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Date: Mon, 19 Oct 2020 16:56:59 -0700
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Subject: Re: [PATCH] powerpc/boot: move the .got section to after the .dynamic
 section
To: Bill Wendling <morbo@google.com>
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Cc: Alan Modra <amodra@gmail.com>, linuxppc-dev@lists.ozlabs.org,
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On Fri, Oct 16, 2020 at 5:01 PM Bill Wendling <morbo@google.com> wrote:
>
> Both .dynamic and .got are RELRO sections and should be placed together,
> and LLD emits an error:
>
>   ld.lld: error: section: .got is not contiguous with other relro sections
>
> Place them together to avoid this.
>
> Cc: Fangrui Song <maskray@google.com>
> Cc: Alan Modra <amodra@gmail.com>
> Signed-off-by: Bill Wendling <morbo@google.com>
> ---
>  arch/powerpc/boot/zImage.lds.S | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/arch/powerpc/boot/zImage.lds.S b/arch/powerpc/boot/zImage.lds.S
> index a21f3a76e06f..d6f072865627 100644
> --- a/arch/powerpc/boot/zImage.lds.S
> +++ b/arch/powerpc/boot/zImage.lds.S
> @@ -34,6 +34,17 @@ SECTIONS
>      __dynamic_start = .;
>      *(.dynamic)
>    }
> +
> +#ifdef CONFIG_PPC64_BOOT_WRAPPER
> +  . = ALIGN(256);
> +  .got :
> +  {
> +    __toc_start = .;
> +    *(.got)
> +    *(.toc)
> +  }
> +#endif
> +
>    .hash : { *(.hash) }
>    .interp : { *(.interp) }
>    .rela.dyn :
> @@ -76,16 +87,6 @@ SECTIONS
>      _esm_blob_end =  .;
>    }
>
> -#ifdef CONFIG_PPC64_BOOT_WRAPPER
> -  . = ALIGN(256);
> -  .got :
> -  {
> -    __toc_start = .;
> -    *(.got)
> -    *(.toc)
> -  }
> -#endif

The kernel does not require this but normally all read-only sections
precede SHF_WRITE sections.
.dynamic and .got have the SHF_WRITE flag and should be placed here.

Ideally, the order is: R RX RW(RELRO) RW(non-RELRO)  (LLD order)

For comparison:
GNU ld -z separate-code order: R RX R RW(RELRO) RW(non-RELRO) (GNU
ld>=2.31 enables -z separate-code by default for Linux x86)
GNU ld -z noseparate-code order: RX RW(RELRO) RW(non-RELRO)

(AFAIK The only reason .dynamic is writable is due to DT_DEBUG (whose
purpose is questionable nowadays). mips .dynamic is read-only. LLD has
an option -z rodynamic to make .dynamic readonly)

>    . = ALIGN(4096);
>    .bss       :
>    {
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>