fix(sandbox): permit non-Ubuntu rootfs and fix path resolution

Previously, only Ubuntu rootfs were allowed to be used. Now, it allows
any file ending in .ext4, with the defaults being AL2023 or Ubuntu in
it's absence.

Also allow host paths to be resolved. The sandbox tool is run from
inside the container, so it expected artifact paths to have /firecracker
to be prepended. Now, it can accept host paths within the Firecracker
workspace and convert them to container paths.

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>

Author: JamesC1305

tools/devtool

@@ -1177,7 +1177,7 @@ cmd_sh() {
 cmd_sandbox() {
     cmd_build --release
     cmd_ensure_current_artifacts
-    cmd_sh "tmux new env PYTEST_ADDOPTS=--pdbcls=IPython.terminal.debugger:TerminalPdb PYTHONPATH=tests IPYTHONDIR=\$PWD/.ipython ipython -i ./tools/sandbox.py $@"
+    cmd_sh "tmux new env HOST_FC_ROOT_DIR=$FC_ROOT_DIR PYTEST_ADDOPTS=--pdbcls=IPython.terminal.debugger:TerminalPdb PYTHONPATH=tests IPYTHONDIR=\$PWD/.ipython ipython -i ./tools/sandbox.py $@"
     cmd_fix_perms ".ipython"
 }
 

tools/sandbox.py

@@ -10,16 +10,17 @@
 
 import argparse
 import json
+import os
 import re
 from pathlib import Path
 
 import host_tools.cargo_build as build_tools
 from framework.artifacts import disks, kernels
-from framework.defs import DEFAULT_BINARY_DIR
+from framework.defs import DEFAULT_BINARY_DIR, FC_WORKSPACE_DIR
 from framework.microvm import MicroVMFactory
 
 kernels = list(kernels("vmlinux-*"))
-rootfs = list(disks("ubuntu*ext4"))
+rootfs = list(disks("*.ext4"))
 
 
 def parse_byte_size(param):
@@ -35,20 +36,51 @@ def parse_byte_size(param):
     return int(match.group("val")) * unit[match.group("unit")]
 
 
+def translate_host_path(p):
+    """Rewrite a host path under HOST_FC_ROOT_DIR to its /firecracker/... equivalent."""
+    if p is None:
+        return None
+    host_root = os.environ.get("HOST_FC_ROOT_DIR")
+    if not host_root:
+        return Path(p)
+    p = Path(p).resolve()
+    if p.is_relative_to(host_root):
+        return FC_WORKSPACE_DIR / p.relative_to(host_root)
+    if p.exists():
+        return p
+    raise SystemExit(
+        f"{p} not found in container and not under host workspace {host_root}."
+    )
+
+
+def pick_default_rootfs(candidates):
+    """Default to AL2023, falling back to Ubuntu, then any rootfs available."""
+    if not candidates:
+        return None
+    for prefix in ("amazonlinux-", "ubuntu-"):
+        matches = [c for c in candidates if c.name.startswith(prefix)]
+        if matches:
+            return matches[-1]
+    return candidates[-1]
+
+
+default_rootfs = pick_default_rootfs(rootfs)
+default_kernel = kernels[-1] if kernels else None
+
 parser = argparse.ArgumentParser()
 parser.add_argument(
     "--kernel",
     type=Path,
-    choices=kernels,
-    default=kernels[-1],
-    help=f"Kernel to use. [{kernels[-1]}]",
+    default=default_kernel,
+    help=f"Kernel to use. Default: {default_kernel}. "
+    f"Available: {[k.name for k in kernels]}",
 )
 parser.add_argument(
     "--rootfs",
     type=Path,
-    choices=rootfs,
-    default=rootfs[-1],
-    help=f"Rootfs to use. [{rootfs[-1]}]",
+    default=default_rootfs,
+    help=f"Rootfs to use. Default: {default_rootfs}. "
+    f"Available: {[r.name for r in rootfs]}",
 )
 parser.add_argument("--vcpus", type=int, default=2)
 parser.add_argument(
@@ -66,8 +98,17 @@ def parse_byte_size(param):
     "--gdb", action="store_true", default=False, help="Connect to Firecracker guest GDB"
 )
 args = parser.parse_args()
+args.kernel = translate_host_path(args.kernel)
+args.rootfs = translate_host_path(args.rootfs)
+args.binary_dir = translate_host_path(args.binary_dir)
+args.cpu_template_path = translate_host_path(args.cpu_template_path)
 print(args)
 
+if args.kernel is None:
+    raise SystemExit("No kernel found and --kernel was not provided.")
+if args.rootfs is None:
+    raise SystemExit("No rootfs found and --rootfs was not provided.")
+
 binary_dir = None
 if args.binary_dir:
     binary_dir = Path(args.binary_dir).resolve()
@@ -81,7 +122,7 @@ def parse_byte_size(param):
 
 cpu_template = None
 if args.cpu_template_path is not None:
-    cpu_template = json.loads(args.cpu_template_path.read_text())
+    cpu_template = json.loads(args.cpu_template_path.read_text("utf-8"))
 vmfcty = MicroVMFactory(binary_dir)
 
 if args.debug or args.gdb: