Re: [PATCH v5 32/36] x86/boot/compressed: Reorganize zero-size section asserts

[Arvind Sankar]

On Fri, Jul 31, 2020 at 04:08:16PM -0700, Kees Cook wrote:
> For readability, move the zero-sized sections to the end after DISCARDS
> and mark them NOLOAD for good measure.
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  arch/x86/boot/compressed/vmlinux.lds.S | 42 +++++++++++++++-----------
>  1 file changed, 25 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 3c2ee9a5bf43..42dea70a5091 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -42,18 +42,16 @@ SECTIONS
>  		*(.rodata.*)
>  		_erodata = . ;
>  	}
> -	.rel.dyn : {
> -		*(.rel.*)
> -	}
> -	.rela.dyn : {
> -		*(.rela.*)
> -	}
> -	.got : {
> -		*(.got)
> -	}
>  	.got.plt : {
>  		*(.got.plt)
>  	}
> +	ASSERT(SIZEOF(.got.plt) == 0 ||
> +#ifdef CONFIG_X86_64
> +	       SIZEOF(.got.plt) == 0x18,
> +#else
> +	       SIZEOF(.got.plt) == 0xc,
> +#endif
> +	       "Unexpected GOT/PLT entries detected!")
>  
>  	.data :	{
>  		_data = . ;
> @@ -85,13 +83,23 @@ SECTIONS
>  	ELF_DETAILS
>  
>  	DISCARDS
> -}
>  
> -ASSERT(SIZEOF(.got) == 0, "Unexpected GOT entries detected!")
> -#ifdef CONFIG_X86_64
> -ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT entries detected!")
> -#else
> -ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, "Unexpected GOT/PLT entries detected!")
> -#endif
> +	/*
> +	 * Sections that should stay zero sized, which is safer to
> +	 * explicitly check instead of blindly discarding.
> +	 */
> +	.got (NOLOAD) : {
> +		*(.got)
> +	}
> +	ASSERT(SIZEOF(.got) == 0, "Unexpected GOT entries detected!")
>  
> -ASSERT(SIZEOF(.rel.dyn) == 0 && SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations detected!")
> +	/* ld.lld does not like .rel* sections being made "NOLOAD". */
> +	.rel.dyn : {
> +		*(.rel.*)
> +	}
> +	ASSERT(SIZEOF(.rel.dyn) == 0, "Unexpected run-time relocations (.rel) detected!")
> +	.rela.dyn : {
> +		*(.rela.*)
> +	}
> +	ASSERT(SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations (.rela) detected!")
> +}
> -- 
> 2.25.1
> 

There's no point in marking zero-size sections NOLOAD -- if the ASSERT's
passed, they won't be present in the file at all anyway.

The only section for which there might be a point is .got.plt, which is
non-empty on 32-bit, and only if it is first moved to the end. That
saves a few bytes.