[PATCH] arm64: ftrace: don't dereference a probably invalid address

[Steven Rostedt]

On Mon, 7 Jun 2021 11:23:30 +0800
Mark-PK Tsai <mark-pk.tsai at mediatek.com> wrote:

> Address in __mcount_loc may be invalid if somthing goes wrong.
> On our arm64 platform, the bug in recordmcount make kernel
> crash in ftrace_init().

How did it crash? The link below doesn't show any crash.

> 
> https://lore.kernel.org/lkml/20210607023839.26387-1-mark-pk.tsai@mediatek.com/
> 
> Return -EFAULT if we are dealing with out-of-range condition
> to prevent dereference the invalid address in ftrace_bug(),
> then the kernel can disable ftrace safely for problematic
> __mcount_loc.

!mod is not an out-of-range condition. It just happened that the other
bug caused this strange side-effect. A !mod does not mean a fault
happened. Just because it may have been caused by a fault in your use
case does not mean that it's a fault in all use cases. That's like
saying that your dog is a poodle, so all dogs are poodles.

A return of -EINVAL should not cause a crash. If it does, then that
needs to be fixed.

-- Steve


> 
> Signed-off-by: Mark-PK Tsai <mark-pk.tsai at mediatek.com>
> ---
>  arch/arm64/kernel/ftrace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index b5d3ddaf69d9..98bec8445a58 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -201,7 +201,7 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
>  			preempt_enable();
>  
>  			if (WARN_ON(!mod))
> -				return -EINVAL;
> +				return -EFAULT;
>  		}
>  
>  		/*