linux-kernel - Re: [PATCH 1/4] cpuset: Remove unneeded NODEMASK_ALLOC() in cpuset_sprintf

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Date:	Fri, 18 Feb 2011 10:22:49 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Paul Menage <menage@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	David Rientjes <rientjes@...gle.com>,
	缪 勰 <miaox@...fujitsu.com>,
	linux-mm@...ck.org
Subject: Re: [PATCH 1/4] cpuset: Remove unneeded NODEMASK_ALLOC() in cpuset_sprintf_memlist()

Paul Menage wrote:
> On Wed, Feb 16, 2011 at 5:49 PM, Li Zefan <lizf@...fujitsu.com> wrote:
>> It's not necessary to copy cpuset->mems_allowed to a buffer
>> allocated by NODEMASK_ALLOC(). Just pass it to nodelist_scnprintf().
>>
>> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> 
> Acked-by: Paul Menage <menage@...gle.com>
> 
> The only downside is that we're now doing more work (and more complex
> work) inside callback_mutex, but I guess that's OK compared to having
> to do a memory allocation. (I poked around in lib/vsprintf.c and I
> couldn't see any cases where it might allocate memory, but it would be
> particularly bad if there was any way to trigger an Oops.)
> 
>> ---
>>  kernel/cpuset.c |   10 +---------
>>  1 files changed, 1 insertions(+), 9 deletions(-)
>>
>> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
>> index 10f1835..f13ff2e 100644
>> --- a/kernel/cpuset.c
>> +++ b/kernel/cpuset.c
>> @@ -1620,20 +1620,12 @@ static int cpuset_sprintf_cpulist(char *page, struct cpuset *cs)
>>
>>  static int cpuset_sprintf_memlist(char *page, struct cpuset *cs)
>>  {
>> -       NODEMASK_ALLOC(nodemask_t, mask, GFP_KERNEL);
>>        int retval;
>>
>> -       if (mask == NULL)
>> -               return -ENOMEM;
>> -
> 
> And this was particularly broken since the only caller of
> cpuset_sprintf_memlist() doesn't handle a negative error response
> anyway and would then overwrite byte 4083 on the preceding page with a
> '\n'. And then since the (size_t)(s-page) that's passed to
> simple_read_from_buffer() would be a very large number, it would write
> arbitrary (user-controlled) amounts of kernel data to the userspace
> buffer.
> 
> Maybe we could also rename 'retval' to 'count' in this function (and
> cpuset_sprintf_cpulist()) to make it clearer that callers don't expect
> negative error values?
> 

Good spot!
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