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Date: Thu, 17 Sep 2009 19:58:02 +0900 (JST)
Subject: Re: [RFC][PATCH][bugfix] more checks for negative f_pos handling v4
From: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>
To: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>
Cc: "Wu Fengguang" <fengguang.wu@intel.com>,
       "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>,
       "viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
       "Andrew Morton" <akpm@linux-foundation.org>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "hugh.dickins@tiscali.co.uk" <hugh.dickins@tiscali.co.uk>,
       "oleg@redhat.com" <oleg@redhat.com>,
       "xiyou.wangcong@gmail.com" <xiyou.wangcong@gmail.com>
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KAMEZAWA Hiroyuki wrote:
> Wu Fengguang wrote:
>> On Thu, Sep 17, 2009 at 03:23:24PM +0800, KAMEZAWA Hiroyuki wrote:
>>> On Thu, 17 Sep 2009 15:14:28 +0800
>>> Wu Fengguang <fengguang.wu@intel.com> wrote:
>>>
>>> > On Thu, Sep 17, 2009 at 02:51:00PM +0800, KAMEZAWA Hiroyuki wrote:
>>> > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>>> > >
>>> > > Now, rw_verify_area() checsk f_pos is negative or not. And if
>>> > > negative, returns -EINVAL.
>>> > >
>>> > > But, some special files as /dev/(k)mem and /proc/<pid>/mem etc..
>>> > > has negative offsets. And we can't do any access via read/write
>>> > > to the file(device).
>>> > >
>>> > > This patch introduce a flag S_VERYBIG and allow negative file
>>> > > offsets for big files. (usual files don't allow it.)
>>> > >
>>> > > Changelog: v3->v4
>>> > >  - make changes in mem.c aligned.
>>> > >  - change __negative_fpos_check() to return int.
>>> > >  - fixed bug in "pos" check.
>>> > >  - added comments.
>>> > >
>>> > > Changelog: v2->v3
>>> > >  - fixed bug in rw_verify_area (it cannot be compiled)
>>> > >
>>> > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>>> > > ---
>>> > >  drivers/char/mem.c |   23 +++++++++++++----------
>>> > >  fs/proc/base.c     |    2 ++
>>> > >  fs/read_write.c    |   22 ++++++++++++++++++++--
>>> > >  include/linux/fs.h |    2 ++
>>> > >  4 files changed, 37 insertions(+), 12 deletions(-)
>>> > >
>>> > > Index: mmotm-2.6.31-Sep14/fs/read_write.c
>>> > > ===================================================================
>>> > > --- mmotm-2.6.31-Sep14.orig/fs/read_write.c
>>> > > +++ mmotm-2.6.31-Sep14/fs/read_write.c
>>> > > @@ -205,6 +205,21 @@ bad:
>>> > >  }
>>> > >  #endif
>>> > >
>>> > > +static int
>>> > > +__negative_fpos_check(struct inode *inode, loff_t pos, size_t
>>> count)
>>> > > +{
>>> > > +	/*
>>> > > +	 * pos or pos+count is negative here, check overflow.
>>> > > +	 * too big "count" will be caught in rw_verify_area().
>>> > > +	 */
>>> > > +	if ((pos < 0) && (pos + count < pos))
>>> > > +		return -EOVERFLOW;
>>> >
>>> > This returns -EOVERFLOW when pos=-10 and count=1. What's the
>>> intention?
>>>   Hmm ?
>>>
>>>   pos+count=-9 > -10 ? it's ok. no -EOVERFLOW
>>>
>>>   pos=-10, count=11,
>>>   pos+count=1 > -10, then overflow.
>>
>> Hmm, it seems less confusing to do
>>
>> static int __negative_fpos_check(struct inode *inode,
>>                                  unsigned long pos,
>>                                  unsigned long count)
>> {
>>         if (pos + count < pos)
>>                 return -EOVERFLOW;
>>         ...
>> }
>>
> have to avoid pos == LONGLONGMAX case.
>
Ah, you ask me to do cast from loff_t to unsigned long long ?

Not making much difference, I think. This is usual math.
But ok, I don't want to explain again.
If I post v5, I'll do.

Thanks,
-Kame


> Thanks,
> -Kame
>
>
>> Thanks,
>> Fengguang
>>
>> --
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