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

[Andrew Melnichenko]

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

To solve the case - we need to "save" the halper from an old package somehow.

>
> With regards,
> Daniel
> --
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