linux-kernel - Re: [RFC PATCH v3 2/6] uretprobes/x86: hijack return address

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Date:	Fri, 1 Mar 2013 16:51:32 +0530
From:	Ananth N Mavinakayanahalli <ananth@...ibm.com>
To:	Anton Arapov <anton@...hat.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Josh Stone <jistone@...hat.com>,
	Frank Eigler <fche@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC PATCH v3 2/6] uretprobes/x86: hijack return address

On Fri, Mar 01, 2013 at 12:00:43PM +0100, Anton Arapov wrote:
> On Fri, Mar 01, 2013 at 11:15:36AM +0530, Ananth N Mavinakayanahalli wrote:
> > On Thu, Feb 28, 2013 at 12:00:11PM +0100, Anton Arapov wrote:

...

> > > +extern unsigned long arch_uretprobe_hijack_return_addr(unsigned long
> > > +		rp_trampoline_vaddr, struct pt_regs *regs)
> > > +{
> > > +	int rasize, ncopied;
> > > +	unsigned long orig_ret_vaddr = 0; /* clear high bits for 32-bit apps */
> > > +
> > > +	rasize = is_ia32_task() ? 4 : 8;
> > > +	ncopied = copy_from_user(&orig_ret_vaddr, (void __user *)regs->sp, rasize);
> > > +	if (unlikely(ncopied))
> > 
> > What if ncopied < rasize? Agreed that the upper order bits can be 0, but should
> > you not validate ncopied == rasize?
> 
>   Function returns 0 in case copy_from_user() was not able to copy
> return address entirely, and "if (ncopied)" makes sure of it. We 
> can't continue if we have no correct return address.
> 
>   copy_from_user() returns number of bytes that were *not* copied,
> thus "ncopied == rasize" means copy_from_user() was not able to copy
> *all* bytes. I don't see the point of such check here.
> 
>   Or am I missing anything?

You are right... my bad.

Ananth

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