From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
To: Mark Brown <broonie@kernel.org>
Cc: mark.rutland@arm.com, 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 2/8] arm64: Implement frame types
Date: Fri, 19 Mar 2021 09:40:46 -0500 [thread overview]
Message-ID: <e8d596c3-b1ec-77a6-f387-92ecd2ebfceb@linux.microsoft.com> (raw)
In-Reply-To: <20210319132208.GD5619@sirena.org.uk>
On 3/19/21 8:22 AM, Mark Brown wrote:
> On Thu, Mar 18, 2021 at 05:22:49PM -0500, Madhavan T. Venkataraman wrote:
>> On 3/18/21 12:40 PM, Mark Brown wrote:
>
>>> Unless I'm misreading what's going on here this is more trying to set a
>>> type for the stack as a whole than for a specific stack frame. I'm also
>>> finding this a bit confusing as the unwinder already tracks things it
>>> calls frame types and it handles types that aren't covered here like
>>> SDEI. At the very least there's a naming issue here.
>
>> Both these frames are on the task stack. So, it is not a stack type.
>
> OTOH it's also not something that applies to every frame but only to the
> base frame from each stack which I think was more where I was coming
> from there. In any case, the issue is also that there's already another
> thing that the unwinder calls a frame type so there's at least that
> collision which needs to be resolved if nothing else.
>
The base frame from each stack as well as intermediate marker frames such
as the EL1 frame and the Ftrace frame.
As for the frame type, I will try to come up with a better name.
>>> Taking a step back though do we want to be tracking this via pt_regs?
>>> It's reliant on us robustly finding the correct pt_regs and on having
>>> the things that make the stack unreliable explicitly go in and set the
>>> appropriate type. That seems like it will be error prone, I'd been
>>> expecting to do something more like using sections to filter code for
>>> unreliable features based on the addresses of the functions we find on
>>> the stack or similar. This could still go wrong of course but there's
>>> fewer moving pieces, and especially fewer moving pieces specific to
>>> reliable stack trace.
>
>> In that case, I suggest doing both. That is, check the type as well
>> as specific functions. For instance, in the EL1 pt_regs, in addition
>> to the above checks, check the PC against el1_sync(), el1_irq() and
>> el1_error(). I have suggested this in the cover letter.
>
>> If this is OK with you, we could do that. We want to make really sure that
>> nothing goes wrong with detecting the exception frame.
>
> ...
>
>> If you dislike the frame type, I could remove it and just do the
>> following checks:
>
>> FP == pt_regs->regs[29]
>> PC == pt_regs->pc
>> and the address check against el1_*() functions
>
>> and similar changes for EL0 as well.
>
>> I still think that the frame type check makes it more robust.
>
> Yeah, we know the entry points so they can serve the same role as
> checking an explicitly written value. It does mean one less operation
> on exception entry, though I'm not sure that's that a big enough
> overhead to actually worry about. I don't have *super* strong opinons
> against adding the explicitly written value other than it being one more
> thing we don't otherwise use which we have to get right for reliable
> stack trace, there's a greater risk of bitrot if it's not something that
> we ever look at outside of the reliable stack trace code.
>
So, I will add the address checks for robustness. I will think some more
about the frame type.
>>>> EL1_FRAME
>>>> EL1 exception frame.
>
>>> We do trap into EL2 as well, the patch will track EL2 frames as EL1
>>> frames. Even if we can treat them the same the naming ought to be
>>> clear.
>
>> Are you referring to ARMv8.1 VHE extension where the kernel can run
>> at EL2? Could you elaborate? I thought that EL2 was basically for
>> Hypervisors.
>
> KVM is the main case, yes - IIRC otherwise it's mainly error handlers
> but I might be missing something. We do recommend that the kernel is
> started at EL2 where possible.
>
> Actually now I look again it's just not adding anything on EL2 entries
> at all, they use a separate set of macros which aren't updated - this
> will only update things for EL0 and EL1 entries so my comment above
> about this tracking EL2 as EL1 isn't accurate.
>
OK.
Madhavan
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next prev parent reply other threads:[~2021-03-19 14:42 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 [this message]
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
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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