linux-kernel - RE: [PATCH] phy: fix compiler array bounds warning on settings[]

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Date:	Thu, 6 Mar 2014 09:13:09 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Bjorn Helgaas' <bhelgaas@...gle.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Florian Fainelli" <f.fainelli@...il.com>
Subject: RE: [PATCH] phy: fix compiler array bounds warning on settings[]

From: Bjorn Helgaas
> I'm stumped.  phy_find_valid() is static and only called from one
> place.  The 'idx' argument is always the result of phy_find_setting(),
> which should always return something between 0 and
> ARRAY_SIZE(settings), so I don't see any way idx can be < 0.
> 
> I stripped this down as far as I could; the resulting test code is at
> http://pastebin.com/pp1zMEWu if anybody else wants to look at it.  I'm
> using gcc 4.8.x 20131105 (prerelease), with "-Warray-bounds -O2"
> flags.
> 
> I hesitate to suspect a compiler bug, but it is very strange.  For
> example, in my test code, replacing "MAX_NUM_SETTINGS" with "2" gets
> rid of the warnings.  MAX_NUM_SETTINGS is known to be 2 at
> compile-time, so I don't know why this should make a difference.

I can't get an array bounds error from the gcc 4.8.1-10ubuntu9 at all.
Not even when I index the array with a constant 3.
I wonder if they compiled it out!

	David