Re: [BUG] 2.6.25-rc3 hangs in early boot on Sun Ultra5

[David Miller <davem@davemloft.net>]

From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 27 Feb 2008 09:02:22 +0100 (CET)

> On Tue, 26 Feb 2008, David Miller wrote:
> > What the FUTEX checking code is doing now is doing a "user" access
> > with set_fs(KERNEL_DS) since it runs from the kernel bootup early init
> > sequence.  And this is illegal according to the existing checks.
> > 
> > When we do set_fs(KERNEL_DS) then pass a "user" pointer down
> > into a system call or something like that, we give it a pointer
> > that "cannot fault".  So if we get into the fault handling
> > path here for a case like that we really do want to scream and
> > print out an OOPS message in my opinion.
> 
> So it would be correct to set_fs(USER_DS) then do the check and switch
> back to KERNEL_DS ?

No, I'm saying it would be better not to take faults purposefully in
the kernel address space.  We don't have a usable user address space
setup at this point in the boot, so using USER_DS would be even worse.

I think I'll just add a different version of the sanity check to this
sparc64 code later on, one that will take into consideration this
KERNEL_DS case because I can see how it could be useful in other
circumstances.

> > Did this FUTEX change go into -stable too?
> 
> It's queued, AFAIK

Crap, I'll need to push my fix there too.