linux-kernel - Re: [PATCH v8 7/8] livepatch: Correctly handle atomic replace for not yet loaded modules

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Date:   Thu, 22 Feb 2018 22:00:28 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Petr Mladek <pmladek@...e.com>
cc:     Jiri Kosina <jikos@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jessica Yu <jeyu@...nel.org>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 7/8] livepatch: Correctly handle atomic replace for
 not yet loaded modules

On Wed, 21 Feb 2018, Petr Mladek wrote:

> The atomic replace feature uses dynamically allocated struct klp_func to
> handle functions that will not longer be patched. These structures are

s/not longer/no longer/, but "handle functions that will not be patched 
any longer" may be even better.

> of the type KLP_FUNC_NOP. They cause the ftrace handler to jump to
> the original code. But the address of the original code is not known
> until the patched module is loaded.
> 
> This patch allows the late initialization. Also it adds a sanity check
> into the ftrace handler.
> 
> Alternative solution would be to do not set the address at all. The ftrace

"Alternative solution would be not to set the address at all." ?

> handler could just return to the original code when NOP struct klp_func
> is used. But this would require another changes. For example, in the stack
> checking. Note that NOP structures might be available even when the patch
> is being disabled. This would happen when the patch enable transition is
> reverted.
> 
> Signed-off-by: Petr Mladek <pmladek@...e.com>
> ---
>  kernel/livepatch/core.c  | 16 ++++++++++++++--
>  kernel/livepatch/patch.c |  5 +++++
>  2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index ad508a86b2f9..da1438d47d83 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -945,8 +945,12 @@ static void klp_free_object_loaded(struct klp_object *obj)
>  
>  	obj->mod = NULL;
>  
> -	klp_for_each_func(obj, func)
> +	klp_for_each_func(obj, func) {
>  		func->old_addr = 0;
> +
> +		if (klp_is_func_type(func, KLP_FUNC_NOP))
> +			func->new_func = NULL;
> +	}
>  }
>  
>  /*
> @@ -984,7 +988,12 @@ static void klp_free_patch(struct klp_patch *patch)
>  
>  static int klp_init_func(struct klp_object *obj, struct klp_func *func)
>  {
> -	if (!func->old_name || !func->new_func)
> +	if (!func->old_name)
> +		return -EINVAL;
> +
> +	/* NOPs do not know the address until the patched module is loaded. */
> +	if (!func->new_func &&
> +	    (!klp_is_func_type(func, KLP_FUNC_NOP) || klp_is_object_loaded(obj)))
>  		return -EINVAL;

If we changed the order of klp_init_func() and klp_init_object_loaded() 
calls in klp_init_object(), the hunk would not be needed. Is that correct? 
But that would require more changes elsewhere, so it's not worth it, I 
think.

>  	INIT_LIST_HEAD(&func->stack_node);
> @@ -1039,6 +1048,9 @@ static int klp_init_object_loaded(struct klp_patch *patch,
>  			return -ENOENT;
>  		}
>  
> +		if (klp_is_func_type(func, KLP_FUNC_NOP))
> +			func->new_func = (void *)func->old_addr;

Is there a reason why you left the same assignment in 
klp_alloc_func_nop()? This one is enough, no?

Miroslav