linux-kernel - Re: [RFC PATCH v8 3/4] arm64: Introduce stack trace reliability checks in the unwinder

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Date:   Tue, 24 Aug 2021 07:19:52 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     "nobuta.keiya@...itsu.com" <nobuta.keiya@...itsu.com>
Cc:     "mark.rutland@....com" <mark.rutland@....com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "ardb@...nel.org" <ardb@...nel.org>,
        "sjitindarsingh@...il.com" <sjitindarsingh@...il.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "pasha.tatashin@...een.com" <pasha.tatashin@...een.com>,
        "jthierry@...hat.com" <jthierry@...hat.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "live-patching@...r.kernel.org" <live-patching@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v8 3/4] arm64: Introduce stack trace reliability
 checks in the unwinder



On 8/24/21 12:55 AM, nobuta.keiya@...itsu.com wrote:
> Hi Madhavan,
> 
>> @@ -245,7 +271,36 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
>>  		fp = thread_saved_fp(task);
>>  		pc = thread_saved_pc(task);
>>  	}
>> -	unwind(consume_entry, cookie, task, fp, pc);
>> +	unwind(consume_entry, cookie, task, fp, pc, false);
>> +}
>> +
>> +/*
>> + * arch_stack_walk_reliable() may not be used for livepatch until all of
>> + * the reliability checks are in place in unwind_consume(). However,
>> + * debug and test code can choose to use it even if all the checks are not
>> + * in place.
>> + */
> 
> I'm glad to see the long-awaited function :)
> 
> Does the above comment mean that this comment will be removed by
> another patch series that about live patch enablement, instead of [PATCH 4/4]?
> 
> It seems to take time... But I start thinking about test code.
> 

Yes. This comment will be removed when livepatch will be enabled eventually.
So, AFAICT, there are 4 pieces that are needed:

- Reliable stack trace in the kernel. I am trying to address that with my patch
  series.

- Mark Rutland's work for making patching safe on ARM64.

- Objtool (or alternative method) for stack validation.

- Suraj Jitindar Singh's patch for miscellaneous things needed to enable live patch.

Once all of these pieces are in place, livepatch can be enabled.

That said, arch_stack_walk_reliable() can be used for test and debug purposes anytime
once this patch series gets accepted.

Thanks.

Madhavan