of itself.  Object
synchronization rules remain exactly the same.  Formerly broken code is
still broken and I don't even think unload inhibition would mask them
too much either.

I think the naming is too ambiguous.  Maybe it should be named something
like "hold_module_for_suicide".

Thanks.

-- 
tejun
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