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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 10 Jan 2018 12:20:40 -0800
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Subject: Re: [PATCH] x86/alternatives: Fix optimize_nops() checking
To: Borislav Petkov <bp@alien8.de>
Cc: David Woodhouse <dwmw2@infradead.org>,
        Andi Kleen <andi@firstfloor.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Andrew Lutomirski <luto@kernel.org>,
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        Tom Lendacky <thomas.lendacky@amd.com>,
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On Wed, Jan 10, 2018 at 12:05 PM, Borislav Petkov <bp@alien8.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