linux-kernel - Re: [RFC][PATCH][bugfix] more checks for negative f

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Date:	Thu, 17 Sep 2009 19:54:00 +0900 (JST)
From:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
To:	"Wu Fengguang" <fengguang.wu@...el.com>
Cc:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hugh.dickins@...cali.co.uk" <hugh.dickins@...cali.co.uk>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>
Subject: Re: [RFC][PATCH][bugfix] more checks for negative f_pos handling v4

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@...el.com> wrote:
>>
>> > On Thu, Sep 17, 2009 at 02:51:00PM +0800, KAMEZAWA Hiroyuki wrote:
>> > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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@...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.

Thanks,
-Kame


> Thanks,
> Fengguang
>
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