linux-kernel - Re: [PATCH v2] x86/build: Mark per-cpu symbols as absolute

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Message-ID: <CAKwvOdn5Syd_pvAmcDy6n6c9=HrpRxEU2fWBa7Rf2QexPZP90g@mail.gmail.com>
Date:   Tue, 8 Jan 2019 12:52:10 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Rafael Avila de Espindola <rafael@...indo.la>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        Bill Wendling <morbo@...gle.com>,
        Dmitry Golovin <dima@...ovin.in>, hpa@...or.com,
        Michael Matz <matz@...e.de>, Tri Vo <trong@...roid.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Joerg Roedel <jroedel@...e.de>,
        Cao jin <caoj.fnst@...fujitsu.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] x86/build: Mark per-cpu symbols as absolute

On Wed, Dec 19, 2018 at 11:02 AM <ndesaulniers@...gle.com> wrote:
>
> From: Rafael Ávila de Espíndola <rafael@...indo.la>
>
> The kernel has many variables that it wants to have per CPU. It is
> similar to how each thread wants a copy of a thread local variable.
>
> To access such variable, the code has to find the offset of that
> variable in the per cpu block and add it to the address of the current
> brock for that cpu.
>
> Section 3.10.8 of ld.bfd's documentation states:
> ```
> For expressions involving numbers, relative addresses and absolute
> addresses, ld follows these rules to evaluate terms:
>
> Other binary operations, that is, between two relative addresses
> not in the same section, or between a relative address and an
> absolute address, first convert any non-absolute term to an
> absolute address before applying the operator."
> ```
>
> Note that ld.lld does not implement this behavior. Instead, we can
> explicitly mark non-absolute terms ABSOLUTE via linker script. This is
> no functional change for ld.bfd which coverts the term to ABSOLUTE
> anyways as specified above.
>
> Reported-by: Dmitry Golovin <dima@...ovin.in>
> Tested-by: Dmitry Golovin <dima@...ovin.in>
> Suggested-by: Borislav Petkov <bp@...en8.de>
> Suggested-by: Michael Matz <matz@...e.de>
> Signed-off-by: Tri Vo <trong@...roid.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Rafael Ávila de Espíndola <rafael@...indo.la>
> [nd: commit message updated as per Boris' and Michael's sugguestions]

Happy new year.  Bumping for review.

> ---
> Changes from v1 -> v2: commit message updated
>
>  arch/x86/kernel/vmlinux.lds.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 0d618ee634ac..ee3b5c7d662e 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -401,7 +401,7 @@ SECTIONS
>   * Per-cpu symbols which need to be offset from __per_cpu_load
>   * for the boot processor.
>   */
> -#define INIT_PER_CPU(x) init_per_cpu__##x = x + __per_cpu_load
> +#define INIT_PER_CPU(x) init_per_cpu__##x = ABSOLUTE(x) + __per_cpu_load
>  INIT_PER_CPU(gdt_page);
>  INIT_PER_CPU(irq_stack_union);
>
> --
> 2.20.1.415.g653613c723-goog
>


-- 
Thanks,
~Nick Desaulniers