Re: [PATCH] usercopy: delete __noreturn from usercopy_abort

[Jiangfeng Xiao]

On 2024/3/6 1:58, Josh Poimboeuf wrote:
>>>> For the usercopy_abort function, whether '__noreturn' is added
>>>> does not affect the internal behavior of the usercopy_abort function.
>>>> Therefore, it is recommended that '__noreturn' be deleted
>>>> so that backtrace can work properly.
>>>
>>> This isn't acceptable. Removing __noreturn this will break
>>> objtool's processing of execution flow for livepatching, IBT, and
>>> KCFI instrumentation. These all depend on an accurate control flow
>>> descriptions, and usercopy_abort is correctly marked __noreturn.
> 
> __noreturn also has the benefit of enabling the compiler to produce more
> compact code for callees.
> 
>> Thank you for providing this information.
>> I'll go back to further understand how __noreturn is used
>> in features such as KCFI and livepatching.
> 
> Adding ARM folks -- see
> https://lkml.kernel.org/lkml/1709516385-7778-1-git-send-email-xiaojiangfeng@xxxxxxxxxx
> for the original bug report.
> 
> 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.
> 

Thanks for your advice. I think I understand. To solve this problem, I need to
fix the off-by-one bug which is common in unwinders. I'll try to fix it later
by referring to your patch.

> 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.
> 
> For ORC (x86), we fixed this by decrementing the PC for call frames (but
> not exception frames).  I've seen user space unwinders do similar, for
> non-signal frames.
> 
> Something like the following might fix your issue (completely untested):
> 
> diff --git a/arch/arm/include/asm/stacktrace.h b/arch/arm/include/asm/stacktrace.h
> index 360f0d2406bf..4891e38cdc1f 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 +35,8 @@ 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
> +		frame->ex_frame = !!regs;
> +
>  }
>  
>  extern int unwind_frame(struct stackframe *frame);
> diff --git a/arch/arm/kernel/stacktrace.c b/arch/arm/kernel/stacktrace.c
> index 620aa82e3bdd..caed7436da09 100644
> --- a/arch/arm/kernel/stacktrace.c
> +++ b/arch/arm/kernel/stacktrace.c
> @@ -154,9 +154,6 @@ 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,
> @@ -167,6 +164,7 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
>  	if (regs) {
>  		start_stack_trace(&frame, NULL, regs->ARM_fp, regs->ARM_sp,
>  				  regs->ARM_lr, regs->ARM_pc);
> +		frame.ex_frame = true;
>  	} else if (task != current) {
>  #ifdef CONFIG_SMP
>  		/*
> @@ -180,6 +178,7 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
>  				  thread_saved_sp(task), 0,
>  				  thread_saved_pc(task));
>  #endif
> +		frame.ex_frame = false;
>  	} else {
>  here:
>  		start_stack_trace(&frame, task,
> @@ -187,6 +186,7 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
>  				  current_stack_pointer,
>  				  (unsigned long)__builtin_return_address(0),
>  				  (unsigned long)&&here);
> +		frame.ex_frame = false;
>  		/* skip this function */
>  		if (unwind_frame(&frame))
>  			return;
> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> index 3bad79db5d6e..46a5b1eb3f0a 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",
> +	printk("%s[<%08lx>] (%ps) from [<%08lx>] (%pB)\n",
>  		loglvl, where, (void *)where, from, (void *)from);
>  #else
> -	printk("%s %ps from %pS\n", loglvl, (void *)where, (void *)from);
> +	printk("%s %ps from %pB\n", loglvl, (void *)where, (void *)from);
>  #endif
>  
>  	if (in_entry_text(from) && end <= ALIGN(frame, THREAD_SIZE))
> diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
> index 9d2192156087..99ded32196af 100644
> --- a/arch/arm/kernel/unwind.c
> +++ b/arch/arm/kernel/unwind.c
> @@ -407,7 +407,7 @@ int unwind_frame(struct stackframe *frame)
>  {
>  	const struct unwind_idx *idx;
>  	struct unwind_ctrl_block ctrl;
> -	unsigned long sp_low;
> +	unsigned long sp_low, pc;
>  
>  	/* store the highest address on the stack to avoid crossing it*/
>  	sp_low = frame->sp;
> @@ -417,19 +417,22 @@ int unwind_frame(struct stackframe *frame)
>  	pr_debug("%s(pc = %08lx lr = %08lx sp = %08lx)\n", __func__,
>  		 frame->pc, frame->lr, frame->sp);
>  
> -	idx = unwind_find_idx(frame->pc);
> +	pc = frame->ex_frame ? frame->pc : frame->pc - 4;
> +
> +	idx = unwind_find_idx(pc);
>  	if (!idx) {
> -		if (frame->pc && kernel_text_address(frame->pc)) {
> -			if (in_module_plt(frame->pc) && frame->pc != frame->lr) {
> +		if (kernel_text_address(pc)) {
> +			if (in_module_plt(pc) && frame->pc != frame->lr) {
>  				/*
>  				 * Quoting Ard: Veneers only set PC using a
>  				 * PC+immediate LDR, and so they don't affect
>  				 * the state of the stack or the register file
>  				 */
>  				frame->pc = frame->lr;
> +				frame->ex_frame = false;
>  				return URC_OK;
>  			}
> -			pr_warn("unwind: Index not found %08lx\n", frame->pc);
> +			pr_warn("unwind: Index not found %08lx\n", pc);
>  		}
>  		return -URC_FAILURE;
>  	}
> @@ -442,7 +445,7 @@ int unwind_frame(struct stackframe *frame)
>  	if (idx->insn == 1)
>  		/* can't unwind */
>  		return -URC_FAILURE;
> -	else if (frame->pc == prel31_to_addr(&idx->addr_offset)) {
> +	else if (frame->ex_frame && pc == prel31_to_addr(&idx->addr_offset)) {
>  		/*
>  		 * Unwinding is tricky when we're halfway through the prologue,
>  		 * since the stack frame that the unwinder expects may not be
> @@ -451,9 +454,10 @@ int unwind_frame(struct stackframe *frame)
>  		 * a function, we are still effectively in the stack frame of
>  		 * the caller, and the unwind info has no relevance yet.
>  		 */
> -		if (frame->pc == frame->lr)
> +		if (pc == frame->lr)
>  			return -URC_FAILURE;
>  		frame->pc = frame->lr;
> +		frame->ex_frame = false;
>  		return URC_OK;
>  	} else if ((idx->insn & 0x80000000) == 0)
>  		/* prel31 to the unwind table */
> @@ -515,6 +519,7 @@ int unwind_frame(struct stackframe *frame)
>  	frame->lr = ctrl.vrs[LR];
>  	frame->pc = ctrl.vrs[PC];
>  	frame->lr_addr = ctrl.lr_addr;
> +	frame->ex_frame = false;
>  
>  	return URC_OK;
>  }
> @@ -544,6 +549,7 @@ void unwind_backtrace(struct pt_regs *regs, struct task_struct *tsk,
>  		 */
>  here:
>  		frame.pc = (unsigned long)&&here;
> +		frame.ex_frame = false;
>  	} else {
>  		/* task blocked in __switch_to */
>  		frame.fp = thread_saved_fp(tsk);
> @@ -554,11 +560,12 @@ void unwind_backtrace(struct pt_regs *regs, struct task_struct *tsk,
>  		 */
>  		frame.lr = 0;
>  		frame.pc = thread_saved_pc(tsk);
> +		frame.ex_frame = false;
>  	}
>  
>  	while (1) {
>  		int urc;
> -		unsigned long where = frame.pc;
> +		unsigned long where = frame.ex_frame ? frame.pc : frame.pc - 4;
>  
>  		urc = unwind_frame(&frame);
>  		if (urc < 0)
> .
>