linux-kernel - Re: [RFC PATCH v3 19/22] arm64: unwinder: Add a reliability check in the unwinder based on ORC

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Message-ID: <56308235-3893-75ac-a19f-497cc203c520@linux.microsoft.com>
Date:   Mon, 6 Mar 2023 10:52:09 -0600
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Suraj Jitindar Singh <sjitindarsingh@...il.com>
Cc:     poimboe@...hat.com, peterz@...radead.org, chenzhongjin@...wei.com,
        mark.rutland@....com, broonie@...nel.org, nobuta.keiya@...itsu.com,
        catalin.marinas@....com, will@...nel.org,
        jamorris@...ux.microsoft.com, linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 19/22] arm64: unwinder: Add a reliability check in
 the unwinder based on ORC



On 2/22/23 22:07, Suraj Jitindar Singh wrote:
> On Thu, 2023-02-02 at 01:40 -0600, madvenka@...ux.microsoft.com wrote:
>> From: "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
>>
>> Introduce a reliability flag in struct unwind_state. This will be set
>> to
>> false if the PC does not have a valid ORC or if the frame pointer
>> computed
>> from the ORC does not match the actual frame pointer.
>>
>> Now that the unwinder can validate the frame pointer, introduce
>> arch_stack_walk_reliable().
>>
>> Signed-off-by: Madhavan T. Venkataraman <madvenka@...ux.microsoft.com
>>>
>> ---
>>  arch/arm64/include/asm/stacktrace/common.h |  15 ++
>>  arch/arm64/kernel/stacktrace.c             | 167
>> ++++++++++++++++++++-
>>  2 files changed, 175 insertions(+), 7 deletions(-)
>>
> 
> [snip]
>  
>> -static void notrace unwind(struct unwind_state *state,
>> +static int notrace unwind(struct unwind_state *state, bool
>> need_reliable,
>>  			   stack_trace_consume_fn consume_entry, void
>> *cookie)
>>  {
>> -	while (1) {
>> -		int ret;
>> +	int ret = 0;
>>  
>> +	while (1) {
>> +		if (need_reliable && !state->reliable)
>> +			return -EINVAL;
>>  		if (!consume_entry(cookie, state->pc))
>>  			break;
>>  		ret = unwind_next(state);
>> +		if (need_reliable && !ret)
>> +			unwind_check_reliable(state);
>>  		if (ret < 0)
>>  			break;
>>  	}
>> +	return ret;
> 
> nit:
> 
> I think you're looking more for comments on the approach and the
> correctness of these patches, but from an initial read I'm still
> putting it all together in my head. So this comment is on the coding
> style.
> 
> The above loop seems to check the current reliability state, then
> unwind a frame then check the reliability, and then break based of
> something which couldn't have been updated by the line immediately
> above. I propose something like:
> 
> unwind(...) {
> 	ret = 0;
> 
> 	while (!ret) {
> 		if (need_reliable) {
> 			unwind_check_reliable(state);
> 			if (!state->reliable)
> 				return -EINVAL;
> 		}
> 		if (!consume_entry(cookie, state->pc))
> 			return -EINVAL;
> 		ret = unwind_next(state);
> 	}
> 
> 	return ret;
> }
> 
> This also removes the need for the call to unwind_check_reliable()
> before the first unwind() below in arch_stack_walk_reliable().
> 

OK. Suggestion sounds reasonable. Will do.

Madhavan

> - Suraj
> 
>>  }
>>  NOKPROBE_SYMBOL(unwind);
>>  
>> @@ -216,5 +337,37 @@ noinline notrace void
>> arch_stack_walk(stack_trace_consume_fn consume_entry,
>>  		unwind_init_from_task(&state, task);
>>  	}
>>  
>> -	unwind(&state, consume_entry, cookie);
>> +	unwind(&state, false, consume_entry, cookie);
>> +}
>> +
>> +noinline notrace int arch_stack_walk_reliable(
>> +				stack_trace_consume_fn consume_entry,
>> +				void *cookie, struct task_struct *task)
>> +{
>> +	struct stack_info stacks[] = {
>> +		stackinfo_get_task(task),
>> +		STACKINFO_CPU(irq),
>> +#if defined(CONFIG_VMAP_STACK)
>> +		STACKINFO_CPU(overflow),
>> +#endif
>> +#if defined(CONFIG_VMAP_STACK) && defined(CONFIG_ARM_SDE_INTERFACE)
>> +		STACKINFO_SDEI(normal),
>> +		STACKINFO_SDEI(critical),
>> +#endif
>> +	};
>> +	struct unwind_state state = {
>> +		.stacks = stacks,
>> +		.nr_stacks = ARRAY_SIZE(stacks),
>> +	};
>> +	int ret;
>> +
>> +	if (task == current)
>> +		unwind_init_from_caller(&state);
>> +	else
>> +		unwind_init_from_task(&state, task);
>> +	unwind_check_reliable(&state);
>> +
>> +	ret = unwind(&state, true, consume_entry, cookie);
>> +
>> +	return ret == -ENOENT ? 0 : -EINVAL;
>>  }