Crash when booting RPi2 / bcm2836 with ftrace=function

[Steven Rostedt]

On Wed, 23 Aug 2017 17:42:14 +0200
Matthias Reichl <hias at horus.com> wrote:

> I haven't verified it myself but Phil Elwells first analysis here
> sounds plausible
> 
> https://github.com/raspberrypi/linux/issues/2166#issuecomment-323355145

I noticed this comment:

"What makes this interesting (and is the reason this doesn't fail
 normally) is that these functions are all called within kernel_init, so
 the __ro_after_init attribute shouldn't have caused the variable to be
 read-only yet."

When ftrace starts, it needs to set the kernel text to write, and when
it finishes it sets it back to readonly. But the kernel is not set to
readonly at boot up.

Can you test this patch? (Warning, I didn't even compile test it)

-- Steve

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index ad80548..fd75f38 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -745,19 +745,29 @@ static int __mark_rodata_ro(void *unused)
 	return 0;
 }
 
+static int kernel_set_to_readonly;
+
 void mark_rodata_ro(void)
 {
+	kernel_set_to_readonly = 1;
+
 	stop_machine(__mark_rodata_ro, NULL, NULL);
 }
 
 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);
 }