linux-kernel - Re: [PATCH] x86/stacktrace: update kconfig help text for reliable unwinders

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Message-ID: <c8bae684-2a46-68e6-dc43-a8c215bdd111@redhat.com>
Date:   Wed, 6 Nov 2019 18:16:28 -0500
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: [PATCH] x86/stacktrace: update kconfig help text for reliable
 unwinders

On 11/6/19 6:05 PM, Josh Poimboeuf wrote:
> On Wed, Nov 06, 2019 at 05:43:44PM -0500, Joe Lawrence wrote:
>> commit 6415b38bae26 ("x86/stacktrace: Enable HAVE_RELIABLE_STACKTRACE
>> for the ORC unwinder") marked the ORC unwinder as a "reliable" unwinder.
>> Update the help text to reflect that change: the frame pointer unwinder
>> is no longer the only one that provides HAVE_RELIABLE_STACKTRACE.
>>
>> Signed-off-by: Joe Lawrence <joe.lawrence@...hat.com>
>> ---
>>   arch/x86/Kconfig.debug | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
>> index bf9cd83de777..69cdf0558c13 100644
>> --- a/arch/x86/Kconfig.debug
>> +++ b/arch/x86/Kconfig.debug
>> @@ -316,10 +316,6 @@ config UNWINDER_FRAME_POINTER
>>   	  unwinder, but the kernel text size will grow by ~3% and the kernel's
>>   	  overall performance will degrade by roughly 5-10%.
>>   
>> -	  This option is recommended if you want to use the livepatch
>> -	  consistency model, as this is currently the only way to get a
>> -	  reliable stack trace (CONFIG_HAVE_RELIABLE_STACKTRACE).
>> -
>>   config UNWINDER_GUESS
>>   	bool "Guess unwinder"
>>   	depends on EXPERT
>> @@ -333,6 +329,10 @@ config UNWINDER_GUESS
>>   	  useful in many cases.  Unlike the other unwinders, it has no runtime
>>   	  overhead.
>>   
>> +	  This option is not recommended if you want to use the livepatch
>> +	  consistency model, as this unwinder cannot guarantee reliable stack
>> +	  traces.
>> +
> 
> I'm not sure whether this last hunk is helpful.  At the very least the
> wording of "not recommended" might be confusing because it's not even
> possible to combine UNWINDER_GUESS+HAVE_RELIABLE_STACKTRACE.
> 
> arch/x86/Kconfig:       select HAVE_RELIABLE_STACKTRACE         if X86_64 && (UNWINDER_FRAME_POINTER || UNWINDER_ORC) && STACK_VALIDATION
> 

Ah good point.  The alternative would be to copy the recommended note to 
both UNWINDER_FRAME_POINTER and UNWINDER_ORC, or at least remove the 
"only" phrasing.  I dunno, nobody has noticed it yet, so maybe the first 
hunk would be good enough.

-- Joe