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From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: broonie@kernel.org, jpoimboe@redhat.com, jthierry@redhat.com,
	catalin.marinas@arm.com, will@kernel.org,
	linux-arm-kernel@lists.infradead.org,
	live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack trace unreliable
Date: Tue, 23 Mar 2021 15:24:14 -0500	[thread overview]
Message-ID: <8aa50127-3f00-818d-d58c-4b3ff7235c74@linux.microsoft.com> (raw)
In-Reply-To: <20210323183053.GH98545@C02TD0UTHF1T.local>



On 3/23/21 1:30 PM, Mark Rutland wrote:
> On Tue, Mar 23, 2021 at 12:23:34PM -0500, Madhavan T. Venkataraman wrote:
>> On 3/23/21 12:02 PM, Mark Rutland wrote:
> 
> [...]
> 
>> I think that I did a bad job of explaining what I wanted to do. It is not
>> for any additional protection at all.
>>
>> So, let us say we create a field in the task structure:
>>
>> 	u64		unreliable_stack;
>>
>> Whenever an EL1 exception is entered or FTRACE is entered and pt_regs get
>> set up and pt_regs->stackframe gets chained, increment unreliable_stack.
>> On exiting the above, decrement unreliable_stack.
>>
>> In arch_stack_walk_reliable(), simply do this check upfront:
>>
>> 	if (task->unreliable_stack)
>> 		return -EINVAL;
>>
>> This way, the function does not even bother unwinding the stack to find
>> exception frames or checking for different return addresses or anything.
>> We also don't have to worry about code being reorganized, functions
>> being renamed, etc. It also may help in debugging to know if a task is
>> experiencing an exception and the level of nesting, etc.
> 
> As in my other reply, since this is an optimization that is not
> necessary for functional correctness, I would prefer to avoid this for
> now. We can reconsider that in future if we encounter performance
> problems.
> 
> Even with this there will be cases where we have to identify
> non-unwindable functions explicitly (e.g. the patchable-function-entry
> trampolines, where the real return address is in x9), and I'd prefer
> that we use one mechanism consistently.
> 
> I suspect that in the future we'll need to unwind across exception
> boundaries using metadata, and we can treat the non-unwindable metadata
> in the same way.
> 
> [...]
> 
>>> 3. Figure out exception boundary handling. I'm currently working to
>>>    simplify the entry assembly down to a uniform set of stubs, and I'd
>>>    prefer to get that sorted before we teach the unwinder about
>>>    exception boundaries, as it'll be significantly simpler to reason
>>>    about and won't end up clashing with the rework.
>>
>> So, here is where I still have a question. Is it necessary for the unwinder
>> to know the exception boundaries? Is it not enough if it knows if there are
>> exceptions present? For instance, using something like num_special_frames
>> I suggested above?
> 
> I agree that it would be legitimate to bail out early if we knew there
> was going to be an exception somewhere in the trace. Regardless, I think
> it's simpler overall to identify non-unwindability during the trace, and
> doing that during the trace aligns more closely with the structure that
> we'll need to permit unwinding across these boundaries in future, so I'd
> prefer we do that rather than trying to optimize for early returns
> today.
> 

OK. Fair enough.

Thanks.

Madhavan

  reply	other threads:[~2021-03-23 20:25 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <5997dfe8d261a3a543667b83c902883c1e4bd270>
2021-03-15 16:57 ` [RFC PATCH v2 0/8] arm64: Implement reliable stack trace madvenka
2021-03-15 16:57   ` [RFC PATCH v2 1/8] arm64: Implement stack trace termination record madvenka
2021-03-18 15:09     ` Mark Brown
2021-03-18 20:26       ` Madhavan T. Venkataraman
2021-03-19 12:30         ` Mark Brown
2021-03-19 14:29           ` Madhavan T. Venkataraman
2021-03-19 18:19             ` Madhavan T. Venkataraman
2021-03-19 22:03               ` Madhavan T. Venkataraman
2021-03-23 10:24                 ` Mark Rutland
2021-03-23 12:39                   ` Madhavan T. Venkataraman
2021-03-15 16:57   ` [RFC PATCH v2 2/8] arm64: Implement frame types madvenka
2021-03-18 17:40     ` Mark Brown
2021-03-18 22:22       ` Madhavan T. Venkataraman
2021-03-19 13:22         ` Mark Brown
2021-03-19 14:40           ` Madhavan T. Venkataraman
2021-03-19 15:02             ` Madhavan T. Venkataraman
2021-03-19 16:20               ` Mark Brown
2021-03-19 16:27                 ` Madhavan T. Venkataraman
2021-03-23 10:34     ` Mark Rutland
2021-03-15 16:57   ` [RFC PATCH v2 3/8] arm64: Terminate the stack trace at TASK_FRAME and EL0_FRAME madvenka
2021-03-18 18:26     ` Mark Brown
2021-03-18 20:29       ` Madhavan T. Venkataraman
2021-03-23 10:36         ` Mark Rutland
2021-03-23 12:40           ` Madhavan T. Venkataraman
2021-03-15 16:57   ` [RFC PATCH v2 4/8] arm64: Detect an EL1 exception frame and mark a stack trace unreliable madvenka
2021-03-23 10:42     ` Mark Rutland
2021-03-23 12:46       ` Madhavan T. Venkataraman
2021-03-23 13:04         ` Mark Rutland
2021-03-23 13:31           ` Madhavan T. Venkataraman
2021-03-23 14:33             ` Mark Rutland
2021-03-23 15:22               ` Madhavan T. Venkataraman
2021-03-15 16:57   ` [RFC PATCH v2 5/8] arm64: Detect an FTRACE " madvenka
2021-03-23 10:51     ` Mark Rutland
2021-03-23 12:56       ` Madhavan T. Venkataraman
2021-03-23 13:36         ` Mark Rutland
2021-03-23 13:38           ` Madhavan T. Venkataraman
2021-03-23 14:15             ` Madhavan T. Venkataraman
2021-03-23 14:57               ` Mark Rutland
2021-03-23 15:26                 ` Madhavan T. Venkataraman
2021-03-23 16:20                   ` Madhavan T. Venkataraman
2021-03-23 17:02                     ` Mark Rutland
2021-03-23 17:23                       ` Madhavan T. Venkataraman
2021-03-23 17:27                         ` Madhavan T. Venkataraman
2021-03-23 18:27                         ` Mark Brown
2021-03-23 20:23                           ` Madhavan T. Venkataraman
2021-03-23 18:30                         ` Mark Rutland
2021-03-23 20:24                           ` Madhavan T. Venkataraman [this message]
2021-03-23 21:04                             ` Madhavan T. Venkataraman
2021-03-23 16:48                   ` Mark Rutland
2021-03-23 16:53                     ` Madhavan T. Venkataraman
2021-03-23 17:09                       ` Mark Rutland
2021-03-15 16:57   ` [RFC PATCH v2 6/8] arm64: Check the return PC of every stack frame madvenka
2021-03-15 16:57   ` [RFC PATCH v2 7/8] arm64: Detect kretprobed functions in stack trace madvenka
2021-03-15 16:58   ` [RFC PATCH v2 8/8] arm64: Implement arch_stack_walk_reliable() madvenka
2021-03-15 19:01   ` [RFC PATCH v2 0/8] arm64: Implement reliable stack trace Madhavan T. Venkataraman

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