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

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Message-ID: <20191106230553.wnyltmkzwk5dph4l@treble>
Date:   Wed, 6 Nov 2019 17:05:53 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Joe Lawrence <joe.lawrence@...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 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

-- 
Josh