From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>,
Mark Rutland <mark.rutland@arm.com>
Cc: broonie@kernel.org, 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,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks
Date: Fri, 9 Apr 2021 17:05:58 -0500 [thread overview]
Message-ID: <8c30ec5f-b51e-494f-5f6c-d2f012135f69@linux.microsoft.com> (raw)
In-Reply-To: <20210409213741.kqmwyajoppuqrkge@treble>
On 4/9/21 4:37 PM, Josh Poimboeuf wrote:
> On Fri, Apr 09, 2021 at 01:09:09PM +0100, Mark Rutland wrote:
>> On Mon, Apr 05, 2021 at 03:43:09PM -0500, madvenka@linux.microsoft.com wrote:
>>> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
>>>
>>> There are a number of places in kernel code where the stack trace is not
>>> reliable. Enhance the unwinder to check for those cases and mark the
>>> stack trace as unreliable. Once all of the checks are in place, the unwinder
>>> can provide a reliable stack trace. But before this can be used for livepatch,
>>> some other entity needs to guarantee that the frame pointers are all set up
>>> correctly in kernel functions. objtool is currently being worked on to
>>> fill that gap.
>>>
>>> Except for the return address check, all the other checks involve checking
>>> the return PC of every frame against certain kernel functions. To do this,
>>> implement some infrastructure code:
>>>
>>> - Define a special_functions[] array and populate the array with
>>> the special functions
>>
>> I'm not too keen on having to manually collate this within the unwinder,
>> as it's very painful from a maintenance perspective.
>
> Agreed.
>
>> I'd much rather we could associate this information with the
>> implementations of these functions, so that they're more likely to
>> stay in sync.
>>
>> Further, I believe all the special cases are assembly functions, and
>> most of those are already in special sections to begin with. I reckon
>> it'd be simpler and more robust to reject unwinding based on the
>> section. If we need to unwind across specific functions in those
>> sections, we could opt-in with some metadata. So e.g. we could reject
>> all functions in ".entry.text", special casing the EL0 entry functions
>> if necessary.
>
> Couldn't this also end up being somewhat fragile? Saying "certain
> sections are deemed unreliable" isn't necessarily obvious to somebody
> who doesn't already know about it, and it could be overlooked or
> forgotten over time. And there's no way to enforce it stays that way.
>
Good point!
> FWIW, over the years we've had zero issues with encoding the frame
> pointer on x86. After you save pt_regs, you encode the frame pointer to
> point to it. Ideally in the same macro so it's hard to overlook.
>
I had the same opinion. In fact, in my encoding scheme, I have additional
checks to make absolutely sure that it is a true encoding and not stack
corruption. The chances of all of those values accidentally matching are,
well, null.
> If you're concerned about debuggers getting confused by the encoding -
> which debuggers specifically? In my experience, if vmlinux has
> debuginfo, gdb and most other debuggers will use DWARF (which is already
> broken in asm code) and completely ignore frame pointers.
>
Yes. I checked gdb actually. It did not show a problem.
>> I think there's a lot more code that we cannot unwind, e.g. KVM
>> exception code, or almost anything marked with SYM_CODE_END().
>
> Just a reminder that livepatch only unwinds blocked tasks (plus the
> 'current' task which calls into livepatch). So practically speaking, it
> doesn't matter whether the 'unreliable' detection has full coverage.
> The only exceptions which really matter are those which end up calling
> schedule(), e.g. preemption or page faults.
>
> Being able to consistently detect *all* possible unreliable paths would
> be nice in theory, but it's unnecessary and may not be worth the extra
> complexity.
>
You do have a point. I tried to think of arch_stack_walk_reliable() as
something that should be implemented independent of livepatching. But
I could not really come up with a single example of where else it would
really be useful.
So, if we assume that the reliable stack trace is solely for the purpose
of livepatching, I agree with your earlier comments as well.
Thanks!
Madhavan
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next prev parent reply other threads:[~2021-04-09 22:24 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <705993ccb34a611c75cdae0a8cb1b40f9b218ebd>
2021-04-05 20:43 ` [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks madvenka
2021-04-05 20:43 ` [RFC PATCH v2 1/4] arm64: Implement infrastructure for " madvenka
2021-04-08 15:15 ` Mark Brown
2021-04-08 17:17 ` Mark Brown
2021-04-08 19:30 ` Madhavan T. Venkataraman
2021-04-08 23:30 ` Madhavan T. Venkataraman
2021-04-09 11:57 ` Mark Brown
2021-04-05 20:43 ` [RFC PATCH v2 2/4] arm64: Mark a stack trace unreliable if an EL1 exception frame is detected madvenka
2021-04-05 20:43 ` [RFC PATCH v2 3/4] arm64: Detect FTRACE cases that make the stack trace unreliable madvenka
2021-04-08 16:58 ` Mark Brown
2021-04-08 19:23 ` Madhavan T. Venkataraman
2021-04-09 11:31 ` Mark Brown
2021-04-09 14:02 ` Madhavan T. Venkataraman
2021-04-09 12:27 ` Mark Rutland
2021-04-09 17:23 ` Madhavan T. Venkataraman
2021-04-05 20:43 ` [RFC PATCH v2 4/4] arm64: Mark stack trace as unreliable if kretprobed functions are present madvenka
2021-04-09 12:09 ` [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks Mark Rutland
2021-04-09 17:16 ` Madhavan T. Venkataraman
2021-04-09 21:37 ` Josh Poimboeuf
2021-04-09 22:05 ` Madhavan T. Venkataraman [this message]
2021-04-09 22:32 ` Josh Poimboeuf
2021-04-09 22:53 ` Josh Poimboeuf
2021-04-11 17:54 ` Madhavan T. Venkataraman
2021-04-12 16:59 ` Mark Brown
2021-04-13 22:53 ` Josh Poimboeuf
2021-04-14 12:24 ` Mark Brown
2021-04-12 17:36 ` Mark Brown
2021-04-12 19:55 ` Madhavan T. Venkataraman
2021-04-13 11:02 ` Mark Brown
2021-04-14 10:23 ` Madhavan T. Venkataraman
2021-04-14 12:35 ` Mark Brown
2021-04-16 14:43 ` Madhavan T. Venkataraman
2021-04-16 15:36 ` Mark Brown
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