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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 11 Feb 2018 10:13:44 -0800
Message-ID: <CA+55aFz3gUKQMr-X-b=ytLPX0AjGUaXNtLSUQ0_xZwZuDU33RQ@mail.gmail.com>
Subject: Re: [RFC PATCH 4/7] kconfig: support new special property shell=
To:     Kees Cook <keescook@chromium.org>
Cc:     Ulf Magnusson <ulfalizer@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nicolas Pitre <nicolas.pitre@linaro.org>,
        "Luis R . Rodriguez" <mcgrof@suse.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Sam Ravnborg <sam@ravnborg.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Pavel Machek <pavel@ucw.cz>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Jiri Kosina <jkosina@suse.cz>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Tejun Heo <tj@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
        Arnd Bergmann <arnd@arndb.de>
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On Sun, Feb 11, 2018 at 9:56 AM, Kees Cook <keescook@chromium.org> wrote:
>
>>     - How common are those broken compilers?
>
> I *thought* it was rare (i.e. gcc 4.2) but while working on ..._AUTO I
> found breakage in akpm's 4.4 gcc, and all of Arnd's gccs due to some
> very strange misconfiguration between the gcc build environment and
> other options. So, it turns out this is unfortunately common. The good
> news is that it does NOT appear to happen with most distro compilers,
> though I've seen Android's compiler regress the global vs %gs at least
> once about a year ago.

Hmm. Ok, so it's not *that* common, and won't affect normal people.

That actually sounds like we could just

 (a) make gcc 4.5 be the minimum required version

 (b) actually error out if we find a bad compiler

Upgrading the minimum required gcc version to 4.5 is pretty much going
to happen _anyway_, because we're starting to rely on "asm goto" for
avoiding speculation.

End result: maybe we can make the configuration phase just use the
standard "does gcc support this flag" logic, and then just have a
separate script that is run to validate that gcc doesn't generate
garbage, and error out loudly if it does.

               Linus