linux-kernel - Re: [PATCH] x86/alternatives: Fix optimize

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Message-ID: <20180110205015.ehahjumt3e3in6q6@pd.tnic>
Date:   Wed, 10 Jan 2018 21:50:16 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Jiri Kosina <jikos@...nel.org>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86/alternatives: Fix optimize_nops() checking

On Wed, Jan 10, 2018 at 12:20:40PM -0800, Linus Torvalds wrote:
> No, that's not valid. That could never work anyway. The ".long 2f"
> would be the absolute address in the alternative section, but opcode
> E9 takes a relative 32-bit offset.

Ah, right, doh. I remember now. We used to do those jmps by computing the
relative offset:

From: 090a3f615524c3f75d09fdb37f15ea1868d79f7e
-       .section .altinstr_replacement,"ax"
-1:     .byte 0xeb                                      /* jmp <disp8> */
-       .byte (copy_page_rep - copy_page) - (2f - 1b)   /* offset */

-- 
Regards/Gruss,
    Boris.

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