linux-kernel - Re: [RFC PATCH v1 0/4] arm64: Implement stack trace reliability checks

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Message-ID: <7dda9af3-1ecf-5e6f-1e46-8870a2a5e550@linux.microsoft.com>
Date:   Mon, 5 Apr 2021 09:56:48 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, mark.rutland@....com,
        broonie@...nel.org, jthierry@...hat.com, catalin.marinas@....com,
        will@...nel.org, linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 0/4] arm64: Implement stack trace reliability
 checks



On 4/5/21 8:24 AM, Masami Hiramatsu wrote:
> Hi Madhaven,
> 
> On Sat, 3 Apr 2021 22:29:12 -0500
> "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com> wrote:
> 
> 
>>>> Check for kretprobe
>>>> ===================
>>>>
>>>> For functions with a kretprobe set up, probe code executes on entry
>>>> to the function and replaces the return address in the stack frame with a
>>>> kretprobe trampoline. Whenever the function returns, control is
>>>> transferred to the trampoline. The trampoline eventually returns to the
>>>> original return address.
>>>>
>>>> A stack trace taken while executing in the function (or in functions that
>>>> get called from the function) will not show the original return address.
>>>> Similarly, a stack trace taken while executing in the trampoline itself
>>>> (and functions that get called from the trampoline) will not show the
>>>> original return address. This means that the caller of the probed function
>>>> will not show. This makes the stack trace unreliable.
>>>>
>>>> Add the kretprobe trampoline to special_functions[].
>>>>
>>>> FYI, each task contains a task->kretprobe_instances list that can
>>>> theoretically be consulted to find the orginal return address. But I am
>>>> not entirely sure how to safely traverse that list for stack traces
>>>> not on the current process. So, I have taken the easy way out.
>>>
>>> For kretprobes, unwinding from the trampoline or kretprobe handler
>>> shouldn't be a reliability concern for live patching, for similar
>>> reasons as above.
>>>
>>
>> Please see previous answer.
>>
>>> Otherwise, when unwinding from a blocked task which has
>>> 'kretprobe_trampoline' on the stack, the unwinder needs a way to get the
>>> original return address.  Masami has been working on an interface to
>>> make that possible for x86.  I assume something similar could be done
>>> for arm64.
>>>
>>
>> OK. Until that is available, this case needs to be addressed.
> 
> Actually, I've done that on arm64 :) See below patch.
> (and I also have a similar code for arm32, what I'm considering is how
> to unify x86/arm/arm64 kretprobe_find_ret_addr(), since those are very
> similar.)
> 
> This is applicable on my x86 series v5
> 
> https://lore.kernel.org/bpf/161676170650.330141.6214727134265514123.stgit@devnote2/
> 
> Thank you,
> 
> 

I took a brief look at your changes. Looks reasonable.

However, for now, I am going to include the kretprobe_trampoline in the special_functions[]
array until your changes are merged. At that point, it is just a matter of deleting
kretprobe_trampoline from the special_functions[] array. That is all.

I hope that is fine with everyone.

Madhavan