Re: [PATCH] ARM: unwind: improve unwinders for noreturn case

[Kees Cook]

On Wed, Mar 20, 2024 at 10:19:29AM +0800, Jiangfeng Xiao wrote:
> This is an off-by-one bug which is common in unwinders,
> due to the fact that the address on the stack points
> to the return address rather than the call address.
> 
> So, for example, when the last instruction of a function
> is a function call (e.g., to a noreturn function), it can
> cause the unwinder to incorrectly try to unwind from
> the function after the callee.
> 
> foo:
> ...
> 	bl	bar
> ... end of function and thus next function ...
> 
> which results in LR pointing into the next function.
> 
> Fixed this by subtracting 1 from frmae->pc in the call frame
> (but not exception frames) like ORC on x86 does.
> 
> Refer to the unwind_next_frame function in the unwind_orc.c
> 
> Suggested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Link: https://lkml.kernel.org/lkml/20240305175846.qnyiru7uaa7itqba@treble/
> Signed-off-by: Jiangfeng Xiao <xiaojiangfeng@xxxxxxxxxx>
> ---
>  arch/arm/include/asm/stacktrace.h |  4 ----
>  arch/arm/kernel/stacktrace.c      |  2 --
>  arch/arm/kernel/traps.c           |  4 ++--
>  arch/arm/kernel/unwind.c          | 18 +++++++++++++++---
>  4 files changed, 17 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm/include/asm/stacktrace.h b/arch/arm/include/asm/stacktrace.h
> index 360f0d2..07e4c16 100644
> --- a/arch/arm/include/asm/stacktrace.h
> +++ b/arch/arm/include/asm/stacktrace.h
> @@ -21,9 +21,7 @@ struct stackframe {
>  	struct llist_node *kr_cur;
>  	struct task_struct *tsk;
>  #endif
> -#ifdef CONFIG_UNWINDER_FRAME_POINTER
>  	bool ex_frame;
> -#endif
>  };
>  
>  static __always_inline
> @@ -37,9 +35,7 @@ void arm_get_current_stackframe(struct pt_regs *regs, struct stackframe *frame)
>  		frame->kr_cur = NULL;
>  		frame->tsk = current;
>  #endif
> -#ifdef CONFIG_UNWINDER_FRAME_POINTER
>  		frame->ex_frame = in_entry_text(frame->pc);
> -#endif
>  }
>  
>  extern int unwind_frame(struct stackframe *frame);
> diff --git a/arch/arm/kernel/stacktrace.c b/arch/arm/kernel/stacktrace.c
> index 620aa82..1abd4f9 100644
> --- a/arch/arm/kernel/stacktrace.c
> +++ b/arch/arm/kernel/stacktrace.c
> @@ -154,9 +154,7 @@ static void start_stack_trace(struct stackframe *frame, struct task_struct *task
>  	frame->kr_cur = NULL;
>  	frame->tsk = task;
>  #endif
> -#ifdef CONFIG_UNWINDER_FRAME_POINTER
>  	frame->ex_frame = in_entry_text(frame->pc);
> -#endif
>  }
>  
>  void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> index 3bad79d..b64e442 100644
> --- a/arch/arm/kernel/traps.c
> +++ b/arch/arm/kernel/traps.c
> @@ -84,10 +84,10 @@ void dump_backtrace_entry(unsigned long where, unsigned long from,
>  	printk("%sFunction entered at [<%08lx>] from [<%08lx>]\n",
>  		loglvl, where, from);
>  #elif defined CONFIG_BACKTRACE_VERBOSE
> -	printk("%s[<%08lx>] (%ps) from [<%08lx>] (%pS)\n",
> +	pr_warn("%s[<%08lx>] (%ps) from [<%08lx>] (%pB)\n",
>  		loglvl, where, (void *)where, from, (void *)from);

This should stay printk("%s...", loglvl, ...) or loglvl should be
dropped when converting to pr_warn():

	pr_warn([<%08lx>] (%ps) from [<%08lx>] (%pB)\n",
		where, (void *)where, from, (void *)from);

Why did you want to force the "warn" log level?

>  #else
> -	printk("%s %ps from %pS\n", loglvl, (void *)where, (void *)from);
> +	pr_warn("%s %ps from %pB\n", loglvl, (void *)where, (void *)from);

Ditto.

-Kees

-- 
Kees Cook