linux-kernel - Re: [PATCH 1/7] usb: trivial cleanups

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Message-ID: <Pine.LNX.4.44L0.0910190956580.3899-100000@iolanthe.rowland.org>
Date:	Mon, 19 Oct 2009 09:59:01 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Felipe Contreras <felipe.contreras@...il.com>
cc:	Jiri Kosina <jkosina@...e.cz>, <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	<linux-usb@...r.kernel.org>
Subject: Re: [PATCH 1/7] usb: trivial cleanups

On Mon, 19 Oct 2009, Felipe Contreras wrote:

> I still have an unfixed warning:
> drivers/usb/core/hcd.c: In function ârh_stringâ:
> /data/public/src/linux/arch/x86/include/asm/string_32.h:74: warning:
> array subscript is above array bounds
> 
> Apparently there's a problem with the optimized memcpy for x86 with this code:
> static char const langids[4] = {4, USB_DT_STRING, 0x09, 0x04};
> len = min_t(unsigned, len, sizeof(langids));
> memcpy(data, langids, len);
> return len;
> 
> gcc 4.4 is trying to optimize the memcpy, but it's not able to realize
> that 'len' will always be <= 4 and the memcpy will not exceed langids.
> One way to solve this is by replacing len with the min_t expression:
> memcpy(data, langids, min_t(unsigned, len, sizeof(langids)));
> 
> However, that looks ugly and we need the expression again for the
> return. Another way is to remove 'const' from langids.
> 
> AFAIK the code is perfectly correct as it is, I think the fact that
> gcc 4.4 complains is a bug on gcc side.

Yes, it is a well-known bug in gcc.  Other places in the kernel get 
similar warnings.

Alan Stern

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