Date: Wed, 2 Jul 2014 18:00:19 -0700
From: Andi Kleen <ak@linux.intel.com>
To: Kees Cook <kees@outflux.net>
Cc: Randy Dunlap <rdunlap@infradead.org>, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, linux-next@vger.kernel.org,
        sfr@canb.auug.org.au, mhocko@suse.cz
Subject: Re: mmotm 2014-07-02-15-07 uploaded (stack protector)
Message-ID: <20140703010019.GS19781@tassilo.jf.intel.com>
References: <53b482d3.gR7nYB/K7hPREviI%akpm@linux-foundation.org>
 <53B493E2.2030905@infradead.org>
 <20140703002231.GR19781@tassilo.jf.intel.com>
 <20140703005100.GI5412@outflux.net>
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> The problem is that if you make kbuild hard-fail when selecting this missing
> compiler option, you can never switch it back because "make menuconfig" will
> refuse to build since the compiler option would be missing. Being silent
> about the missing option (and/or falling back to other options) means that
> you could get two different kernel features selection with the same CONFIG_*
> set, depending on the kernel, which is extremely bad ("I selected
> stack-protector-strong but it built without it?!").

The assumption that every flag in a .config has been consciouscly 
selected by a human is a quite dubious one ...

LTO just turns itself off if the toolchain doesn't support it.

> So, the middle ground was to warn about it during the kbuild logic so
> you could find the source of the problem, but ultimately fail the build
> when the compiler doesn't support it so there weren't any silent failure
> modes.

Longer term it would be of course best to move all the cc-options
probing into Kconfig. I bet that would speed up builds too.

-Andi
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