>> > - I tried adding `--shared-dir /tmp/ff:ff:type=9p` to share a host
>> > directory. Then `mount -t 9p -o trans=virtio,version=9p2000.L ff /tmp`
>> > in the VM seemed to work, but `ls /tmp` crashed the VM.
>>
>> Yeah, this is a known issue.  I have a patch[1] for it but didn't add it
>> to the package since I mostly have been working with my own source
>> builds of crosvm.
>>
>> [1]: https://spectrum-os.org/git/crosvm/commit/?id=1e318da5b57c12f67bed3b528100dbe4ec287ac5
>
> Ah, I didn't realise it was using seccomp too. I'm not sure how to
> compile specific versions of crosvm. I tried with:
>
>   srcs = lib.genAttrs [
>     "src/third_party/adhd"
>     "src/aosp/external/minijail"
>   ] getSrc // { "src/platform/crosvm" = /home/.../crosvm; };
>
> and blanked out the hash as it requested, but then:
>
> error: failed to sync Caused by: failed to load pkg lockfile Caused
> by: failed to resolve patches for
> `https://github.com/rust-lang/crates.io-index` Caused by: failed to
> load source for dependency `libvda` Caused by: Unable to update
> /build/src/platform2/arc/vm/libvda/rust Caused by: failed to read
> `/build/src/platform2/arc/vm/libvda/rust/Cargo.toml`
>
> Looks like this happens since 57df6a0ab23c3b2ba233b9aa5886ecf47ba3f91f
> (added a dependency?). Commit 460406d10bbfaa890d56d616b4610813da63a312
> just before that gets further, but:
>
> error: the lock file /build/src/platform/crosvm/Cargo.lock needs to be
> updated but --frozen was passed to prevent this
>
> How do you build it?
>
> (sorry for these basic Nix/Rust questions)
>
> However, I could get 9p to work by running the previous version with
> --seccomp-log-failures. With that, I can read and write files from the
> console, but I can't chown things and so can't write from the terminal
> window, which is running as a user. I guess it needs uidmap set, but
> I'm not sure how to make that work.

Yeah, crosvm isn't a very nice program to build or package. :(  I tried
to get the libvda stuff working some time in the past, but it was very
complicated.  I think you might be able to disable it with

    cargoBuildFlags = [ "--no-default-features" ];

but my knowledge here is a few months out of date.  I can have a look in
more detail once I get back from my break. :)

>> Yeah, crosvm needs to be CAP_NET_ADMIN for that (which is difficult to
>> do with Nix).  You can make a TAP device yourself iproute2 and use
>> --tap-fd to tell crosvm to use it, or you can use the mktuntap program I
>> wrote (with a privelege drop after running mktuntap), like this:
>>
>>     sudo mktuntap -pvB 3 \
>>         sudo -u $USER -C 4 result/bin/spectrum-vm -- --tap-fd 3
>
> OK, I tried like this:
>
> exec sudo "$mktuntap" -pvB 3 \
>   sudo -u "$USER" -C 4 \
>    "$crosvm" run \
>     -p init=/sbin/init \
>     -p "spectrumcmd=$(printf %s "$command" | base64 -w0)" \
>     --tap-fd 3 \
>     --seccomp-log-failures \
>     --root "$rootfs" \
>     --host_ip 10.0.0.1 \
>     --netmask 255.0.0.0 \
>     --mac c0:ff:ee:c0:ff:ee \
>     -m 4096 \
>     "$@" \
>     "$kernel"
>
> I got "sudo: you are not permitted to use the -C option", which I
> fixed by editing the sudoers file. Then it fails with:
>
> [ERROR:src/main.rs:1351] The architecture failed to build the vm:
> error creating devices: failed to set up virtio networking: failed to
> open tap device: failed to create tap interface: Operation not
> permitted (os error 1)
>
> Strace shows:
>
> openat(AT_FDCWD, "/dev/net/tun", O_RDWR|O_NONBLOCK|O_CLOEXEC) = 31
> ioctl(31, TUNSETIFF, 0x7ffee7ede238) = -1 EPERM (Operation not permitted)
>
> Maybe it's just because my crosvm is too old?

This is because if you specify --host_ip, --netmask, or --mac, crosvm
will try to create its own TAP device.  If you omit all those arguments
I think it should work.

>> Hope that's all clear -- please ask more questions if you have them,
>> although if it's anything particularly in the weeds I might wait until
>> I'm back from my break to answer. :)
>
> I have many questions :-) But don't feel pressured to answer them; I
> need to figure out how to make this all work myself anyway, and it's
> just a bonus if you've already done the work for me...

Well, my ultimate goal is to provide a distribution so that people don't
need to figure this stuff out for themselves, but we are a little while
away from that. ;)