Re: [PATCH v2 5/9] x86: Make sure _etext includes function sections

[Kees Cook <keescook@xxxxxxxxxxxx>]

On Thu, May 21, 2020 at 09:56:36AM -0700, Kristen Carlson Accardi wrote:
> When using -ffunction-sections to place each function in
> it's own text section so it can be randomized at load time, the
> linker considers these .text.* sections "orphaned sections", and
> will place them after the first similar section (.text). In order
> to accurately represent the end of the text section and the
> orphaned sections, _etext must be moved so that it is after both
> .text and .text.* The text size must also be calculated to
> include .text AND .text.*
> 
> Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
> Tested-by: Tony Luck <tony.luck@xxxxxxxxx>
> ---
>  arch/x86/kernel/vmlinux.lds.S     | 18 +++++++++++++++++-
>  include/asm-generic/vmlinux.lds.h |  2 +-
>  2 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 1bf7e312361f..044f7528a2f0 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -147,8 +147,24 @@ SECTIONS
>  #endif
>  	} :text =0xcccc
>  
> -	/* End of text section, which should occupy whole number of pages */
> +#ifdef CONFIG_FG_KASLR
> +	/*
> +	 * -ffunction-sections creates .text.* sections, which are considered
> +	 * "orphan sections" and added after the first similar section (.text).
> +	 * Adding this ALIGN statement causes the address of _etext
> +	 * to be below that of all the .text.* orphaned sections
> +	 */
> +	. = ALIGN(PAGE_SIZE);
> +#endif
>  	_etext = .;
> +
> +	/*
> +	 * the size of the .text section is used to calculate the address
> +	 * range for orc lookups. If we just use SIZEOF(.text), we will
> +	 * miss all the .text.* sections. Calculate the size using _etext
> +	 * and _stext and save the value for later.
> +	 */
> +	text_size = _etext - _stext;
>  	. = ALIGN(PAGE_SIZE);

I don't think there's any need for this #ifdef (nor the trailing ALIGN).
I think leave the comment to explain why the ALIGN is being moved before
the _etext.

A repeated ALIGN won't move the position:

--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -147,9 +147,11 @@ SECTIONS
 #endif
        } :text =0xcccc
 
+       . = ALIGN(PAGE_SIZE);
        /* End of text section, which should occupy whole number of * pages */
        _etext = .;
        . = ALIGN(PAGE_SIZE);
+       _ktext = .;
 
        X86_ALIGN_RODATA_BEGIN
        RO_DATA(PAGE_SIZE)


$ nm vmlinux | grep '_[ek]text'
ffffffff81e02000 R _etext
ffffffff81e02000 R _ktext

-- 
Kees Cook