linux-kernel - Re: [PATCH 1/2] livepatch: Initialize shadow variables by init function safely

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Date:   Wed, 14 Mar 2018 14:28:57 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Jiri Kosina <jikos@...nel.org>, Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jessica Yu <jeyu@...nel.org>, Nicolai Stange <nstange@...e.de>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] livepatch: Initialize shadow variables by init
 function safely

On Tue, Mar 13, 2018 at 04:54:47PM +0100, Petr Mladek wrote:
> The existing API allows to pass a sample data to initialize the shadow
> data. It works well when the data are position independent. But it fails
> miserably when we need to set a pointer to the shadow structure itself.
> 
> Unfortunately, we might need to initialize the pointer surprisingly
> often because of struct list_head. It is even worse because the list
> might be hidden in other common structures, for example, struct mutex,
> struct wait_queue_head.

Can you provide a specific example in the changelog of where this was
needed?

> This patch makes the API more safe. A custom init function and data
> are passed to klp_shadow_*alloc() functions instead of the sample data.
> 
> Note that the init_data are not longer a template for the shadow->data.
> It might point to any data that might be necessary when the init
> function is called.
> 
> In addition, the newly allocated shadow structure is initialized
> only when it is really used.

I don't understand this sentence.  It makes it sound like the init
function is called when you do klp_shadow_get().  However, looking at
the code, the init function is always called after allocation.

> For this, the init function must be
> called under klp_shadow_lock. On one hand, this adds a risk of
> ABBA deadlocks. On the other hand, it allows to do some operations
> safely. For example, we could add the new structure into an
> existing list.

> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> index 4754f01c1abb..fc7c64ce0992 100644
> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -186,11 +186,20 @@ static inline bool klp_have_reliable_stack(void)
>  	       IS_ENABLED(CONFIG_HAVE_RELIABLE_STACKTRACE);
>  }
>  
> +struct klp_shadow;

Why is this forward struct declaration needed?

> @@ -150,6 +145,23 @@ static void *__klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data,
>  		goto exists;
>  	}
>  
> +	new_shadow->obj = obj;
> +	new_shadow->id = id;
> +
> +	if (init_func) {
> +		int err;
> +
> +		err = init_func(obj, new_shadow->data, init_data);

Am I hallucinating, or will new_shadow->data always be NULL?  How did it
even work before?

-- 
Josh