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Date:   Tue, 19 Jul 2022 11:06:40 -0700
From:   Sami Tolvanen <samitolvanen@google.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     David Laight <David.Laight@aculab.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Joao Moreira <joao@overdrivepizza.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        "Cooper, Andrew" <andrew.cooper3@citrix.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Johannes Wikner <kwikner@ethz.ch>,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Jann Horn <jannh@google.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        "Moreira, Joao" <joao.moreira@intel.com>,
        "Nuzman, Joseph" <joseph.nuzman@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Gross, Jurgen" <jgross@suse.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Peter Collingbourne <pcc@google.com>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
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On Tue, Jul 19, 2022 at 10:27:00AM -0700, Linus Torvalds wrote:
> On Tue, Jul 19, 2022 at 10:23 AM Sami Tolvanen <samitolvanen@google.com> wrote:
> >
> > The preamble hash is encoded into an instruction just to avoid special
> > casing objtool, which would otherwise get confused about the random
> > bytes. On arm64, we just emit a bare constant before the function.
> 
> Ahh.
> 
> I think objtool would want to understand about kCFI anyway, so I think
> in the long run that hack isn't a goog idea.
> 
> But I get why you'd do it as a "do this as just a compiler thing and
> hide it from objtool" as a development strategy.

I believe it was actually Peter's idea to use an instruction. :) In
earlier revisions of KCFI, I did teach objtool about the preambles, but
that was just so it can ignore them.

Sami