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

[Nick Desaulniers]

On Wed, Dec 19, 2018 at 11:02 AM <ndesaulniers@xxxxxxxxxx> wrote:
>
> From: Rafael Ãvila de EspÃndola <rafael@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Tested-by: Dmitry Golovin <dima@xxxxxxxxxx>
> Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
> Suggested-by: Michael Matz <matz@xxxxxxx>
> Signed-off-by: Tri Vo <trong@xxxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Signed-off-by: Rafael Ãvila de EspÃndola <rafael@xxxxxxxxxx>
> [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