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tundra/tools/ramdisk_reference.py

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+# SPDX-License-Identifier: MPL-2.0
+"""Read verified newc ramdisks, resolving hardlinks without extracting secrets."""
+import gzip
+from pathlib import PurePosixPath
+import stat
+import subprocess
+import re
+import os
+
+
+def read_newc(data):
+    if data[:2] == b'\x1f\x8b':
+        data = gzip.decompress(data)
+    entries, payloads, offset = {}, {}, 0
+    while offset + 110 <= len(data):
+        header = data[offset:offset+110]
+        if header[:6] != b'070701':
+            raise ValueError('Expected newc ramdisk')
+        fields = [int(header[6+i*8:14+i*8],16) for i in range(13)]
+        inode, mode, uid, gid, links, mtime, size, major, minor, rmajor, rminor, namesize, check = fields
+        name = data[offset+110:offset+110+namesize-1].decode()
+        start = (offset+110+namesize+3)&~3
+        offset = (start+size+3)&~3
+        if name == 'TRAILER!!!':
+            break
+        path = PurePosixPath(name)
+        if path.is_absolute() or '..' in path.parts or offset > len(data):
+            raise ValueError('Invalid ramdisk entry')
+        if name == '.':
+            continue
+        key = (major,minor,inode)
+        payload = data[start:start+size]
+        entries[str(path)] = {'mode':mode,'data':payload,'key':key,'links':links}
+        if stat.S_ISREG(mode) and payload:
+            payloads[key] = payload
+    for entry in entries.values():
+        if stat.S_ISREG(entry['mode']) and entry['links'] > 1 and not entry['data']:
+            entry['data'] = payloads.get(entry['key'],b'')
+    return entries
+
+
+def resolve(entries, name):
+    import posixpath
+    seen = set()
+    while stat.S_ISLNK(entries[name]['mode']):
+        if name in seen:
+            raise ValueError('Cyclic ramdisk link')
+        seen.add(name)
+        link = entries[name]['data'].decode()
+        name = posixpath.normpath(link.lstrip('/') if link.startswith('/') else posixpath.join(posixpath.dirname(name),link))
+        if name.startswith('../'):
+            raise ValueError('Link escapes ramdisk')
+    return entries[name]['data']
+
+
+def select_helpers(entries, scratch, extra=()):
+    """Copy only explicit programs and their ELF dependencies, never /etc or /root."""
+    selected = {}
+    queue = ['bin/busybox','sbin/dropbear','sbin/dmsetup','lib/aarch64-linux-gnu/libnss_files.so.2', *extra]
+    while queue:
+        name = queue.pop()
+        if name in selected:
+            continue
+        content = resolve(entries,name)
+        selected[name] = content
+        probe = scratch/'elf';probe.write_bytes(content)
+        info = subprocess.check_output(['readelf','-ld',str(probe)],text=True,env={**os.environ, 'LC_ALL':'C'})
+        interpreter = re.search(r'Requesting program interpreter: ([^\]]+)',info)
+        if interpreter:
+            queue.append(interpreter.group(1).lstrip('/'))
+        for library in re.findall(r'\(NEEDED\).*\[([^\]]+)\]',info):
+            found = next((base+library for base in ('lib/aarch64-linux-gnu/','usr/lib/aarch64-linux-gnu/','lib/','usr/lib/') if base+library in entries),None)
+            if found is None:
+                raise ValueError('Missing ramdisk library: '+library)
+            queue.append(found)
+    return selected