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

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Date:	Thu, 17 Sep 2009 17:42:03 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"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

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;
        ...
}

Thanks,
Fengguang

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