linux-kernel - Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe

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Message-ID: <20120302161247.GO8681@moon>
Date:	Fri, 2 Mar 2012 20:12:47 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file

On Fri, Mar 02, 2012 at 04:26:22PM +0100, Oleg Nesterov wrote:
...
> > Hi Oleg, sure you were right. I even think testing for
> > num_exe_file_vmas is not needed, since until real
> > executable VMA is read and mapped it might remain
> > zero. So I guess something like below should work.
> 
> No, you need to check num_exe_file_vmas != 0.
> 
> > +static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
> > +{
> > +	struct file *new_exe_file, *old_exe_file;
> > +
> > +	new_exe_file = fget(fd);
> > +	if (!new_exe_file)
> > +		return -EBADF;
> > +
> > +	down_write(&mm->mmap_sem);
> > +	old_exe_file = mm->exe_file;
> > +	mm->exe_file = new_exe_file;
> > +	up_write(&mm->mmap_sem);
> 
> This changes the current rule: mm->exe_file goes away once
> num_exe_file_vmas becomes zero.

Yes, Oleg, you're right of course! The set_mm_exe_file confused
me a bit (I didn't reply to your previous emails simply because
you were asking me to prove what exactly is broken when num_exe_file_vmas
greater 2 and we reassign new exe_file, and I didn't know what to reply
since I knew I was wrong in first assumption).

As a conclusion -- sure I need to check for num_exe_file_vmas to
not break current logic, thanks!

	Cyrill
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