linux-kernel - Re: [PATCH v3 0/2] livepatch: Allocate and free shadow variables more safely

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Date:   Mon, 16 Apr 2018 14:57:42 -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 v3 0/2] livepatch: Allocate and free shadow variables
 more safely

On Mon, Apr 16, 2018 at 01:36:45PM +0200, Petr Mladek wrote:
> Nikolai started to use shadow variables and found that list_head
> is not initialized correctly using the existing API.
> 
> The problem is that LIST_HEAD_INIT() set pointers to itself.
> klp_shadow_alloc() then copies these pointers to another location.
> 
> It would be possible to solve this by a late initialization. But
> this does not work well with klp_shadow_get_or_alloc(). Also this
> behavior is error prone. Note that list_head might be hidden in
> another structure, for example mutex.
> 
> I suggest to change the API and allow to initialize the shadow
> variables using a custom callback. I think that we have already
> talked about this but we decided to go the easier way. It seems
> that it was a bad decision.
> 
> I send also patch that add a symmetric callback to the klp_shadow_free()
> functions. It might be handy especially for klp_shadow_free_all().
> 
> 
> Changes against v2:
> 
>   + Use pr_err() instead of WARN() when ctor() fails [Josh]
>   + Some typos [Mirek]
> 
> 
> Changes against v1:
> 
>   + Rename init/free callbacks to ctor/dtor
>   + Misc update of comments and commit messages
>   + Fix sizeof() in the sample code in shadow-vars.txt

Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>

-- 
Josh