linux-kernel - Re: [PATCH 01/25] x86: Make SMAP 64-bit only

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Date:   Mon, 18 Mar 2019 09:58:20 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, Peter Anvin <hpa@...or.com>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 01/25] x86: Make SMAP 64-bit only

On Mon, Mar 18, 2019 at 8:54 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> We rely on objtool to verify AC=1 doesn't escape. However there is no
> objtool support for x86_32, and thus we cannot guarantee the
> correctness of the 32bit code.

Absolutely not.

This is just crazy. We had working SMAP long before objtool, and we
will have it regardless of objtool.

This is like saying "ok, I don't have an oxygen sensor, so I don't
know that the air I'm breathing is sufficient to maintain life, so
I'll just stop breathing".

So no way in hell do we make SMAP go away on 32-bit for no sane reason
what-so-ever.

Besides, the x86-64 objtool coverage will cover 99% of all 32-bit code
too, simply because we share it. In fact, it will cover most of the
code for other architectures too.

                  Linus