linux-kernel - Re: [PATCH] module: make module symbols visible after init

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Message-ID: <20200604085712.GD22497@linux-b0ei>
Date:   Thu, 4 Jun 2020 10:57:12 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Cheng Jian <cj.chengjian@...wei.com>
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        chenwandun@...wei.com, xiexiuqi@...wei.com,
        bobo.shaobowang@...wei.com, huawei.libin@...wei.com,
        jeyu@...nel.org, jikos@...nel.org
Subject: Re: [PATCH] module: make module symbols visible after init

On Wed 2020-06-03 14:12:00, Cheng Jian wrote:
> When lookup the symbols of module by module_kallsyms_lookup_name(),
> the symbols address is visible only if the module's status isn't
> MODULE_STATE_UNFORMED, This is problematic.
> 
> When complete_formation is done, the state of the module is modified
> to MODULE_STATE_COMING, and the symbol of module is visible to the
> outside.
> 
> At this time, the init function of the module has not been called,
> so if the address of the function symbol has been found and called,
> it may cause some exceptions.

It is really handful that module symbols can be found already when
the module is MODULE_STATE_COMING state. It is used by livepatching,
ftrace, and maybe some other subsystems.

The problem is that nobody is allowed to use (call) module symbols
before mod->init() is called and the module is moved to
MODULE_STATE_LIVE.

By other words. Any code that calls module symbols before the module
is fully initialized is buggy. The caller should get fixed,
not the kallsyms side.

Have you seen such a problem in the real life, please?

Best Regards,
Petr