Re: [PATCH 3/5] qmp: Added the helper stamp check.

[Toke Høiland-Jørgensen]

Andrew Melnichenko <andrew@daynix.com> writes:

> Hi all,
>
> On Mon, Feb 20, 2023 at 11:50 AM Daniel P. Berrangé <berrange@redhat.com> 
> wrote:
>>
>> On Sun, Feb 19, 2023 at 06:20:58PM +0200, Andrew Melnychenko wrote:
>> > Added a function to check the stamp in the helper.
>> > eBPF helper should have a special symbol that generates during the build.
>> > QEMU checks the helper and determines that it fits, so the helper
>> > will produce proper output.
>>
>> I think this is quite limiting for in place upgrades.
>>
>> Consider this scenario
>>
>>  * Host has QEMU 8.1.0 installed
>>  * VM is running QEMU 8.1.0
>>  * QEMU 8.1.1 is released with a bug fix in the EBF program
>>  * Host is upgraded to QEMU 8.1.1
>>  * User attempts to hotplug a NIC to the running VM
>>
>> IIUC this last step is going to fail because we'll be loading
>> the EBF program from 8.1.1 and so its hash is different from
>> that expected by the QEMU 8.1.0 that the pre-existing VM is
>> running.
>>
>> If some changes to the EBF program are not going to be back
>> compatible from the POV of the QEMU process, should we instead
>> be versioning the EBF program. eg so new QEMU will ship both
>> the old and new versions of the EBF program.
>>
>
> I think it's too complicated to maintain backward compatibility with
> eBPF programs in "one package".
> As we can see, the eBPF skeleton may be changed not only by bugfix but
> with changes in libbpf(g.e. libbpf 1.0.1+).

Hmm, what change in libbpf 1.0.1 affects the skeleton format?

> This may lead to issues when with a newer environment you can't
> consistently recreate the "old" skeleton.

This should be detectable, though? As in, if you know that a new version
breaks compatibility you just bump the version number regardless of the
underlying application version?

-Toke