linux-kernel - Re: [PATCH v4] livepatch: Prevent to apply the patch once coming module notifier fails

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Message-ID: <20150514143031.GA28717@treble.redhat.com>
Date:	Thu, 14 May 2015 09:30:31 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Minfei Huang <mnfhuang@...il.com>
Cc:	sjenning@...hat.com, jkosina@...e.cz, vojtech@...e.cz,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
	mhuang@...hat.com
Subject: Re: [PATCH v4] livepatch: Prevent to apply the patch once coming
 module notifier fails

On Thu, May 14, 2015 at 09:51:07AM +0800, Minfei Huang wrote:
> The previous patches can be applied, once the corresponding module is
> loaded. In general, the patch will do relocation (if necessary) and
> obtain/verify function address before we start to enable patch.
> 
> There are three different situations in which the coming module notifier
> can fail:
> 
> 1) relocations are not applied for some reason. In this case kallsyms
> for module symbol is not called at all. The patch is not applied to the
> module. If the user disable and enable patch again, there is possible
> bug in klp_enable_func. If the user specified func->old_addr for some
> function in the module (and he shouldn't do that, but nevertheless) our
> warning would not catch it, there will be something wrong with the
> ftrace.
> 
> 2) relocations are applied successfully, but kallsyms lookup fails. In
> this case func->old_addr can be correct for all previous lookups, 0 for
> current failed one, and "unspecified" for the rest. If we undergo the
> same scenario as in 1, the behaviour differs for three cases, but the
> patch is not enable anyway.
> 
> 3) the object is initialized, but klp_enable_object fails in the
> notifier due to possible ftrace error. Since it is improbable that
> ftrace would heal itself in the future, we would get those errors
> everytime the patch is enabled.
> 
> In order to fix above situations, we can make obj->mod to NULL, if the
> coming modified notifier fails.
> 
> Signed-off-by: Minfei Huang <mnfhuang@...il.com>
> ---
> v3:
> - modify the code style
> v2:
> - add the error message to make it more friendly
> - modify the commit log, base on the mbenes@...e.cz suggesting
> v1:
> - modify the commit log, describe the issue more details
> ---
>  kernel/livepatch/core.c | 30 +++++++++++++++++++-----------
>  1 file changed, 19 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index 284e269..d4603e7 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -883,7 +883,7 @@ int klp_register_patch(struct klp_patch *patch)
>  }
>  EXPORT_SYMBOL_GPL(klp_register_patch);
>  
> -static void klp_module_notify_coming(struct klp_patch *patch,
> +static int klp_module_notify_coming(struct klp_patch *patch,
>  				     struct klp_object *obj)
>  {
>  	struct module *pmod = patch->mod;
> @@ -891,22 +891,24 @@ static void klp_module_notify_coming(struct klp_patch *patch,
>  	int ret;
>  
>  	ret = klp_init_object_loaded(patch, obj);
> -	if (ret)
> -		goto err;
> +	if (ret) {
> +		pr_warn("failed to initialize patch '%s' for module '%s' (%d)\n",
> +			pmod->name, mod->name, ret);
> +		goto out;
> +	}
>  
>  	if (patch->state == KLP_DISABLED)
> -		return;
> +		goto out;
>  
>  	pr_notice("applying patch '%s' to loading module '%s'\n",
>  		  pmod->name, mod->name);
>  
>  	ret = klp_enable_object(obj);
> -	if (!ret)
> -		return;
> -
> -err:
> -	pr_warn("failed to apply patch '%s' to module '%s' (%d)\n",
> -		pmod->name, mod->name, ret);
> +	if (ret)
> +		pr_warn("failed to apply patch '%s' to module '%s' (%d)\n",
> +			pmod->name, mod->name, ret);
> +out:
> +	return ret;

One more minor comment: the out label isn't needed.  Instead of "goto
out", they can just return directly.

Other than that, it looks good to me.

Thanks!

-- 
Josh
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