fix(installer): identify live loop backing after dracut switch-root

hydra/installer.py

@@ -10,6 +10,7 @@ from __future__ import annotations
 import argparse
 from dataclasses import dataclass
 import getpass
+import fcntl
 import json
 import os
 from pathlib import Path, PurePosixPath
@@ -17,6 +18,7 @@ import re
 import shlex
 import shutil
 import stat
+import struct
 import subprocess
 import sys
 import tempfile
@@ -241,17 +243,12 @@ class SystemProbe:
         # Mounted loops may hide the physical filesystem that stores their file.
         for path, node in topo.nodes.items():
             if node['type'] == 'loop' and path in topo.protected:
-                backing_file = Path('/sys/dev/block') / node['maj:min'] / 'loop/backing_file'
-                try:
-                    backing = backing_file.read_text().strip()
-                except OSError as exc:
-                    raise InstallError(f'Cannot inspect mounted loop backing file: {path}') from exc
-                backing = re.sub(r'\\([0-7]{3})', lambda m: chr(int(m[1], 8)), backing)
-                if not backing.startswith('/'):
-                    backing = '/' + backing
-                source = self._source_for_path(backing, topo)
-                if not source:
-                    raise InstallError(f'Unknown physical backing for mounted loop: {path}')
+                identity = self._loop_backing_identity(path, node['maj:min'])
+                sources = [p for p, n in topo.nodes.items() if n['maj:min'] == identity]
+                if len(sources) != 1:
+                    raise InstallError(f'Unknown physical backing for mounted loop: {path} ({identity})')
+                source = sources[0]
+                topo.protect(source)
                 topo.link(path, source)
         if not Path(payload).is_file():
             raise InstallError(f'Prepared squashfs payload is not a regular file: {payload}')
@@ -262,6 +259,29 @@ class SystemProbe:
         topo.protect(source)
         return topo
 
+    @staticmethod
+    def _loop_backing_identity(path, expected_identity):
+        # LOOP_GET_STATUS64 reports the backing inode's filesystem device,
+        # independent of names which dracut switch_root may have relocated.
+        # lo_rdevice is nonzero when the backing inode is itself a block device.
+        try:
+            descriptor = os.open(path, os.O_RDONLY | os.O_CLOEXEC)
+            try:
+                info = os.fstat(descriptor)
+                identity = f'{os.major(info.st_rdev)}:{os.minor(info.st_rdev)}'
+                if not stat.S_ISBLK(info.st_mode) or identity != expected_identity:
+                    raise InstallError(f'Mounted loop device identity changed: {path}')
+                status = fcntl.ioctl(descriptor, 0x4C05, bytes(232))
+                if not isinstance(status, bytes) or len(status) != 232:
+                    raise InstallError(f'Malformed loop status: {path}')
+                filesystem_device, _, block_device = struct.unpack_from('=QQQ', status)
+            finally:
+                os.close(descriptor)
+        except (OSError, struct.error) as exc:
+            raise InstallError(f'Cannot inspect mounted loop backing identity: {path}') from exc
+        device = block_device or filesystem_device
+        return f'{os.major(device)}:{os.minor(device)}'
+
     @staticmethod
     def _mounts(raw):
         try: