Re: [Xen-devel] [livepatch-build-tools part3 1/3] create-diff-object: Do not create empty .livepatch.funcs section

["Wieczorkiewicz, Pawel" <wipawel@xxxxxxxxx>]

> On 29. Apr 2019, at 16:37, Ross Lagerwall <ross.lagerwall@xxxxxxxxxx> wrote:
> 
> On 4/25/19 5:51 AM, Konrad Rzeszutek Wilk wrote:
>> On Tue, Apr 16, 2019 at 12:22:39PM +0000, Pawel Wieczorkiewicz wrote:
>>> When there is no changed function in the generated payload, do not
>>> create an empty .livepatch.funcs section. Hypervisor code considers
>>> such payloads as broken and rejects to load them.
>>> 
>>> Such payloads without any changed functions may appear when only
>>> hooks are specified.
>> Ross, I am going to push this in next week unless you have other thoughts?
> Reviewed-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> 
> This code change looks OK to me. However:
> 

Thank you for reviewing.

> 1) I think that the hypervisor should treat an empty .livepatch.funcs section 
> the same as it treats a non-present .livepatch.funcs section (i.e. it allows 
> it) which would make this change unnecessary.
> 

I do not have a strong opinion here, but it felt unnecessary (and a bit 
confusing) to generate an empty section.
Also I did not want to touch hypervisor code for this.

> 2) Unless I'm being stupid, I don't see how this change would work anyway. 
> Surely this code at the start of prepare_payload() would fail if the section 
> were missing?
> 
>    sec = livepatch_elf_sec_by_name(elf, ELF_LIVEPATCH_FUNC);
>    ASSERT(sec);
>    if ( !section_ok(elf, sec, sizeof(*payload->funcs)) )
>        return -EINVAL;
> 

In my soon-to-be-upstreamed backlog I have the following commit:
livepatch: Do not enforce ELF_LIVEPATCH_FUNC section presence
Where I actually make use of this new functionality.

The main idea behind this change, was to enable generation of hooks-only 
hotpatch modules
(i.e. modules that does not patch anything, just trigger actions).

> Regards,
> -- 
> Ross Lagerwall


Best Regards,
Pawel Wieczorkiewicz



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