linux-kernel - Re: [PATCH] generic_nvram: Turn nvram

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Message-Id: <200910120025.05208.arnd@arndb.de>
Date:	Mon, 12 Oct 2009 00:25:05 +0200
From:	Arnd Bergmann <arndbergmann@...glemail.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	John Kacur <jkacur@...hat.com>,
	"Sven-Thorsten Dietrich" <sven@...bigcorporation.com>,
	Jonathan Corbet <corbet@....net>,
	Alessio Igor Bogani <abogani@...ware.it>,
	Arnd Bergmann <arndbergmann@...glemail.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] generic_nvram: Turn nvram_ioctl into an unlocked ioctl

On Monday 12 October 2009, Frederic Weisbecker wrote:
> nvram_ioctl is a bkl locked ioctl, but it can be an unlocked ioctl.
> 
> - part is provided by the user
> - offset is provided by pmac_get_partition() which is safe as it only
>   touches nvram_partitions, an array inistialized on __init time and
>   read-only the rest of the time.
> - nvram_sync() only relies on core99_nvram_sync() which checks
>   is_core_99, nvram_data, nvram_image. Those are variables initialized
>   on __init time only and their direct values are not touched further.
>   The rest modifies the nvram image header, protected by nv_lock
>   already.
> 
> So it's safe to call nvram_ioctl without the big kernel lock held.
> 
> Reported-by: Arnd Bergmann <arndbergmann@...glemail.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: John Kacur <jkacur@...hat.com>
> Cc: Sven-Thorsten Dietrich <sven@...bigcorporation.com>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Alessio Igor Bogani <abogani@...ware.it>
> Cc: Arnd Bergmann <arndbergmann@...glemail.com>
> Cc: Alan Cox <alan@...rguk.ukuu.org.uk>

Hmm, I wish I had not started using the gmail MTA for sending out
mail, not that address is public forever.

>  drivers/char/generic_nvram.c |   10 +++++-----
>  1 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/char/generic_nvram.c b/drivers/char/generic_nvram.c
> index c49200e..fd448aa 100644
> --- a/drivers/char/generic_nvram.c
> +++ b/drivers/char/generic_nvram.c
> @@ -118,11 +118,11 @@ static int nvram_ioctl(struct inode *inode, struct file *file,
>  }
>  
>  const struct file_operations nvram_fops = {
> -	.owner		= THIS_MODULE,
> -	.llseek		= nvram_llseek,
> -	.read		= read_nvram,
> -	.write		= write_nvram,
> -	.ioctl		= nvram_ioctl,
> +	.owner			= THIS_MODULE,
> +	.llseek			= nvram_llseek,
> +	.read			= read_nvram,
> +	.write			= write_nvram,
> +	.unlocked_ioctl		= nvram_ioctl,
>  };
>  
>  static struct miscdevice nvram_dev = {

The ioctl and unlocked_ioctl functions have a different signature, so you
need to adapt that, not just rename it.

Not sure if we should do it in the same patch, but this driver should also 
have a compat_ioctl method pointing to the same function. The ioctl numbers
in this driver are all 32/64 bit clean, but are not listed in fs/compat_ioctl.c,
so adding a .compat_ioctl pointer is the easiest way to fix 32 bit userland
accessing the device.

	Arnd <><
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