linux-kernel - Re: [PATCH] usb: use memdup

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Date:	Tue, 5 May 2009 10:22:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	David Brownell <david-b@...bell.net>,
	Li Hong <lihong.hi@...il.com>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: use memdup_user()

On Tue, 5 May 2009 12:44:01 +0200 Oliver Neukum <oliver@...kum.org> wrote:

> Am Dienstag, 5. Mai 2009 08:11:57 schrieb Andrew Morton:
> > On Mon, 4 May 2009 16:01:51 +0200 Oliver Neukum <oliver@...kum.org> wrote:
> 
> > > I want people to be forced to think about memory allocations.
> > > We had endless trouble during 2.4 with storage deadlocking.
> > > We simply need full control of this.
> >
> > thou-shalt-use-GFP_NOFS is a very common pattern in many filesystems.
> > And thou-shalt-use-GFP_NOIO is a very common pattern in block drivers.
> 
> USB drivers are interface level yet some functions, reset and power
> management, are on a device level. As it is unpredictable whether
> a driver will share a device with a storage driver, all USB drivers as far as
> these functions are concerned must be considered block device drivers.
> That's the reason GFP_NOIO is so prevalent in USB.

There must be some particular action which flips the thread of control
from one state to the other.  eg, taking of a lock.

> > I wonder how hard it would be to add runtime debugging checks?  If
> 
> I'd prefer compile time checks. Ideally we'd annotate a function with an
> attribute making the compiler barf if copy_to/from_user or an inappropriate
> kmalloc is used. It can't be perfect due to function pointers, but it would
> be a good start.

I don't think that would have enough coverage - bugs in this area tend
to come from calling some function which looks innocent, but which
calls some function which calls some function which calls some function
which uses GFP_KERNEL.

And then there's stuff like "usb takes a mutex which is also taken by
some other thread which does a GFP_KERNEL allocation while holding that
mutex".

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