linux-kernel - Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

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Date:   Thu, 21 Jul 2022 11:06:42 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joao Moreira <joao@...rdrivepizza.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        "Moreira, Joao" <joao.moreira@...el.com>,
        "Nuzman, Joseph" <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Gross, Jurgen" <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Peter Collingbourne <pcc@...gle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

On Thu, Jul 21, 2022 at 10:56 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> this seems to work, let me go hack the kernel..

Am I missing something?

Isn't this generating

        movl $~IMM,%r10d
        negl %r10d
        cmpl %r10d,-4(%calldest)

for the sequence?

That seems bogus for two reasons:

 (a) 'neg' is not the opposite of '~'. Did you mean 'notl' or did you mean '-'?

     Or am I missing something entirely?

 (b) since you have that r10 use anyway, why can't you just generate the simpler

        movl $-IMM,%r10d
        addl -4(%calldest),%r10d

     instead? You only need ZF anyway.

     Maybe you need to add some "r10 is clobbered" thing, I don't know.

But again: I don't know llvm, so the above is basically me just doing
the "pattern matching monkey" thing.

             Linus