[PATCH] afs: fix tracepoint string placement with built-in AFS

[Alexey Dobriyan]

I was adding custom tracepoint to the kernel, grabbed full F34 kernel
.config, disabled modules and booted whole shebang as VM kernel.

Then did

	perf record -a -e ...

It crashed:

	general protection fault, probably for non-canonical address 0x435f5346592e4243: 0000 [#1] SMP PTI
	CPU: 1 PID: 842 Comm: cat Not tainted 5.12.6+ #26
	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-1.fc33 04/01/2014
	RIP: 0010:t_show+0x22/0xd0

Then reproducer was narrowed to 

	# cat /sys/kernel/tracing/printk_formats

Original F34 kernel with modules didn't crash.

So I started to disable options and after disabling AFS everything
started working again.

The root cause is that AFS was placing char arrays content into a section
full of _pointers_ to strings with predictable consequences.

Non canonical address 435f5346592e4243 is "CB.YFS_" which came from
CM_NAME macro.

The fix is to create char array and pointer to it separatedly.

Steps to reproduce:

	CONFIG_AFS=y
	CONFIG_TRACING=y

	# cat /sys/kernel/tracing/printk_formats

Signed-off-by: Alexey Dobriyan (SK hynix) <adobriyan at gmail.com>
---

 fs/afs/cmservice.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/fs/afs/cmservice.c
+++ b/fs/afs/cmservice.c
@@ -30,8 +30,9 @@ static void SRXAFSCB_TellMeAboutYourself(struct work_struct *);
 static int afs_deliver_yfs_cb_callback(struct afs_call *);
 
 #define CM_NAME(name) \
-	char afs_SRXCB##name##_name[] __tracepoint_string =	\
-		"CB." #name
+	const char afs_SRXCB##name##_name[] = "CB." #name;		\
+	static const char *_afs_SRXCB##name##_name __tracepoint_string =\
+		afs_SRXCB##name##_name
 
 /*
  * CB.CallBack operation type