linux-kernel - Re: [PATCH] ftrace/module: Allow ftrace to make only loaded module text read-write

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Date:   Thu, 10 Oct 2019 11:03:26 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Jessica Yu <jeyu@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] ftrace/module: Allow ftrace to make only loaded module
 text read-write

On Thu, 10 Oct 2019 10:55:15 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> But we have an issue with the state of the module here, as it is still
> set as MODULE_STATE_UNFORMED. Let's look at what happens if we have:
> 
> 
> 	CPU0				CPU1
> 	----				----
>  echo function > current_tracer
> 				modprobe foo
> 				  enable foo functions to be traced
> 				  (foo function records not disabled)
>  echo nop > current_tracer
> 
>    disable all functions being
>    traced including foo functions
> 
>    arch calls set_all_modules_text_rw()
>     [skips UNFORMED modules, which foo still is ]
> 
> 				  set foo's text to read-only
> 				  foo's state to COMING
> 
>    tries to disable foo's functions
>    foo's text is read-only
> 
>    BUG trying to write to ro text!!!
> 
> 
> Like I said, this is very subtle. It may no longer be a bug on x86
> with your patches, but it will bug on ARM or anything else that still
> uses set_all_modules_text_rw() in the ftrace prepare code.

I guess I should have commented this, but the big reason for the split
between ftrace_module_init() and ftrace_module_enable() is that we add
the nops when the module is still UNFORMED, but we only enable it when
the module is COMING or beyond, and not UNFORMED.

-- Steve