Message-ID: <20110304231031.GA27628@kroah.com>
Date:	Fri, 4 Mar 2011 15:10:31 -0800
From:	Greg KH <greg@...ah.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Matthew Wilcox <willy@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [REVIEW] NVM Express driver

On Fri, Mar 04, 2011 at 10:33:29PM +0000, Alan Cox wrote:
> > > How do most other apps do it.
> > 
> > They write to the sysfs file with the firmware when they feel like it.
> 
> Very few seem to do this. Quite a lot also have a much more complex
> interaction anyway than 'firmware please', 'zap' , 'ok' to be fair.
> 
> > > > So, what could be changed in the current firmware interface to fix this
> > > > problem in a manner which would solve these perceived issues?
> > > 
> > > I'm not sure it can. The basis of the interface is driver requests
> > > firmware. That is done by using a pathname which in a namespaced
> > > environment isn't global and has races
> > 
> > Of course, but those races don't show up in real systems, right?
> 
> They do. That's why I know they exist. If enough people use your distro
> then eventually someone catches it.

Ok, I guess I must be doing support for distros that never get used :)

Have pointers to bug reports?

> > > (Several things break quite spectacularly if you request_firmware while
> > > updating a package, but of course nobody considered such details even for
> > > automatic stuff in many cases. Really there is some locking needed).
> > 
> > I've never heard of this race before as you usually do firmware upload
> > either at boot time, or when a user specifically asks for it.  Neither
> > of those times is when packages are usually getting updated.
> 
> Usually is not a good answer for correctness and security with firmware.
> It's really really not a good answer when flashing new firmware. "Usually
> your expensive hardware is not turned into a valueless brick" lacks a
> certain something.
> 
> For dynamically requested firmware the request_firmware interface is
> excellent stuff, and the right way to fix the races is to lock between
> the daemon and package manager. For flashing stuff it's not up to the job.

Fair enough.

> > > For manual updating of a block of firmware the interface most stuff wants
> > > is
> > > 
> > > 	copy_from_user()
> > > 
> > > or if you wanted to wrap it up nicely
> > > 
> > > 	x = vmalloc_from_user(void __user *ptr, ssize_t len);
> > > 
> > > The app knows which firmware it is talking about and can inspect and
> > > compare it while holding an open file handle on the deivce. That protects
> > > against hotplug races and getting the wrong device second access, and
> > > ensures that the firmware, device and userspace are all talking about
> > > exactly the same thing.
> > 
> > But you do get this type of buffer from the firware interface to your
> > driver today, right?
> 
> Providing you jump through a small army of hoops, get the right sysfs
> nodes, dodge any race conditions, hotplug and the like yes. But it should
> be robust and simple. Using request_firmware for this case is neither
> robust nor simple, nor is it easy to get the sysfs/driver open/hotplug
> race handling right so instead of the kernel code being very occasionally
> buggy (which we can spot) the user apps will be horribly buggy and we
> don't read those so your push for a complex mis-standard in the kernel
> actually makes the problem far worse than it would be without.
> 
> > Still, I don't want this to all of a sudden be "open season" for every
> > individual driver deciding to want to create an ioctl call for firmware
> > updating just because the authors don't like the existing in-kernel
> > interface.  Please fix up the in-kernel one instead to meet your needs.
> 
> You are not creating an open season. The every driver having its own
> ioctl for firmware updating is the norm, and its in some ways a rather
> good norm because when it comes to flash updating there isn't much
> standardisation and you don't want standardisation as it just asks for
> the wrong tool to work in an unfortunate race or user mix-up. Flash
> updating is special - it's good that one app doesn't work on another
> device !
> 
> You could have a standardised helper easily enough I guess. Pick a
> standard struct and firmware descriptor and provide a
> 
> request_firmware_from_user(struct firmware_something __user *fw)
> 
> which hands back stuff in the form the rest of the firmware logic likes
> to play and has a standard user side struct format.

Ok, patches gladly accepted :)

As well as the general "The firmware subsystem needs an active
maintainer" plea, as the previous maintainer passed away a number of
years ago :(

thanks,

greg k-h
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