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

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Message-ID: <CA+55aFwxK6=LEyOR+3+wd7Lepxjmy4K-SduvQkEp+v7F=bd0_Q@mail.gmail.com>
Date:   Wed, 10 Jan 2018 12:20:40 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Borislav Petkov <bp@...en8.de>
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 12:05 PM, Borislav Petkov <bp@...en8.de> wrote:
>
> I did this:
>
>         alternative("", "xor %%rdi, %%rdi; .byte 0xe9; .long 2f; 2: jmp startup_64", X86_FEATURE_K8);

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.

So the error you get isn't because relocations wouldn't work in
alternatives, it's because you tried to fit an absolute 64-biy value
in a 32-bit relocation, and it was wrong _regardless_ of which section
you were in.

                   Linus