linux-kernel - Re: [PATCH 02/24] objtool: Add retpoline validation

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Date:   Fri, 26 Jan 2018 09:54:50 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>
Subject: Re: [PATCH 02/24] objtool: Add retpoline validation

On Tue, 2018-01-23 at 16:25 +0100, Peter Zijlstra wrote:
> 
> +               if (insn->type != INSN_JUMP_DYNAMIC &&
> +                   insn->type != INSN_CALL_DYNAMIC) {
> +                       WARN_FUNC("retpoline_safe hint not a indirect jump/call",
> +                                 insn->sec, insn->offset);
> +                       return -1;


...

        case 0xff:
                if (modrm_reg == 2 || modrm_reg == 3)

                        *type = INSN_CALL_DYNAMIC;

                else if (modrm_reg == 4)

                        *type = INSN_JUMP_DYNAMIC;

                else if (modrm_reg == 5)

                        /* jmpf */
                        *type = INSN_CONTEXT_SWITCH;


I *think* your check includes far calls (FF/3), although not far jumps?
It shouldn't, because I don't believe far calls are subject to the same
speculation?

Other than that, which you can probably ignore if you didn't have to
explicitly annotate [m]any safe far calls anyway,

Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>

Thanks for doing this.
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