linux-kernel - Re: [patch 2.6.27 mmotm] rtc-cmos: export second NVRAM bank

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Message-Id: <200809121213.45877.david-b@pacbell.net>
Date:	Fri, 12 Sep 2008 12:13:45 -0700
From:	David Brownell <david-b@...bell.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	rtc-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [patch 2.6.27 mmotm] rtc-cmos: export second NVRAM bank

On Friday 12 September 2008, Andrew Morton wrote:
> > +#define can_bank2	true
> 
> It would be more idiomatic to make this upper-case: CAN_BANK2.

Think of it as:  "static const bool can_bank2 = true;" ...


> : static ssize_t
> : cmos_nvram_read(struct kobject *kobj, struct bin_attribute *attr,
> : 		char *buf, loff_t off, size_t count)
> : {
> : 	int	retval;
> : 
> : 	if (unlikely(off >= attr->size))
> : 		return 0;
> : 	if (unlikely(off < 0))
> : 		return -EINVAL;
> : 	if ((off + count) > attr->size)
> : 		count = attr->size - off;
> : 
> : 	off += NVRAM_OFFSET;
> : 
> 
> The VFS will (hopefully) prevent ->read methods from being called with
> a negative file offset.  What prompted the additional test for that?

I was chasing down a problem related to offsets.  Now fixed;
that bit could probably go away ... except that so long as the
interface allows negative/bogus values, it seems appropriate to
defend against them.  If the interface passed an unsigned type,
my paranoia-meter would not fire in this case.

 
> I did't look at it exhaustively but I suspect that the above code won't
> work right if attr->size has a value of around (2^31 - 42) and `offset'
> is (2^31 - 54) and NVRAM_OFFSET==54.  Or something like that.  It looks
> holey ;)

In this case, size over 256 would be impossible.  A relevant
case study may be:

  http://thedailywtf.com/Articles/Rule-Number-One.aspx

:)
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