linux-kernel - Re: [PATCH] Documentation/livepatch: remove the limitation for schedule() patching

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Message-ID: <20170106150140.GX14894@pathway.suse.cz>
Date:   Fri, 6 Jan 2017 16:01:40 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     jpoimboe@...hat.com, jeyu@...hat.com, jikos@...nel.org,
        corbet@....net, live-patching@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Documentation/livepatch: remove the limitation for
 schedule() patching

On Fri 2017-01-06 15:00:45, Miroslav Benes wrote:
> The Limitations section of the documentation describes the impossibility
> to livepatch anything that is inlined to __schedule() function. This had
> been true till 4.9 kernel came. Thanks to commit 0100301bfdf5
> ("sched/x86: Rewrite the switch_to() code") from Brian Gerst there is
> __switch_to_asm function now (implemented in assembly) called properly
> from context_switch(). RIP is thus saved on the stack and a task would
> return to proper version of __schedule() et al. functions.
> 
> Of course __switch_to_asm() is not patchable for the reason described in
> the section. But there is no __fentry__ call and I cannot imagine a
> reason to do it anyway.
> 
> Therefore, remove the paragraphs from the section.
> 
> Signed-off-by: Miroslav Benes <mbenes@...e.cz>

It is great to get a feature for free ;-)

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr

---
> FWIW, I also tested this to be sure on top of the consistency model
> patch set. I patched schedule() function which calls __schedule() (it is
> impossible to patch it directly due to notrace attribute). It works well
> except...
> 
> 1. the patching process does not finish, because many tasks sleep in
> schedule. STOP/CONT signal does not help. I'll investigate.

Are these userspace processes or kthreads? Kthreads would cause
problems because they do not handle signals.


> 2. reversion of the process does not work as expected. The kernel
> crashes after the removal of the module. A task very likely slept in
> schedule and was not migrated properly. It might be because of the races
> in klp_reverse_transition() described by Petr, or might be somewhere
> else. I'll look into it.

I hope that I will be able to do another dive into the consistency
model patchset the following week.

Best Regards,
Petr