linux-kernel - Re: [PATCH v3 2/2] livepatch: add atomic replace

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Date:   Fri, 6 Oct 2017 17:32:47 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jason Baron <jbaron@...mai.com>
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        jeyu@...nel.org, jikos@...nel.org, mbenes@...e.cz, pmladek@...e.com
Subject: Re: [PATCH v3 2/2] livepatch: add atomic replace

On Wed, Sep 27, 2017 at 11:41:30PM -0400, Jason Baron wrote:
> Since 'atomic replace' has completely replaced all previous livepatch
> modules, it explicitly disables all previous livepatch modules. However,
> previous livepatch modules that have been replaced, can be re-enabled
> if they have the 'replace' flag set. In this case the set of 'nop'
> functions is re-calculated, such that it replaces all others.
> 
> For example, if kpatch-a.ko and kpatch-b.ko have the 'replace' flag set
> then:
> 
> # insmod kpatch-a.ko
> # insmod kpatch-b.ko
> 
> At this point we have:
> 
> # cat /sys/kernel/livepatch/kpatch-a/enabled
> 0
> 
> # cat /sys/kernel/livepatch/kpatch-b/enabled
> 1
> 
> To revert to the kpatch-a state we can do:
> 
> echo 1 > /sys/kernel/livepatch/kpatch-a/enabled
> 
> And now we have:
> 
> # cat /sys/kernel/livepatch/kpatch-a/enabled
> 1
> 
> # cat /sys/kernel/livepatch/kpatch-b/enabled
> 0

I don't really like allowing a previously replaced patch to replace the
current patch.  It's just more unnecessary complexity.  If the user
wants to atomically revert back to kpatch-a, they should be able to:

  rmmod kpatch-a
  insmod kpatch-a.ko

> Note that it may be possible to unload (rmmod) replaced patches in some
> cases based on the consistency model, when we know that all the functions
> that are contained in the patch may no longer be in used, however its
> left as future work, if this functionality is desired.

If you don't allow a previously replaced patch to be enabled again, I
think it would be trivial to let it be unloaded.

> Also, __klp_enable_patch() calls klp_add_nops(), which necessitated moving
> a bunch of existing functions before __klp_enable_patch(). So there is a
> bit of churn in moving functions that are not modified.

To make review easier, can you put the moving of functions into a
separate patch?

-- 
Josh