fixup! fix(tar) absolute symlink extraction

Author: e3krisztian

unblob/handlers/archive/_safe_tarfile.py

@@ -77,23 +77,33 @@ def extract(self, tarinfo: tarfile.TarInfo, extract_root: Path):  # noqa: C901
         # prevent traversal attempts through links
         if tarinfo.islnk() or tarinfo.issym():
             if Path(tarinfo.linkname).is_absolute():
-                self.record_problem(
-                    tarinfo,
-                    "Absolute path as link target.",
-                    "Converted to extraction relative path.",
-                )
 
                 def calculate_linkname():
                     root = extract_root.resolve()
                     path = (extract_root / tarinfo.name).resolve()
-                    common_path = Path(os.path.commonpath([root, path]))
-                    # normally root == common_path
-                    # if it is not, the output will be bad
-                    depth = max(0, len(path.parts) - len(common_path.parts) - 1)
+
+                    if path.parts[: len(root.parts)] != root.parts:
+                        return None
+
+                    depth = max(0, len(path.parts) - len(root.parts) - 1)
                     return ("/".join([".."] * depth) or ".") + tarinfo.linkname
 
-                tarinfo.linkname = calculate_linkname()
-                assert not Path(tarinfo.linkname).is_absolute()
+                relative_linkname = calculate_linkname()
+                if relative_linkname is None:
+                    self.record_problem(
+                        tarinfo,
+                        "Absolute path conversion to extraction relative failed - would escape root.",
+                        "Skipped.",
+                    )
+                    return
+
+                assert not Path(relative_linkname).is_absolute()
+                self.record_problem(
+                    tarinfo,
+                    "Absolute path as link target.",
+                    "Converted to extraction relative path.",
+                )
+                tarinfo.linkname = relative_linkname
 
             resolved_path = (extract_root / tarinfo.name).parent / tarinfo.linkname
             if not is_safe_path(basedir=extract_root, path=resolved_path):