linux-kernel - Re: [PATCH net-next v2 3/8] x86/jump_label: expect default_nop if static

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Message-ID: <20131005200558.3be2f841@gandalf.local.home>
Date:	Sat, 5 Oct 2013 20:05:58 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>, x86@...nel.org
Subject: Re: [PATCH net-next v2 3/8] x86/jump_label: expect default_nop if
 static_key gets enabled on boot-up

On Sun,  6 Oct 2013 01:20:53 +0200
Hannes Frederic Sowa <hannes@...essinduktion.org> wrote:

> net_get_random_once(intrduced in the next patch) uses static_keys in
> a way that they get enabled on boot-up instead of replaced with an
> ideal_nop. So check for default_nop on initial enabling.
> 
> Other architectures don't check for this.

But they should.

> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Jason Baron <jbaron@...hat.com>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: x86@...nel.org
> Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
> ---
>  arch/x86/kernel/jump_label.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
> index ee11b7d..26d5a55 100644
> --- a/arch/x86/kernel/jump_label.c
> +++ b/arch/x86/kernel/jump_label.c
> @@ -42,15 +42,27 @@ static void __jump_label_transform(struct jump_entry *entry,
>  				   int init)
>  {
>  	union jump_code_union code;
> +	const unsigned char default_nop[] = { STATIC_KEY_INIT_NOP };
>  	const unsigned char *ideal_nop = ideal_nops[NOP_ATOMIC5];
>  
>  	if (type == JUMP_LABEL_ENABLE) {
> -		/*
> -		 * We are enabling this jump label. If it is not a nop
> -		 * then something must have gone wrong.
> -		 */
> -		if (unlikely(memcmp((void *)entry->code, ideal_nop, 5) != 0))
> -			bug_at((void *)entry->code, __LINE__);
> +		if (init) {
> +			/*
> +			 * Jump label is enabled for the first time.
> +			 * So we expect a default_nop...
> +			 */
> +			if (unlikely(memcmp((void *)entry->code, default_nop, 5)
> +				     != 0))
> +				bug_at((void *)entry->code, __LINE__);
> +		} else {
> +			/*
> +			 * ...otherwise expect an ideal_nop. Otherwise
> +			 * something went horribly wrong.
> +			 */
> +			if (unlikely(memcmp((void *)entry->code, ideal_nop, 5)
> +				     != 0))
> +				bug_at((void *)entry->code, __LINE__);
> +		}

I don't know if I like this change. This is similar to a bug we had
with the Xen folks, where they didn't realize that jump labels are not
suppose to be used (or set) before jump_label_init() is called.

I'll have to take a deeper look at this on Monday.

-- Steve

>  
>  		code.jump = 0xe9;
>  		code.offset = entry->target -
> @@ -63,7 +75,6 @@ static void __jump_label_transform(struct jump_entry *entry,
>  		 * are converting the default nop to the ideal nop.
>  		 */
>  		if (init) {
> -			const unsigned char default_nop[] = { STATIC_KEY_INIT_NOP };
>  			if (unlikely(memcmp((void *)entry->code, default_nop, 5) != 0))
>  				bug_at((void *)entry->code, __LINE__);
>  		} else {

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