linux-kernel - Re: [PATCH] lguest: explicitly setup /dev/lguest private

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Date:	Tue, 7 Apr 2015 10:36:16 +0200
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Tom Van Braeckel <tomvanbraeckel@...il.com>
Cc:	rusty@...tcorp.com.au, lguest@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, fengguang.wu@...el.com, lkp@...org
Subject: Re: [PATCH] lguest: explicitly setup /dev/lguest private_data

On Tue, Apr 07, 2015 at 10:18:18AM +0200, Tom Van Braeckel wrote:
> The private_data member of the /dev/lguest device file is used to hold
> the current struct lguest and needs to be set to NULL to signify that
> no initialization has taken place.
> 
> We explicitly set it to NULL to be independent of whatever value the
> misc subsystem initializes it to.
> 
> Signed-off-by: Tom Van Braeckel <tomvanbraeckel@...il.com>
> ---
> Backstory:
> ==========
> The misc subsystem used to initialize a file's private_data to point to
> the misc device when a driver had registered a custom open file
> operation and initialized it to NULL when a custom open file operation
> had *not* been provided.
> 
> This subtle quirk was confusing, to the point where kernel code
> registered *empty* file open operations to have private_data point to
> the misc device structure.
> 
> And it lead to bugs, where the addition or removal of a custom open
> file operation surprisingly changed the initial contents of a file's
> private_data structure.
> 
> The misc subsystem is currently underdoing changes to *always* set
> private_data to point to the misc device instead of only doing this
> when a custom open file operation has been registered.
> 
> Intel's 0day kernel testing robot discovered that the lguest driver
> depended on it implicitly being initialized to NULL, as Fengguang Wu
> reported. Thanks a lot for all the hard work!
> 
>  drivers/lguest/lguest_user.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)

I can take this through my char-misc tree, where this misc core change
was, if the lguest maintainer (i.e. Rusty) acks it.

Tom, thanks for tracking this down.

greg k-h
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