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Message-ID: <alpine.DEB.2.21.1906261643200.32342@nanos.tec.linutronix.de>
Date: Wed, 26 Jun 2019 16:44:45 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
cc: Miroslav Benes <mbenes@...e.cz>,
Steven Rostedt <rostedt@...dmis.org>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Jessica Yu <jeyu@...nel.org>, Jiri Kosina <jikos@...nel.org>,
Joe Lawrence <joe.lawrence@...hat.com>,
linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
Johannes Erdfelt <johannes@...felt.com>,
Ingo Molnar <mingo@...nel.org>, mhiramat@...nel.org,
torvalds@...ux-foundation.org
Subject: Re: [PATCH 1/3] module: Fix livepatch/ftrace module text permissions
race
On Wed, 26 Jun 2019, Petr Mladek wrote:
> On Wed 2019-06-26 10:22:45, Miroslav Benes wrote:
> It is similar problem that has been solved by 2d1e38f56622b9bb5af8
> ("kprobes: Cure hotplug lock ordering issues"). This commit solved
> it by always taking cpu_hotplug_lock.rw_sem before text_mutex inside.
>
> If we follow the lock ordering then ftrace has to take text_mutex
> only when stop_machine() is not called or from code called via
> stop_machine() parameter.
>
> This is not easy with the current design. For example, arm calls
> set_all_modules_text_rw() already in ftrace_arch_code_modify_prepare(),
> see arch/arm/kernel/ftrace.c. And it is called:
>
> + outside stop_machine() from ftrace_run_update_code()
> + without stop_machine() from ftrace_module_enable()
>
> A conservative solution for 5.2 release would be to move text_mutex
> locking from the generic kernel/trace/ftrace.c into
> arch/x86/kernel/ftrace.c:
>
> ftrace_arch_code_modify_prepare()
> ftrace_arch_code_modify_post_process()
>
> It should be enough to fix the original problem because
> x86 is the only architecture that calls set_all_modules_text_rw()
> in ftrace path and supports livepatching at the same time.
Looks correct, but I've paged out all the gory details vs. lock ordering in
that area.
Thanks,
tglx
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