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Date: Mon, 2 Mar 2015 11:21:54 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, rusty@rustcorp.com.au, mathieu.desnoyers@efficios.com,
        oleg@redhat.com, linux-kernel@vger.kernel.org, andi@firstfloor.org,
        rostedt@goodmis.org, tglx@linutronix.de,
        Seth Jennings <sjenning@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Miroslav Benes <mbenes@suse.cz>, Petr Mladek <pmladek@suse.cz>,
        Jiri Kosina <jkosina@suse.cz>
Subject: Re: [RFC][PATCH 1/9] klp: Fix obvious RCU fail
Message-ID: <20150302192154.GQ15405@linux.vnet.ibm.com>
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References: <20150228212447.381543289@infradead.org>
 <20150228213109.893891745@infradead.org>
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On Sat, Feb 28, 2015 at 10:24:48PM +0100, Peter Zijlstra wrote:
> While one must hold RCU-sched (aka. preempt_disable) for find_symbol()
> one must equally hold it over the use of the object returned.
> 
> The moment you release the RCU-sched read lock, the object can be dead
> and gone.
> 
> Cc: Seth Jennings <sjenning@redhat.com>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> Cc: Miroslav Benes <mbenes@suse.cz>
> Cc: Petr Mladek <pmladek@suse.cz>
> Cc: Jiri Kosina <jkosina@suse.cz>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: Rusty Russell <rusty@rustcorp.com.au>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> ---
>  kernel/livepatch/core.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -248,11 +248,12 @@ static int klp_find_external_symbol(stru
>  	/* first, check if it's an exported symbol */
>  	preempt_disable();
>  	sym = find_symbol(name, NULL, NULL, true, true);
> -	preempt_enable();
>  	if (sym) {
>  		*addr = sym->value;
> +		preempt_enable();
>  		return 0;
>  	}
> +	preempt_enable();
> 
>  	/* otherwise check if it's in another .o within the patch module */
>  	return klp_find_object_symbol(pmod->name, name, addr);
> 
>