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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.03.2022 17:36, Roger Pau Monné wrote:
> On Mon, Mar 07, 2022 at 05:28:20PM +0100, Jan Beulich wrote:
>> 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.
> 
> I was tempted to place it in xen/Kconfig. The logic for the current
> suggested placement is to be closer to it's current only user
> (LIVEPATCH), but I'm not opposed to moving it somewhere else if
> there's consensus.

I guess the main question is whether this option is intended to gain
a prompt. If so, xen/common/Kconfig is likely the better place. If
not, I think it better fits in xen/Kconfig.

Jan