[VM] Override address size limit to unlimited

[Ionic]

The "build" script limits PRLIMIT_AS to (2/3) * (mem_size + swap_size),
which breaks asan, which tries to opportunistically allocate multiple TB
of memory, but crucially never even closely uses this.

This commit eliminates the memory limit right before spawning VMs (and
containers) if necessary, so that stage2 can run unrestricted
(notwithstanding the memory limit imposed by hypervisors) and use memory
overcommitment within the build process.

See the linked bug for a discussion of this.

Fixes: #826

[adrianschroeter]

the ulimit calls would fail when using build script as non-root?

I agree that a limit when doing VM builds seem not to be necessary, not sure about this approach though.

[dirkmueller]

is this still needed after 52cec99 ?

[Ionic]

the ulimit calls would fail when using build script as non-root?

Probably, yes, but doesn't spawning a build require root access to begin with? The actual build process is being run unprivileged in most cases, but the setup typically requires elevated privileges.

is this still needed after 52cec99 ?

Very good question. That one (obviously) is completely new to me. Looking at it and reading up, it does look like my PR is obsolete after the mentioned commit and that it fixes asan usage within VMs/containers. I'll test that and close the PR and bug report if it turns out to essentially allow memory overcommitment.

[Ionic]

Unfortunately, even 52cec99 breaks address sanitizers with LXC builds.

After that change, I'll have to override lxc.prlimit.data to get packages with address sanitizers working, of course, but it essentially boils down to the same breakage.

I can't really test the other virtualization/containerization options (I'd prefer using Qemu/KVM in the first place, but the VM I'm running an OBS appliance on doesn't support nested virtualization, so I'm out of luck on that front).

Will update this PR accordingly.