Dynamic ftrace self test broken on ARM

[rostedt@goodmis.org (Steven Rostedt)]

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);
 }