Dynamic ftrace self test broken on ARM

[stefan@agner.ch (Stefan Agner)]

On 21.06.2018 03:29, Steven Rostedt wrote:
> On Thu, 21 Jun 2018 00:07:34 +0100
> Russell King - ARM Linux <linux@armlinux.org.uk> wrote:
> 
> 
>> > Any input from ARM folks?
>>
>> The same issues must exist on other architectures as ARM is not the only
>> architecture to implement read-only kernel text and dynamic ftrace, so
>> surely this problem isn't unique to ARM.
> 
> Probably because of the way set_kernel_text_ro() is implemented in
> other archs. For example, in x86, we have:
> 
> void set_kernel_text_ro(void)
> {
> 	unsigned long start = PFN_ALIGN(_text);
> 	unsigned long end = PFN_ALIGN(__stop___ex_table);
> 
> 	if (!kernel_set_to_readonly)
> 		return;
> 
> 	/*
> 	 * Set the kernel identity mapping for text RO.
> 	 */
> 	set_memory_ro(start, (end - start) >> PAGE_SHIFT);
> }
> 
> and arm has:
> 
> void set_kernel_text_ro(void)
> {
> 	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), true,
> 				current->active_mm);
> }
> 
> Where x86's set_kernel_text_ro() is a nop until the
> kernel_set_to_readonly is set.
> 
> Perhaps this may fix things?
> 
> [ Not even compiled tested ]
> 
> -- Steve
> 
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index c186474422f3..0cc8e04295a4 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -736,20 +736,29 @@ static int __mark_rodata_ro(void *unused)
>  	return 0;
>  }
>  
> +static int kernel_set_to_readonly __read_mostly;
> +
>  void mark_rodata_ro(void)
>  {
> +	kernel_set_to_readonly = 1;
>  	stop_machine(__mark_rodata_ro, NULL, NULL);
>  	debug_checkwx();
>  }
>  
>  void set_kernel_text_rw(void)
>  {
> +	if (!kernel_set_to_readonly)
> +		return;
> +
>  	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), false,
>  				current->active_mm);
>  }
>  
>  void set_kernel_text_ro(void)
>  {
> +	if (!kernel_set_to_readonly)
> +		return;
> +
>  	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), true,
>  				current->active_mm);
>  }

This aligns nicely with how this has been solved with commit
162396309745 ("ftrace, x86: make kernel text writable only for
conversions") a while ago.
Reviewed-by: Stefan Agner <stefan@agner.ch>

Compiled and tested here, selftest as well as ftrace at runtime seems to
work fine.
Tested-by: Stefan Agner <stefan@agner.ch>

Thanks Steven! Can you send a patch or should I do it?

--
Stefan