linux-kernel - Re: fs/pipe.c null pointer dereference

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Date:	Wed, 14 Oct 2009 15:50:40 -0700
From:	Earl Chew <earl_chew@...lent.com>
To:	Frans Pop <elendil@...net.nl>
CC:	linux-kernel@...r.kernel.org
Subject: Re: fs/pipe.c null pointer dereference

Frans Pop wrote:
> 2.6.21 is really rather ancient for the kernel community and I doubt anyone 
> here will really want to look into issues with it, given that there's a 
> realistic chance it has already been fixed since then. The current stable 
> version is 2.6.31 after all.
> 
> Can you reproduce the oops with a current kernel? Have you checked if there 
> have been any changes in the code you identified since 2.6.21?

I was mostly after some guidance. I'm happy to dig around to figure
out what the problem is and log a bug if indeed I find one.

Yes, I have searched for obvious patches but didn't turn up anything.

I have looked at the code for more recent 2.6.31 kernel on
lxr.linux.no and haven't turned up any appreciable difference.

I have given the matter more thought, and I believe it is unlikely
that the problem is in fs/fifo.c. As outlined in my previous
email, the code looks ok.

I'm a little confused about the switch between the fifo_open()
and the subsequent call to pipe_rdwr_open(). My understanding
is fifo_open() is called during open("/fifo", O_RDWR) which
fills in filp->f_op to point at rdwr_fifo_fops.

Thereafter, a call to close() will vector through filp->f_op
to pipe_rdwr_release().

I'm still puzzled as to how pipe_rdwr_open() is called. The
stack trace says:

Call Trace:
  [<ffffffff8028125c>] __dentry_open+0x13c/0x230
  [<ffffffff8028143d>] do_filp_open+0x2d/0x40
  [<ffffffff802814aa>] do_sys_open+0x5a/0x100
  [<ffffffff8021faf3>] sysenter_do_call+0x1b/0x67

I figure that the fileoperation for the fifo inode must be
revectored to rdwr_fifo_fops --- but I can't see that's
done exactly.

Any hints as to where to look for this ?

In any case, my current guess as to how this happens is that
there might be window of time between do_sys_open() ... pipe_rdwr_open()
where the inode is obtained and the fileoperation pointer is
followed to get to pipe_rdwr_open().

During that window of time, might it be possible for the fifo
to be closed and the underlying pipe released?

Where can I look to see if this is possible?

Earl

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