linux-kernel - Re: module: save load_info for livepatch modules

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Message-ID: <20151114020950.GA21414@packer-debian-8-amd64.digitalocean.com>
Date:	Fri, 13 Nov 2015 21:09:50 -0500
From:	Jessica Yu <jeyu@...hat.com>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Petr Mladek <pmladek@...e.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>, linux-api@...r.kernel.org,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: module: save load_info for livepatch modules

+++ Miroslav Benes [13/11/15 13:56 +0100]:
>On Fri, 13 Nov 2015, Miroslav Benes wrote:
>
>> I agree this seems like the best approach. So if we preserve
>> mod_arch_syminfo (in case of s390) we should free it not in
>> module_finalize, but somewhere in free_module... where
>> module_arch_cleanup() is called... and also module_arch_freeing_init() is
>> called there too. And what you find there for s390 is
>>
>> 	vfree(mod->arch.syminfo);
>> 	mod->arch.syminfo = NULL;
>>
>> Well, it does nothing here, because mod->arch.syminfo is already NULL. It
>> was freed in module_finalize. So we can even remove this code from
>> module_finalize and all should be fine. At least for s390.
>
>Which is not true because module_arch_freeing_init is also called from
>do_init_module, called from load_module. So we should move it to
>module_arch_cleanup.
>
>That code is like a maze without Ariadne's thread.

Heh, I agree with that sentiment.

I am slightly confused about the s390 code, and whether the authors
originally intended for that double vfree() to happen in both
module_finalize() and module_arch_freeing_init() (called from
do_init_module). Seems like a mistake. If module load succeeds,
do_init_module calls module_arch_freeing_init(). And if load_module
fails halfway through, both module_deallocate() and free_module() will
also call module_arch_freeing_init(). I feel like that vfree should
only happen once in module_arch_freeing_init() and not in
module_finalize(). If we can remove the double vfree() code from
module_finalize(), we can copy the mod_arch_specific safely before the
call to do_init_module().

Jessica
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