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 16:04:47 -0500 [thread overview]
Message-ID: <5f32abc2-5759-5fb9-a626-cce962ac275a@linux.microsoft.com> (raw)
In-Reply-To: <8aa50127-3f00-818d-d58c-4b3ff7235c74@linux.microsoft.com>
Thanks for all the input - Mark Rutland and Mark Brown.
I will send out the stack termination patch next. Since I am splitting
the original series into 3 separate series, I will change the titles and
start with version 1 in each case, if there is no objection.
Again, Thanks.
Madhavan
On 3/23/21 3:24 PM, Madhavan T. Venkataraman wrote:
>
>
> 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
>
next prev parent reply other threads:[~2021-03-23 21:05 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
2021-03-23 21:04 ` Madhavan T. Venkataraman [this message]
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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