linux-kernel - Re: [PATCH] Disable non-ABI-compliant optimisations for live patching

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Message-ID: <20160626223720.GC21026@amd>
Date:	Mon, 27 Jun 2016 00:37:20 +0200
From:	Pavel Machek <pavel@....cz>
To:	Torsten Duwe <duwe@....de>
Cc:	Jiri Kosina <jkosina@...e.cz>, matz@...e.de,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Disable non-ABI-compliant optimisations for live patching

On Wed 2016-06-22 16:24:41, Torsten Duwe wrote:
> Live patching, as we use it, deliberately disrupts the fabric of
> compile units; thus all assumptions a compiler can make about the
> control flow may be invalid. As an example, it could analyse that a
> callee does not touch a caller-saved register at all, so why waste
> memory bandwidth saving it? The register allocations for the live
> patch replacement function may however be quite different.
> 
> Starting with this example, disable all compiler optimisations that
> do not strictly comply with the established calling conventions.

I thought that in such case, person creating the live patch should
notice and adjust patch appropriately, at assembly level if
neccessary..?

If this is not true, and we want gcc to help us, what other
optimalizations do we need to disable? Even changes inside one
compiler unit can be "interesting"...

								Pavel

> Signed-off-by: Torsten Duwe <duwe@...e.de>
> ---
> 
> Working on the arm64 ftrace-with-regs/livepatch, it struck me that
> this is a general problem: with live patching, certain optimisations
> must be switched off for all architectures, the new(?) IPA register
> allocator in gcc6 is only one example. We should tackle this
> well before it bites us.
> 
> 	Torsten
> 
> ---
>  Makefile | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index b409076..424d2e6 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -743,6 +743,13 @@ KBUILD_CFLAGS 	+= $(call cc-option, -femit-struct-debug-baseonly) \
>  		   $(call cc-option,-fno-var-tracking)
>  endif
>  
> +ifdef CONFIG_LIVEPATCH
> +# The compiler might generate ABI "shortcuts" to speed up the code,
> +# making assumptions which are no longer valid when live patching
> +# is enabled. Disable all of them.
> +KBUILD_CFLAGS	+= $(call cc-option,-fno-ipa-ra)
> +endif
> +
>  ifdef CONFIG_FUNCTION_TRACER
>  ifndef CC_FLAGS_FTRACE
>  CC_FLAGS_FTRACE := -pg

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html