gh-107845: Fix symlink handling for tarfile.data_filter (GH-107846)

Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Lumír 'Frenzy' Balhar <frenzy.madness@gmail.com>

Author: 3 people

Doc/library/tarfile.rst

@@ -740,6 +740,11 @@ A ``TarInfo`` object has the following public data attributes:
    Name of the target file name, which is only present in :class:`TarInfo` objects
    of type :const:`LNKTYPE` and :const:`SYMTYPE`.
 
+   For symbolic links (``SYMTYPE``), the *linkname* is relative to the directory
+   that contains the link.
+   For hard links (``LNKTYPE``), the *linkname* is relative to the root of
+   the archive.
+
 
 .. attribute:: TarInfo.uid
    :type: int

Lib/tarfile.py

@@ -742,7 +742,7 @@ def __init__(self, tarinfo):
 class AbsoluteLinkError(FilterError):
     def __init__(self, tarinfo):
         self.tarinfo = tarinfo
-        super().__init__(f'{tarinfo.name!r} is a symlink to an absolute path')
+        super().__init__(f'{tarinfo.name!r} is a link to an absolute path')
 
 class LinkOutsideDestinationError(FilterError):
     def __init__(self, tarinfo, path):
@@ -802,7 +802,14 @@ def _get_filtered_attrs(member, dest_path, for_data=True):
         if member.islnk() or member.issym():
             if os.path.isabs(member.linkname):
                 raise AbsoluteLinkError(member)
-            target_path = os.path.realpath(os.path.join(dest_path, member.linkname))
+            if member.issym():
+                target_path = os.path.join(dest_path,
+                                           os.path.dirname(name),
+                                           member.linkname)
+            else:
+                target_path = os.path.join(dest_path,
+                                           member.linkname)
+            target_path = os.path.realpath(target_path)
             if os.path.commonpath([target_path, dest_path]) != dest_path:
                 raise LinkOutsideDestinationError(member, target_path)
     return new_attrs

Lib/test/test_tarfile.py

@@ -3337,10 +3337,12 @@ def __exit__(self, *exc):
         self.bio = None
 
     def add(self, name, *, type=None, symlink_to=None, hardlink_to=None,
-            mode=None, **kwargs):
+            mode=None, size=None, **kwargs):
         """Add a member to the test archive. Call within `with`."""
         name = str(name)
         tarinfo = tarfile.TarInfo(name).replace(**kwargs)
+        if size is not None:
+            tarinfo.size = size
         if mode:
             tarinfo.mode = _filemode_to_int(mode)
         if symlink_to is not None:
@@ -3416,7 +3418,8 @@ def check_context(self, tar, filter):
                 raise self.raised_exception
             self.assertEqual(self.expected_paths, set())
 
-    def expect_file(self, name, type=None, symlink_to=None, mode=None):
+    def expect_file(self, name, type=None, symlink_to=None, mode=None,
+                    size=None):
         """Check a single file. See check_context."""
         if self.raised_exception:
             raise self.raised_exception
@@ -3445,6 +3448,8 @@ def expect_file(self, name, type=None, symlink_to=None, mode=None):
             self.assertTrue(path.is_fifo())
         else:
             raise NotImplementedError(type)
+        if size is not None:
+            self.assertEqual(path.stat().st_size, size)
         for parent in path.parents:
             self.expected_paths.discard(parent)
 
@@ -3491,8 +3496,15 @@ def test_parent_symlink(self):
         # Test interplaying symlinks
         # Inspired by 'dirsymlink2a' in jwilk/traversal-archives
         with ArchiveMaker() as arc:
+
+            # `current` links to `.` which is both:
+            #   - the destination directory
+            #   - `current` itself
             arc.add('current', symlink_to='.')
+
+            # effectively points to ./../
             arc.add('parent', symlink_to='current/..')
+
             arc.add('parent/evil')
 
         if os_helper.can_symlink():
@@ -3534,9 +3546,46 @@ def test_parent_symlink(self):
     def test_parent_symlink2(self):
         # Test interplaying symlinks
         # Inspired by 'dirsymlink2b' in jwilk/traversal-archives
+
+        # Posix and Windows have different pathname resolution:
+        # either symlink or a '..' component resolve first.
+        # Let's see which we are on.
+        if os_helper.can_symlink():
+            testpath = os.path.join(TEMPDIR, 'resolution_test')
+            os.mkdir(testpath)
+
+            # testpath/current links to `.` which is all of:
+            #   - `testpath`
+            #   - `testpath/current`
+            #   - `testpath/current/current`
+            #   - etc.
+            os.symlink('.', os.path.join(testpath, 'current'))
+
+            # we'll test where `testpath/current/../file` ends up
+            with open(os.path.join(testpath, 'current', '..', 'file'), 'w'):
+                pass
+
+            if os.path.exists(os.path.join(testpath, 'file')):
+                # Windows collapses 'current\..' to '.' first, leaving
+                # 'testpath\file'
+                dotdot_resolves_early = True
+            elif os.path.exists(os.path.join(testpath, '..', 'file')):
+                # Posix resolves 'current' to '.' first, leaving
+                # 'testpath/../file'
+                dotdot_resolves_early = False
+            else:
+                raise AssertionError('Could not determine link resolution')
+
         with ArchiveMaker() as arc:
+
+            # `current` links to `.` which is both the destination directory
+            # and `current` itself
             arc.add('current', symlink_to='.')
+
+            # `current/parent` is also available as `./parent`,
+            # and effectively points to `./../`
             arc.add('current/parent', symlink_to='..')
+
             arc.add('parent/evil')
 
         with self.check_context(arc.open(), 'fully_trusted'):
@@ -3550,6 +3599,7 @@ def test_parent_symlink2(self):
 
         with self.check_context(arc.open(), 'tar'):
             if os_helper.can_symlink():
+                # Fail when extracting a file outside destination
                 self.expect_exception(
                         tarfile.OutsideDestinationError,
                         "'parent/evil' would be extracted to "
@@ -3560,10 +3610,24 @@ def test_parent_symlink2(self):
                 self.expect_file('parent/evil')
 
         with self.check_context(arc.open(), 'data'):
-            self.expect_exception(
-                    tarfile.LinkOutsideDestinationError,
-                    """'current/parent' would link to ['"].*['"], """
-                    + "which is outside the destination")
+            if os_helper.can_symlink():
+                if dotdot_resolves_early:
+                    # Fail when extracting a file outside destination
+                    self.expect_exception(
+                            tarfile.OutsideDestinationError,
+                            "'parent/evil' would be extracted to "
+                            + """['"].*evil['"], which is outside """
+                            + "the destination")
+                else:
+                    # Fail as soon as we have a symlink outside the destination
+                    self.expect_exception(
+                            tarfile.LinkOutsideDestinationError,
+                            "'current/parent' would link to "
+                            + """['"].*outerdir['"], which is outside """
+                            + "the destination")
+            else:
+                self.expect_file('current/')
+                self.expect_file('parent/evil')
 
     @symlink_test
     def test_absolute_symlink(self):
@@ -3593,12 +3657,30 @@ def test_absolute_symlink(self):
         with self.check_context(arc.open(), 'data'):
             self.expect_exception(
                 tarfile.AbsoluteLinkError,
-                "'parent' is a symlink to an absolute path")
+                "'parent' is a link to an absolute path")
+
+    def test_absolute_hardlink(self):
+        # Test hardlink to an absolute path
+        # Inspired by 'dirsymlink' in https://github.com/jwilk/traversal-archives
+        with ArchiveMaker() as arc:
+            arc.add('parent', hardlink_to=self.outerdir / 'foo')
+
+        with self.check_context(arc.open(), 'fully_trusted'):
+            self.expect_exception(KeyError, ".*foo. not found")
+
+        with self.check_context(arc.open(), 'tar'):
+            self.expect_exception(KeyError, ".*foo. not found")
+
+        with self.check_context(arc.open(), 'data'):
+            self.expect_exception(
+                tarfile.AbsoluteLinkError,
+                "'parent' is a link to an absolute path")
 
     @symlink_test
     def test_sly_relative0(self):
         # Inspired by 'relative0' in jwilk/traversal-archives
         with ArchiveMaker() as arc:
+            # points to `../../tmp/moo`
             arc.add('../moo', symlink_to='..//tmp/moo')
 
         try:
@@ -3649,6 +3731,56 @@ def test_sly_relative2(self):
                     + """['"].*moo['"], which is outside the """
                     + "destination")
 
+    @symlink_test
+    def test_deep_symlink(self):
+        # Test that symlinks and hardlinks inside a directory
+        # point to the correct file (`target` of size 3).
+        # If links aren't supported we get a copy of the file.
+        with ArchiveMaker() as arc:
+            arc.add('targetdir/target', size=3)
+            # a hardlink's linkname is relative to the archive
+            arc.add('linkdir/hardlink', hardlink_to=os.path.join(
+                'targetdir', 'target'))
+            # a symlink's  linkname is relative to the link's directory
+            arc.add('linkdir/symlink', symlink_to=os.path.join(
+                '..', 'targetdir', 'target'))
+
+        for filter in 'tar', 'data', 'fully_trusted':
+            with self.check_context(arc.open(), filter):
+                self.expect_file('targetdir/target', size=3)
+                self.expect_file('linkdir/hardlink', size=3)
+                if os_helper.can_symlink():
+                    self.expect_file('linkdir/symlink', size=3,
+                                     symlink_to='../targetdir/target')
+                else:
+                    self.expect_file('linkdir/symlink', size=3)
+
+    @symlink_test
+    def test_chains(self):
+        # Test chaining of symlinks/hardlinks.
+        # Symlinks are created before the files they point to.
+        with ArchiveMaker() as arc:
+            arc.add('linkdir/symlink', symlink_to='hardlink')
+            arc.add('symlink2', symlink_to=os.path.join(
+                'linkdir', 'hardlink2'))
+            arc.add('targetdir/target', size=3)
+            arc.add('linkdir/hardlink', hardlink_to='targetdir/target')
+            arc.add('linkdir/hardlink2', hardlink_to='linkdir/symlink')
+
+        for filter in 'tar', 'data', 'fully_trusted':
+            with self.check_context(arc.open(), filter):
+                self.expect_file('targetdir/target', size=3)
+                self.expect_file('linkdir/hardlink', size=3)
+                self.expect_file('linkdir/hardlink2', size=3)
+                if os_helper.can_symlink():
+                    self.expect_file('linkdir/symlink', size=3,
+                                     symlink_to='hardlink')
+                    self.expect_file('symlink2', size=3,
+                                     symlink_to='linkdir/hardlink2')
+                else:
+                    self.expect_file('linkdir/symlink', size=3)
+                    self.expect_file('symlink2', size=3)
+
     def test_modes(self):
         # Test how file modes are extracted
         # (Note that the modes are ignored on platforms without working chmod)

Misc/NEWS.d/next/Library/2023-08-10-17-36-22.gh-issue-107845.dABiMJ.rst

@@ -0,0 +1,3 @@
+:func:`tarfile.data_filter` now takes the location of symlinks into account
+when determining their target, so it will no longer reject some valid
+tarballs with ``LinkOutsideDestinationError``.