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

[Andrew Morton]

On Fri, 28 May 2021 01:04:46 +0300 Alexey Dobriyan <adobriyan at gmail.com> wrote:

> 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

I'll add

Fixes: 8e8d7f13b6d5a9 ("afs: Add some tracepoints")

although Andi's d2abfa86ff373 gets in the way a bit.

> --- 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

Should/can afs_SRXCB##name##_name[] be static?


__tracepoint_string is very rarely used.  I wonder if there's much
point in it existing?


kernel/rcu/tree.h does

static char rcu_name[] = RCU_NAME_RAW;
static const char *tp_rcu_varname __used __tracepoint_string = rcu_name;

which is asking the compiler to place a copy of these into each
compilation unit which includes tree.h, which probably isn't what was
intended.