Re: [PATCH v3 2/2] livepatch: set -f{function,data}-sections compiler option

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.03.2022 14:49, Roger Pau Monne wrote:
> If livepatching support is enabled build the hypervisor with
> -f{function,data}-sections compiler options, which is required by the
> livepatching tools to detect changes and create livepatches.
> 
> This shouldn't result in any functional change on the hypervisor
> binary image, but does however require some changes in the linker
> script in order to handle that each function and data item will now be
> placed into its own section in object files. As a result add catch-all
> for .text, .data and .bss in order to merge each individual item
> section into the final image.
> 
> The main difference will be that .text.startup will end up being part
> of .text rather than .init, and thus won't be freed. .text.exit will
> also be part of .text rather than dropped. Overall this could make the
> image bigger, and package some .text code in a sub-optimal way.
> 
> On Arm the .data.read_mostly needs to be moved ahead of the .data
> section like it's already done on x86, so the .data.* catch-all
> doesn't also include .data.read_mostly. The alignment of
> .data.read_mostly also needs to be set to PAGE_SIZE so it doesn't end
> up being placed at the tail of a read-only page from the previous
> section. While there move the alignment of the .data section ahead of
> the section declaration, like it's done for other sections.
> 
> The benefit of having CONFIG_LIVEPATCH enable those compiler option
> is that the livepatch build tools no longer need to fiddle with the
> build system in order to enable them. Note the current livepatch tools
> are broken after the recent build changes due to the way they
> attempt to set  -f{function,data}-sections.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> --- a/xen/arch/x86/xen.lds.S
> +++ b/xen/arch/x86/xen.lds.S
> @@ -88,6 +88,9 @@ SECTIONS
>         *(.text.unlikely .text.*_unlikely .text.unlikely.*)
>  
>         *(.text)
> +#ifdef CONFIG_CC_SPLIT_SECTIONS
> +       *(.text.*)
> +#endif
>         *(.text.__x86_indirect_thunk_*)
>         *(.text.page_aligned)

These last two now will not have any effect anymore when
CC_SPLIT_SECTIONS=y. This may have undesirable effects on the
overall size when there is more than one object with a
.text.page_aligned contribution. In .data ...

> @@ -292,9 +295,7 @@ SECTIONS
>  
>    DECL_SECTION(.data) {
>         *(.data.page_aligned)
> -       *(.data)
> -       *(.data.rel)
> -       *(.data.rel.*)
> +       *(.data .data.*)
>    } PHDR(text)

... this continues to be named first. I wonder whether we wouldn't
want to use SORT_BY_ALIGNMENT (if available) instead in both places.

Jan