From 6ca47a7157a908525af80a18e5dfd7c68b31169f Mon Sep 17 00:00:00 2001
From: icanhasmath <marcg@activestate.com>
Date: Thu, 22 Oct 2026 13:31:37 -0500
Subject: [PATCH] Review: let a symlink resolve to an explicit archive root

_tar_member_paths never recorded ".", so is_symlink_target_in_tar refused a
symlink that resolves to the archive root even when the archive has an
explicit "." member. A "tar -C dir ." archive containing
"./sub/root -> .." extracted before this review round (tar.getmember(".")
found the member) and on upstream 26.2.2, but failed with InstallationError
after the member-name normalization change.

Record "." only when a member normalizes to it, never implicitly, which
matches the previous behaviour and upstream: without an explicit "." member
the link is still refused. No security impact: this only removes a false
rejection, and the realpath containment checks are unchanged (a link to the
root resolves to the extract directory itself, which is inside it).

Adds tests for the explicit-root (accepted) and implicit-root (refused)
cases.

Also restore the second blank line before
test_tar_member_paths_terminates_on_doubled_slash (flake8 E302), which the
previous review commit left out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 src/pip/_internal/utils/unpacking.py |  6 ++++
 tests/unit/test_utils_unpacking.py   | 45 ++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/src/pip/_internal/utils/unpacking.py b/src/pip/_internal/utils/unpacking.py
index 740390c109..6039db965e 100644
--- a/src/pip/_internal/utils/unpacking.py
+++ b/src/pip/_internal/utils/unpacking.py
@@ -124,6 +124,12 @@ def _tar_member_paths(tar):
     paths = set()  # type: Set[str]
     for member in tar.getmembers():
         name = posixpath.normpath(member.name.replace("\\", "/"))
+        if name == ".":
+            # An explicit "." / "./" member is the archive root. Record it so a
+            # symlink resolving to the root (e.g. "./sub/root -> ..") is found,
+            # as tar.getmember(".") did. The root is never added implicitly.
+            paths.add(name)
+            continue
         while name not in ("", ".", "/"):
             paths.add(name)
             parent = posixpath.dirname(name)
diff --git a/tests/unit/test_utils_unpacking.py b/tests/unit/test_utils_unpacking.py
index 76d2e5bc73..b68d7de913 100644
--- a/tests/unit/test_utils_unpacking.py
+++ b/tests/unit/test_utils_unpacking.py
@@ -13,6 +13,7 @@
 from pip._internal.exceptions import InstallationError
 from pip._internal.utils.unpacking import (
     _tar_member_paths,
+    is_symlink_target_in_tar,
     is_within_directory,
     unpack_file,
     untar_file,
@@ -424,6 +425,7 @@ def test_unpack_polyglot_ambiguous_rejected(tmpdir):
     with pytest.raises(InstallationError):
         unpack_file(archive, os.path.join(str(tmpdir), 'out'))
 
+
 def test_tar_member_paths_terminates_on_doubled_slash():
     """posixpath.dirname("//") == "//"; collecting parents must not loop."""
     buf = io.BytesIO()
@@ -440,3 +442,46 @@ def test_tar_member_paths_terminates_on_doubled_slash():
         tar.close()
     assert "//abs/f.txt" in paths
     assert "//abs" in paths
+
+
+def _root_link_tar(explicit_root):
+    # type: (bool) -> tarfile.TarFile
+    buf = io.BytesIO()
+    with tarfile.open(fileobj=buf, mode="w") as tar:
+        names = (["./"] if explicit_root else []) + ["./sub/"]
+        for name in names:
+            info = tarfile.TarInfo(name)
+            info.type = tarfile.DIRTYPE
+            tar.addfile(info)
+        link = tarfile.TarInfo("./sub/root")
+        link.type = tarfile.SYMTYPE
+        link.linkname = ".."
+        tar.addfile(link)
+    buf.seek(0)
+    return tarfile.open(fileobj=buf)
+
+
+def test_symlink_to_explicit_archive_root_is_in_tar():
+    """A "tar -C dir ." archive with "./sub/root -> .." keeps unpacking."""
+    tar = _root_link_tar(explicit_root=True)
+    try:
+        paths = _tar_member_paths(tar)
+        assert "." in paths
+        assert is_symlink_target_in_tar(
+            tar, tar.getmember("./sub/root"), member_paths=paths
+        )
+    finally:
+        tar.close()
+
+
+def test_symlink_to_implicit_archive_root_is_not_in_tar():
+    """Without an explicit "." member the root is not added (as upstream)."""
+    tar = _root_link_tar(explicit_root=False)
+    try:
+        paths = _tar_member_paths(tar)
+        assert "." not in paths
+        assert not is_symlink_target_in_tar(
+            tar, tar.getmember("./sub/root"), member_paths=paths
+        )
+    finally:
+        tar.close()