linux-kernel - Re: [PATCH 0/7] livepatch,module: Remove .klp.arch and module_disable

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Message-ID: <20200416153131.GC6164@linux-8ccs.fritz.box>
Date:   Thu, 16 Apr 2020 17:31:31 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] livepatch,module: Remove .klp.arch and
 module_disable_ro()

+++ Josh Poimboeuf [15/04/20 11:17 -0500]:
>On Wed, Apr 15, 2020 at 04:24:15PM +0200, Peter Zijlstra wrote:
>> > It bothers me that both the notifiers and the module init() both see the
>> > same MODULE_STATE_COMING state, but only in the former case is the text
>> > writable.
>> >
>> > I think it's cognitively simpler if MODULE_STATE_COMING always means the
>> > same thing, like the comments imply, "fully formed" and thus
>> > not-writable:
>> >
>> > enum module_state {
>> > 	MODULE_STATE_LIVE,	/* Normal state. */
>> > 	MODULE_STATE_COMING,	/* Full formed, running module_init. */
>> > 	MODULE_STATE_GOING,	/* Going away. */
>> > 	MODULE_STATE_UNFORMED,	/* Still setting it up. */
>> > };
>> >
>> > And, it keeps tighter constraints on what a notifier can do, which is a
>> > good thing if we can get away with it.
>>
>> Moo! -- but jump_label and static_call are on the notifier chain and I
>> was hoping to make it cheaper for them. Should we perhaps weane them off the
>> notifier and, like ftrace/klp put in explicit calls?
>>
>> It'd make the error handling in prepare_coming_module() a bigger mess,
>> but it should work.
>
>So you're wanting to have jump labels and static_call do direct writes
>instead of text pokes, right?  Makes sense.
>
>I don't feel strongly about "don't let module notifiers modify text".
>
>But I still not a fan of the fact that COMING has two different
>"states".  For example, after your patch, when apply_relocate_add() is
>called from klp_module_coming(), it can use memcpy(), but when called
>from klp module init() it has to use text poke.  But both are COMING so
>there's no way to look at the module state to know which can be used.

This is a good observation, thanks for bringing it up. I agree that we
should strive to be consistent with what the module states mean. In my
head, I think it is easiest to assume/establish the following meanings
for each module state:

MODULE_STATE_UNFORMED - no protections. relocations, alternatives,
ftrace module initialization, etc. any other text modifications are
in the process of being applied. Direct writes are permissible.

MODULE_STATE_COMING - module fully formed, text modifications are
done, protections applied, module is ready to execute init or is
executing init.

I wonder if we could enforce the meaning of these two states more
consistently without needing to add another module state.

Regarding Peter's patches, with the set_all_modules_text_*() api gone,
and ftrace reliance on MODULE_STATE_COMING gone (I think?), is there
anything preventing ftrace_module_init+enable from being called
earlier (i.e., before complete_formation()) while the module is
unformed? Then you don't have to move module_enable_ro/nx later and we
keep the MODULE_STATE_COMING semantics. And if we're enforcing the
above module state meanings, I would also be OK with moving jump_label
and static_call out of the coming notifier chain and making them
explicit calls while the module is still writable.

Sorry in advance if I missed anything above, I'm still trying to wrap
my head around which callers need what module state and what module
permissions :/

Jessica