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

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Date:   Wed, 14 Mar 2018 15:43:01 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Petr Mladek <pmladek@...e.com>
Cc:     Jiri Kosina <jikos@...nel.org>, Miroslav Benes <mbenes@...e.cz>,
        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 03/14/2018 03:28 PM, Josh Poimboeuf wrote:
> On Tue, Mar 13, 2018 at 04:54:47PM +0100, Petr Mladek wrote:
> 
>> 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?

Compiles ok w/o it, so shouldn't be 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?
>
struct klp_shadow {
        struct hlist_node node;
        struct rcu_head rcu_head;
        void *obj;
        unsigned long id;
        char data[];            << not a pointer
};

In the past, this function would allocate the klp_shadow struct size
accordingly, then memcpy in its data contents.  This patch pushes the
responsibility of data initialization out to the init_func().

-- Joe