[PATCH 21/21] selftests/filesystems: check that reading the root of an empty mount namespace stalls nobody

[Christian Brauner <brauner@kernel.org>]

A readdir of the root of an empty mount namespace stuck in the page
fault of its buffer must stall neither a create nor a lookup in that
directory. The test needs userfaultfd and skips without it.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 .../selftests/filesystems/empty_mntns/.gitignore   |  1 +
 .../selftests/filesystems/empty_mntns/Makefile     |  5 ++-
 .../filesystems/empty_mntns/root_readdir_test.c    | 45 ++++++++++++++++++++++
 3 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/filesystems/empty_mntns/.gitignore b/tools/testing/selftests/filesystems/empty_mntns/.gitignore
index 8ea166068d6b..27bcbcaeb1d1 100644
--- a/tools/testing/selftests/filesystems/empty_mntns/.gitignore
+++ b/tools/testing/selftests/filesystems/empty_mntns/.gitignore
@@ -4,3 +4,4 @@ empty_mntns_test
 overmount_chroot_test
 internal_sb_reconfigure_test
 nullfs_atime_test
+root_readdir_test
diff --git a/tools/testing/selftests/filesystems/empty_mntns/Makefile b/tools/testing/selftests/filesystems/empty_mntns/Makefile
index 4b2ea6acec1c..22af2164c54e 100644
--- a/tools/testing/selftests/filesystems/empty_mntns/Makefile
+++ b/tools/testing/selftests/filesystems/empty_mntns/Makefile
@@ -4,7 +4,9 @@ CFLAGS += -Wall -O2 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
 LDLIBS += -lcap
 
 TEST_GEN_PROGS := empty_mntns_test overmount_chroot_test clone3_empty_mntns_test
-TEST_GEN_PROGS += internal_sb_reconfigure_test nullfs_atime_test
+TEST_GEN_PROGS += internal_sb_reconfigure_test nullfs_atime_test root_readdir_test
+
+LOCAL_HDRS += ../readdir_hold.h
 
 include ../../lib.mk
 
@@ -12,3 +14,4 @@ $(OUTPUT)/empty_mntns_test: ../utils.c
 $(OUTPUT)/overmount_chroot_test: ../utils.c
 $(OUTPUT)/clone3_empty_mntns_test: ../utils.c
 $(OUTPUT)/internal_sb_reconfigure_test: ../utils.c
+$(OUTPUT)/root_readdir_test: LDLIBS += -pthread
diff --git a/tools/testing/selftests/filesystems/empty_mntns/root_readdir_test.c b/tools/testing/selftests/filesystems/empty_mntns/root_readdir_test.c
new file mode 100644
index 000000000000..0ff544247027
--- /dev/null
+++ b/tools/testing/selftests/filesystems/empty_mntns/root_readdir_test.c
@@ -0,0 +1,45 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Reading the root of an empty mount namespace holds nothing that anybody
+ * else waits for: the directory never has an entry, so a readdir stuck in
+ * the page fault of its buffer stalls neither a create nor a lookup in it.
+ */
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include "../../kselftest_harness.h"
+#include "../readdir_hold.h"
+#include "empty_mntns.h"
+
+TEST(readdir_blocks_nobody)
+{
+	struct readdir_hold hold;
+	bool stalled;
+	int dfd;
+
+	if (geteuid())
+		SKIP(return, "test requires root");
+	if (readdir_hold_init(&hold))
+		SKIP(return, "test requires userfaultfd");
+	if (unshare(UNSHARE_EMPTY_MNTNS)) {
+		readdir_hold_destroy(&hold);
+		if (errno == EINVAL)
+			SKIP(return, "UNSHARE_EMPTY_MNTNS not supported");
+		ASSERT_TRUE(false)
+			TH_LOG("unshare(UNSHARE_EMPTY_MNTNS): %m");
+	}
+	dfd = open("/", O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+	ASSERT_GE(dfd, 0);
+	ASSERT_EQ(readdir_hold_check(&hold, dfd, &stalled), 0);
+	/* the lookup came back while the readdir was stuck in its fault */
+	EXPECT_FALSE(stalled);
+	close(dfd);
+	readdir_hold_destroy(&hold);
+}
+
+TEST_HARNESS_MAIN

-- 
2.53.0