linux-kernel - Re: [PATCH 1/9] objtool,x86: Replace alternatives with .retpoline

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Message-ID: <YWbfPxkMdpr96lqx@zn.tnic>
Date:   Wed, 13 Oct 2021 15:29:35 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH 1/9] objtool,x86: Replace alternatives with
 .retpoline_sites

On Wed, Oct 13, 2021 at 02:22:18PM +0200, Peter Zijlstra wrote:
> Instead of writing complete alternatives, simply provide a list of all
> the retpoline thunk calls. Then the kernel is free to do with them as
> it pleases. Simpler code all-round.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/x86/kernel/vmlinux.lds.S       |   12 +++
>  arch/x86/lib/retpoline.S            |   42 ------------
>  tools/objtool/arch/x86/decode.c     |  126 +++++++-----------------------------
>  tools/objtool/elf.c                 |   84 ------------------------
>  tools/objtool/include/objtool/elf.h |    1 
>  5 files changed, 38 insertions(+), 227 deletions(-)
> 
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -273,6 +273,18 @@ SECTIONS
>  	}

#ifdef CONFIG_RETPOLINE around it I guess...

>  	/*
> +	 * List of instructions that call/jmp/jcc to retpoline thunks
> +	 * __x86_indirect_thunk_*(). These instructions can be patched along
> +	 * with alternatives, after which the section can be freed.
> +	 */
> +	. = ALIGN(8);
> +	.retpoline_sites : AT(ADDR(.retpoline_sites) - LOAD_OFFSET) {
> +		__retpoline_sites = .;
> +		*(.retpoline_sites)
> +		__retpoline_sites_end = .;
> +	}

#endif

-- 
Regards/Gruss,
    Boris.

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