[Bug]: youki silently drops sync and dirsync on tmpfs mounts #3818

Bug Description

The tmpfs mount succeeds, but /proc/self/mountinfo lacks the requested sync or dirsync superblock option. The container prints the actual mountinfo line.

The directly observed defect is a missing flag. This tmpfs test is not a disk persistence or performance benchmark.

The minimal configuration below was rechecked on 2026-10-01 using the fixed runtime commit recorded below. This does not claim a retest of current upstream HEAD.

Steps to Reproduce

Linux, root with namespace/cgroup permissions, and a statically linked BusyBox matching the host architecture (for example the busybox-static package). Use a fresh directory and rootfs for every candidate and control.

Save this complete configuration as config.json in the fresh directory:

{
  "ociVersion": "1.3.0",
  "root": {
    "path": "rootfs"
  },
  "process": {
    "terminal": false,
    "user": {
      "uid": 0,
      "gid": 0
    },
    "args": [
      "/busybox",
      "sh",
      "-c",
      "/busybox grep ' /stack ' /proc/self/mountinfo; exec /busybox sleep 300"
    ],
    "env": [
      "PATH=/"
    ],
    "cwd": "/"
  },
  "linux": {
    "namespaces": [
      {
        "type": "mount"
      },
      {
        "type": "pid"
      }
    ]
  },
  "mounts": [
    {
      "destination": "/proc",
      "type": "proc",
      "source": "proc"
    },
    {
      "destination": "/stack",
      "type": "tmpfs",
      "source": "tmpfs",
      "options": [
        "size=1m",
        "mode=777",
        "sync"
      ]
    }
  ]
}

Prepare the rootfs and any required fixture:

mkdir -p rootfs/dev
cp /path/to/static-busybox rootfs/busybox
export RUNTIME=/absolute/path/to/youki

Run individually without set -e, because a candidate may intentionally fail:

runtime=${RUNTIME:-youki}
"$runtime" --root "$PWD/state" create --bundle "$PWD" repro-youki >process.log 2>create.stderr
echo "create exit: $?"
cat create.stderr
"$runtime" --root "$PWD/state" state repro-youki
"$runtime" --root "$PWD/state" start repro-youki
echo "start exit: $?"
"$runtime" --root "$PWD/state" state repro-youki
sleep 0.1
cat process.log

After saving the observation, clean up only this test container:

"${RUNTIME:-youki}" --root "$PWD/state" delete --force repro-youki

Expectation

No response

System and Setup Info

youki version: 0.7.0
commit: 0.7.0-993a50ffb788be1a1a879f3446c10dd0c0b6a5a1
spec: 1.1.0
rustc: 1.96.0

Source commit: 993a50ffb788be1a1a879f3446c10dd0c0b6a5a1.

Host OS information

Ubuntu 24.04.1 LTS, x86_64, WSL2.

Host kernel information

6.6.114.1-microsoft-standard-WSL2.

Additional Context

youki-sync-mount-flags-reproducer.tar.gz