linux-kernel - Re: [PATCH 7/7] uprobes/x86: Introduce uprobe_xol_ops and arch

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Date:	Wed, 02 Apr 2014 12:46:24 -0700
From:	Jim Keniston <jkenisto@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	David Long <dave.long@...aro.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Jonathan Lebon <jlebon@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/7] uprobes/x86: Introduce uprobe_xol_ops and
 arch_uprobe->ops

On Mon, 2014-03-31 at 21:44 +0200, Oleg Nesterov wrote:
...
> +/*
> + * Adjust the return address pushed by a call insn executed out of line.
> + */
> +static int adjust_ret_addr(unsigned long sp, long correction)
> +{
> +	int rasize, ncopied;
> +	long ra = 0;
> +
> +	if (is_ia32_task())
> +		rasize = 4;
> +	else
> +		rasize = 8;
> +
> +	ncopied = copy_from_user(&ra, (void __user *)sp, rasize);
> +	if (unlikely(ncopied))
> +		return -EFAULT;
> +
> +	ra += correction;
> +	ncopied = copy_to_user((void __user *)sp, &ra, rasize);
> +	if (unlikely(ncopied))
> +		return -EFAULT;
> +
> +	return 0;
> +}

This isn't your bug, Oleg -- you're just moving code -- but consider
taking this opportunity to fix it...

"ncopied" is a misnomer here.  copy_from_user() and copy_to_user()
return the number of bytes that could NOT be copied.  Once upon a time
(in uprobes's pre-upstream days), this was called "nleft" -- i.e., the
number of bytes left uncopied.  A more accurate name like "nleft" or
"nmissed" or "nr_uncopied" might yield less confusion in the future --
or just dispense with the variable altogether.

arch_uretprobe_hijack_return_addr() has this same problem, although
there we need the variable, because if zero bytes of the return address
are overwritten, we can fail more gracefully.

Jim

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