linux-kernel - Re: [PATCH 0/4] Was: deferring _

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Message-ID: <20120702120259.GG22927@ZenIV.linux.org.uk>
Date:	Mon, 2 Jul 2012 13:02:59 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	". James Morris" <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 0/4] Was: deferring __fput()

On Mon, Jul 02, 2012 at 07:49:50AM -0400, Mimi Zohar wrote:

> > > > pid=1 uid=0 d_name=init comm=swapper/0 dev="rootfs" mode=100775
> > > > pid=1 uid=0 d_name=bash comm=swapper/0 dev="rootfs" mode=100755
> > > 
> > > OK...  Here's what I suspect is going on:
> > > 	* populating initramfs writes binaries there.  We open files (for write) from
> > > the kernel thread (there's nothing other than kernel threads at that point), write to
> > > them, then close().  Final fput() gets delayed.
> > > 	* Then we proceed to execve().  Which means mapping the binary with MAP_DENYWRITE.
> > > Which fails, since there's a struct file still opened for write on that sucker.
> > > 
> > > Your patch did not delay those fput() - they were done without ->mmap_sem held.  So
> > > it survived.  Booting without initramfs always survives; booting with initramfs may
> > > or may not survive, depending on the timings - if that scheduled work manages to
> > > run by the time we do those execve(), we win.  Note that async_synchronize_full()
> > > done in init_post() might easily affect that, depending on config.
> > > 
> > > As a quick test, could you try slapping a delay somewhere around the beginning
> > > of init_post() and see if it rescues the system?
> > 
> > Ho-hum...  How about this (modulo missing documentation of the whole sad mess):
> 
> Sorry, neither adding the delay or this patch helped.

Really odd.  Could you print the error returned by kernel_execve() in run_init_process()?
At least that way we'll get some indication of what's going on there.  Another thing:
could you slap matching printks into the nested if() in fput() and the loop in
delayed_fput(), just to see if we do get __fput() done on all the right struct file?
Just "fput: %p", file and "delayed_fput: %p", file would probably be enough.

I'm assuming that I hadn't misparsed what you wrote and that __fput() in nested
if() in fput() was enough to get the thing working.  Could you confirm that?

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