linux-kernel - Re: [PATCH V2 3/6] mpcore_wdt: Fix WDIOC

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Message-ID: <20110706134806.GA8286@n2100.arm.linux.org.uk>
Date:	Wed, 6 Jul 2011 14:48:06 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Vitaly Kuzmichev <vkuzmichev@...sta.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	linux-watchdog@...r.kernel.org,
	Marc Zyngier <marc.zyngier@....com>,
	Wim Van Sebroeck <wim@...ana.be>,
	Arnd Bergmann <arnd@...db.de>,
	Nicolas Pitre <nico@...xnic.net>,
	John Stultz <johnstul@...ibm.com>,
	linux-kernel@...r.kernel.org, arm@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH V2 3/6] mpcore_wdt: Fix WDIOC_SETOPTIONS handling

On Tue, Jul 05, 2011 at 11:00:37PM +0400, Vitaly Kuzmichev wrote:
> According to the include/linux/watchdog.h WDIOC_SETOPTIONS is
> classified as 'read from device' ioctl call:
>   #define WDIOC_SETOPTIONS        _IOR(WATCHDOG_IOCTL_BASE, 4, int)
> 
> However, the driver 'mpcore_wdt' performs 'copy_from_user' only if
> _IOC_WRITE is set, thus the local variable 'uarg' which is used in
> WDIOC_SETOPTIONS handling remains uninitialized.
> 
> The proper way to fix this is to bind WDIOC_SETOPTIONS to _IOW,
> but this will break compatibility.
> So adding additional condition for performing 'copy_from_user'.

That looks like something which needs fixing in watchdog land.  And it's
not too difficult:

1. Change the existing WDIOC_SETOPTIONS to WDIOC_SETOPTIONS_OLD
2. Add a new, correctly defined WDIOC_SETOPTIONS.
3. Update all drivers to use WDIOC_SETOPTIONS.
4. Provide a helper which deals with WDIOC_SETOPTIONS_OLD and translates
   it to the proper WDIOC_SETOPTIONS call (this could be just:

	if (cmd == WDIOC_SETOPTIONS_OLD) {
		static int first = 1;
		if (first)
			pr_warn("%s:%d used old WDIOC_SETOPTIONS call.  Please rebuild\n",
				current->comm, current->pid);
		first = 0;
		cmd = WDIOC_SETOPTIONS;
	}

   before any argument copies.

This means you preserve existing compatibility with userspace, yet gain
a path to a non-broken ioctl interface.

As stuff gets rebuilt and replaced, you'll eventually be able to remove
the above.  That period used to be two years for such simple changes.
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