linux-kernel - Re: [PATCH] Consolidate CONFIG_DEBUG_STRICT_USER_COPY

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Date:	Wed, 27 Feb 2013 14:52:57 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Stephen Rothwell <sfr@...b.auug.org.au>
CC:	Stephen Boyd <sboyd@...eaurora.org>, Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	linux-parisc@...r.kernel.org, linux-s390@...r.kernel.org,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Helge Deller <deller@....de>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Chris Metcalf <cmetcalf@...era.com>
Subject: Re: [PATCH] Consolidate CONFIG_DEBUG_STRICT_USER_COPY_CHECK

On 02/27/2013 02:45 PM, Stephen Rothwell wrote:
> Hi all,
> 
> On Wed, 27 Feb 2013 14:19:16 -0800 "H. Peter Anvin" <hpa@...or.com>
> wrote:
>> 
>> Although some of the cases I have seen being flagged as "false 
>> positives" have been real bugs.
> 
> [hijacking the thread :-)]
> 
> I have been getting this warning for a very long time ( which would
> be an error if CONFIG_DEBUG_STRICT_USER_COPY_CHECK was set):
> 
> i386 defconfig i386-linux-gcc (GCC) 4.6.3
> 
> In file included from arch/x86/include/asm/uaccess.h:537:0, from
> include/linux/uaccess.h:5, from include/linux/highmem.h:8, from
> include/linux/pagemap.h:10, from fs/binfmt_misc.c:27: 
> arch/x86/include/asm/uaccess_32.h: In function
> 'parse_command.part.2': arch/x86/include/asm/uaccess_32.h:211:26:
> warning: call to 'copy_from_user_overflow' declared with attribute
> warning: copy_from_user() buffer size is not provably correct
> [enabled by default]
> 

OK, that is surprising, because that copy is very clearly properly
guarded:

static int parse_command(const char __user *buffer, size_t count)
{
	char s[4];

	if (!count)
		return 0;
	if (count > 3)
		return -EINVAL;
	if (copy_from_user(s, buffer, count))
		return -EFAULT;

It isn't possible for count to be anything other than 1, 2 or 3 there,
and it is very surprising that gcc can't see it.

This might be worth filing a gcc bug for.

	-hpa

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