Re: [PATCH v2] livepatch: set -f{function,data}-sections compiler option

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.03.2022 16:55, 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.
> 
> Note that placement of the sections inside of .text is also slightly
> adjusted to be more similar to the position found in the default GNU
> ld linker script. This requires having a separate section for the
> header in order to place it at the begging of the output image,
> followed with the unlikely and related sections, and finally the main
> .text section.
> 
> On Arm the .data.read_mostly needs to be moved ahead of the .data
> section like it's already done on x86, and the alignment 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>

The x86 part of this looks fine to me, just one other remark:

> --- a/xen/common/Kconfig
> +++ b/xen/common/Kconfig
> @@ -350,10 +350,14 @@ source "common/sched/Kconfig"
>  config CRYPTO
>       bool
>  
> +config CC_SPLIT_SECTIONS
> +     bool

I think this wants to live higher up in the file, perhaps between
ALTERNATIVE_CALL and HAS_ALTERNATIVE. (CRYPTO, as seen in context
here, imo also would better live higher up.) Or alternatively it may
want to move to xen/Kconfig, next to CC_HAS_VISIBILITY_ATTRIBUTE.

Jan