linux-kernel - Re: strict copy_from

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Message-Id: <201001051445.26149.arnd@arndb.de>
Date:	Tue, 5 Jan 2010 14:45:25 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: strict copy_from_user checks issues?

On Tuesday 05 January 2010, Arjan van de Ven wrote:
> > In file included
> > from /home2/heicarst/cfu/arch/s390/include/asm/mmu_context.h:13,
> > from /home2/heicarst/cfu/arch/s390/include/asm/elf.h:133, from
> > include/linux/elf.h:7, from include/linux/module.h:14, from
> > drivers/net/tun.c:42: In function 'copy_from_user',
> >     inlined from '__tun_chr_ioctl' at drivers/net/tun.c:1124:
> > /home2/heicarst/cfu/arch/s390/include/asm/uaccess.h:299: warning:
> > call to 'copy_from_user_overflow' declared with attribute warning:
> > copy_from_user() buffer size is not provably correct
> 
> this one is ... interesting btw... I have trouble myself finding where
> the check is done... so I can understand gcc having trouble too.
> 

I think it will get inlined on 32 bit machines or without CONFIG_COMPAT,
but not when CONFIG_COMPAT is enabled, because then there are two
call-sites.

The tun_chr_compat_ioctl was only merged in 2.6.33-rc1, so 2.6.32 could
still inline the function all the time.

If the compiler is really smart (haven't tried), it can optimize away
tun_chr_compat_ioctl entirely on i386 and make it an alias to
tun_chr_ioctl, but not on s390 because that uses a nontrivial compat_ptr()
function.

	Arnd
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