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 19:01:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.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, Oct 10, 2019 at 10:55:15AM -0400, Steven Rostedt wrote:

> OK, so basically this moves the enabling of function tracing from
> within the ftrace_module_enable() code without releasing the
> ftrace_lock mutex.
> 
> 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 can't immediately follow, but I think we really should go there.

For now, something like this might work:

--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -34,6 +34,8 @@
 
 #ifdef CONFIG_DYNAMIC_FTRACE
 
+static int ftrace_poke_late = 0;
+
 int ftrace_arch_code_modify_prepare(void)
     __acquires(&text_mutex)
 {
@@ -43,12 +45,15 @@ int ftrace_arch_code_modify_prepare(void
 	 * ftrace has it set to "read/write".
 	 */
 	mutex_lock(&text_mutex);
+	ftrace_poke_late = 1;
 	return 0;
 }
 
 int ftrace_arch_code_modify_post_process(void)
     __releases(&text_mutex)
 {
+	text_poke_finish();
+	ftrace_poke_late = 0;
 	mutex_unlock(&text_mutex);
 	return 0;
 }
@@ -116,7 +121,10 @@ ftrace_modify_code_direct(unsigned long
 		return ret;
 
 	/* replace the text with the new text */
-	text_poke_early((void *)ip, new_code, MCOUNT_INSN_SIZE);
+	if (ftrace_poke_late)
+		text_poke_queue((void *)ip, new_code, MCOUNT_INSN_SIZE, NULL);
+	else
+		text_poke_early((void *)ip, new_code, MCOUNT_INSN_SIZE);
 	return 0;
 }