uktil: add libc.memfd_create() wrapper

This is required for python3.6 where there is no `os.memfd_create()`
yet. Can be removed once we move to python3.8+.

osbuild/util/linux.py

@@ -490,6 +490,29 @@ def __init__(self, lib: ctypes.CDLL):
         setattr(proto, "errcheck", self._errcheck_errno)
         setattr(proto, "__name__", "futimens")
         self.futimens = proto
+        # prototype: _memfd_create() (takes a byte type name)
+        # (can be removed once we move to python3.8)
+        proto = ctypes.CFUNCTYPE(
+            ctypes.c_int,  # restype (return type)
+            ctypes.c_char_p,
+            ctypes.c_uint,
+            use_errno=True,
+        )(
+            ("memfd_create", self._lib),
+            (
+                (1, "name"),
+                (1, "flags", 0),
+            ),
+        )
+        setattr(proto, "errcheck", self._errcheck_errno)
+        setattr(proto, "__name__", "memfd_create")
+        self._memfd_create = proto
+
+    # (can be removed once we move to python3.8)
+    def memfd_create(self, name: str, flags: int = 0) -> int:
+        """ create an anonymous file """
+        char_p_name = name.encode()
+        return self._memfd_create(char_p_name, flags)
 
     @staticmethod
     def make() -> "Libc":

test/mod/test_util_linux.py

@@ -187,6 +187,7 @@ def test_libc():
     assert libc0.RENAME_NOREPLACE
     assert libc0.RENAME_WHITEOUT
     assert libc0.renameat2
+    assert libc0.memfd_create
 
 
 def test_libc_renameat2_errcheck():
@@ -226,6 +227,23 @@ def test_libc_renameat2_exchange(tmpdir):
         assert f.read() == "foo"
 
 
+def test_libc_memfd_create_errcheck():
+    libc = linux.Libc.default()
+    with pytest.raises(OSError) as exc:
+        libc.memfd_create("foo", -1)
+    assert "Invalid argument" in str(exc.value)
+
+
+def test_libc_memfd_create():
+    libc = linux.Libc.default()
+    fd = libc.memfd_create("foo", 0)
+    os.write(fd, b"file content")
+    fd2 = os.dup(fd)
+    os.close(fd)
+    os.lseek(fd2, 0, 0)
+    assert os.read(fd2, 4096) == b"file content"
+
+
 def test_proc_boot_id():
     #
     # Test the `proc_boot_id()` function which reads the current boot-id