micro-activate: generate machine-id randomly

D-Bus is supposed to (?) use it to decide whether it can use FD passing,
shared memory, etc. and while we do a lot of cross-domain magic it's not
quite seamless :) so let's not reuse the host one.

micro-activate.rs

@@ -16,6 +16,20 @@ unsafe extern "C" {
         flags: c_ulong,
         data: *const c_void,
     ) -> c_int;
+    fn getrandom(buf: *mut u8, buflen: usize, flags: u32) -> c_int;
+}
+
+fn gen_machine_id() -> String {
+    use std::fmt::Write as _;
+    let mut bytes: [u8; 16] = [0; 16];
+    if unsafe { getrandom(bytes.as_mut_ptr(), 16, 0) } == -1 {
+        eprintln!("[micro-activate] getrandom failed!");
+    }
+    let mut result = String::with_capacity(32);
+    for b in bytes {
+        let _ = write!(result, "{:02x}", b);
+    }
+    result
 }
 
 fn parse_tmpfiles_line(line: &str) -> Option<(&str, &str)> {
@@ -57,7 +71,6 @@ fn main() -> Result<(), std::io::Error> {
     //
     // Let's preserve the fixed passed-in files and set up the NixOS symlinks in the new mount.
     let resolv_conf = std::fs::read("/run/resolv.conf")?;
-    let machine_id = std::fs::read("/run/machine-id")?;
     assert_eq!(
         unsafe {
             mount(
@@ -71,7 +84,7 @@ fn main() -> Result<(), std::io::Error> {
         0
     );
     std::fs::write("/run/resolv.conf", &resolv_conf)?;
-    std::fs::write("/run/machine-id", &machine_id)?;
+    std::fs::write("/run/machine-id", &gen_machine_id())?;
     std::os::unix::fs::symlink(&closure, "/run/current-system")?;
     if let Ok(tmp_graphics) =
         std::fs::read(format!("{closure}/etc/tmpfiles.d/graphics-driver.conf"))

munix

@@ -195,7 +195,6 @@ bwrap --unshare-all --share-net \
   --symlink "$MICROVM_CLOSURE/sw/bin/env" /usr/bin/env \
   --symlink "$MICROVM_CLOSURE" /run/current-system \
   --ro-bind /nix/store /nix/store \
-  --file 12 /run/machine-id \
   --file 13 /run/resolv.conf \
   --dir "$XDG_RUNTIME_DIR" \
   --setenv PATH "/run/munix/muvm:/run/munix/passt:$MICROVM_CLOSURE/sw/bin" \
@@ -207,5 +206,4 @@ bwrap --unshare-all --share-net \
     -e MICROVM_CLOSURE="$MICROVM_CLOSURE" \
     -e MICROVM_UID="$MICROVM_UID" -e MICROVM_GID="$MICROVM_GID" \
     -i -t "${MICROVM_COMMAND[@]}" \
-  12< /etc/machine-id \
   13< /etc/resolv.conf