linux-kernel - Re: BUG? "Call fasync() functions without the BKL" is racy

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Date:	Mon, 1 Dec 2008 12:34:05 -0700
From:	Jonathan Corbet <corbet@....net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andi Kleen <ak@...ux.intel.com>, Al Viro <viro@...iv.linux.org.uk>,
	Vitaly Mayatskikh <vmayatsk@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: BUG? "Call fasync() functions without the BKL" is racy

On Mon, 1 Dec 2008 20:15:55 +0100
Oleg Nesterov <oleg@...hat.com> wrote:

> > Hmm, about checking for this case and retrying?
> >
> > Or put a fasync mutex into files_struct.  
> 
> Perhaps, we can add O_LOCK_FLAGS, then something like
> 
> 	--- a/fs/fcntl.c
> 	+++ b/fs/fcntl.c
> 	@@ -175,6 +175,15 @@ static int setfl(int fd, struct file * f
> 		if (error)
> 			return error;
> 	 
> 	+	spin_lock(&current->files->file_lock);
> 	+	if (!(filp->f_flags & O_LOCK_FLAGS))
> 	+		filp->f_flags |= O_LOCK_FLAGS;
> 	+	else
> 	+		error = -EAGAIN;
> 	+	spin_unlock(&current->files->file_lock);
> 	+	if (error) /* pretend ->f_flags was changed after us
> */
> 	+		return 0;
> 	+

This strikes me as overkill.  What we really want to do is to protect
against concurrent access to f_flags - something which could come about
in a couple of other situations (nfsd/vfs.c tweaks it, for example).
We *could* just extend files_lock to cover f_flags too, but that comes
at the cost of making ->fasync() atomic when it never has been before -
doesn't seem like a good idea.

Perhaps we just need a single f_flags_mutex?  For code changing
f_flags only (it woudn't be needed to query the flags)? Then
ioctl_fionbio() and ioctl_fioasync() could use it too. It's hard to
imagine that there's enough contention to warrant any more than that,
especially given that it all (except ioctl_fionbio()) has been under
the BKL until now.

jon
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