diff --git a/.gitignore b/.gitignore
index 4cc9c807..da48e6f8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -180,6 +180,7 @@
 /src/aio-dio-regress/aiodio_sparse2
 /src/idmapped-mounts/idmapped-mounts
 /src/idmapped-mounts/mount-idmapped
+/src/idmapped-mounts/fscaps-in-ancestor-userns
 /src/log-writes/replay-log
 /src/perf/*.pyc
 
diff --git a/src/idmapped-mounts/Makefile b/src/idmapped-mounts/Makefile
index ad4ddc99..1bab6471 100644
--- a/src/idmapped-mounts/Makefile
+++ b/src/idmapped-mounts/Makefile
@@ -3,7 +3,9 @@
 TOPDIR = ../..
 include $(TOPDIR)/include/builddefs
 
-TARGETS = idmapped-mounts mount-idmapped
+BINS = idmapped-mounts mount-idmapped
+LINKS = fscaps-in-ancestor-userns
+TARGETS = $(BINS) $(LINKS)
 CFILES_IDMAPPED_MOUNTS = idmapped-mounts.c utils.c
 CFILES_MOUNT_IDMAPPED = mount-idmapped.c utils.c
 
@@ -29,12 +31,18 @@ idmapped-mounts: $(CFILES_IDMAPPED_MOUNTS)
 	@echo "    [CC]    $@"
 	$(Q)$(LTLINK) $(CFILES_IDMAPPED_MOUNTS) -o $@ $(CFLAGS) $(LDFLAGS) $(LDLIBS)
 
+fscaps-in-ancestor-userns:
+	ln -sf idmapped-mounts fscaps-in-ancestor-userns
+
 mount-idmapped: $(CFILES_MOUNT_IDMAPPED)
 	@echo "    [CC]    $@"
 	$(Q)$(LTLINK) $(CFILES_MOUNT_IDMAPPED) -o $@ $(CFLAGS) $(LDFLAGS) $(LDLIBS)
 
 install:
 	$(INSTALL) -m 755 -d $(PKG_LIB_DIR)/src/idmapped-mounts
-	$(INSTALL) -m 755 $(TARGETS) $(PKG_LIB_DIR)/src/idmapped-mounts
+	$(INSTALL) -m 755 $(BINS) $(PKG_LIB_DIR)/src/idmapped-mounts
+	cd $(PKG_LIB_DIR)/src/idmapped-mounts && \
+		rm -f $(LINKS) && \
+		$(LN_S) idmapped-mounts fscaps-in-ancestor-userns
 
 -include .dep
diff --git a/src/idmapped-mounts/idmapped-mounts.c b/src/idmapped-mounts/idmapped-mounts.c
index 4d93b721..94b83c01 100644
--- a/src/idmapped-mounts/idmapped-mounts.c
+++ b/src/idmapped-mounts/idmapped-mounts.c
@@ -3201,6 +3201,121 @@ out:
 	return fret;
 }
 
+static int fscaps_idmapped_mounts_in_userns_valid_in_ancestor_userns(void)
+{
+	int fret = -1;
+	int file1_fd = -EBADF, file1_fd2 = -EBADF, open_tree_fd = -EBADF;
+	struct mount_attr attr = {
+		.attr_set = MOUNT_ATTR_IDMAP,
+	};
+	pid_t pid;
+
+	file1_fd = openat(t_dir1_fd, FILE1, O_CREAT | O_EXCL | O_CLOEXEC, 0644);
+	if (file1_fd < 0) {
+		log_stderr("failure: openat");
+		goto out;
+	}
+
+	/* Skip if vfs caps are unsupported. */
+	if (set_dummy_vfs_caps(file1_fd, 0, 1000))
+		return 0;
+
+	if (fremovexattr(file1_fd, "security.capability")) {
+		log_stderr("failure: fremovexattr");
+		goto out;
+	}
+	if (expected_dummy_vfs_caps_uid(file1_fd, -1)) {
+		log_stderr("failure: expected_dummy_vfs_caps_uid");
+		goto out;
+	}
+	if (errno != ENODATA) {
+		log_stderr("failure: errno");
+		goto out;
+	}
+
+	/* Changing mount properties on a detached mount. */
+	attr.userns_fd	= get_userns_fd(0, 10000, 10000);
+	if (attr.userns_fd < 0) {
+		log_stderr("failure: get_userns_fd");
+		goto out;
+	}
+
+	open_tree_fd = sys_open_tree(t_dir1_fd, "",
+				     AT_EMPTY_PATH |
+				     AT_NO_AUTOMOUNT |
+				     AT_SYMLINK_NOFOLLOW |
+				     OPEN_TREE_CLOEXEC |
+				     OPEN_TREE_CLONE);
+	if (open_tree_fd < 0) {
+		log_stderr("failure: sys_open_tree");
+		goto out;
+	}
+
+	if (sys_mount_setattr(open_tree_fd, "", AT_EMPTY_PATH, &attr, sizeof(attr))) {
+		log_stderr("failure: sys_mount_setattr");
+		goto out;
+	}
+
+	file1_fd2 = openat(open_tree_fd, FILE1, O_RDWR | O_CLOEXEC, 0);
+	if (file1_fd2 < 0) {
+		log_stderr("failure: openat");
+		goto out;
+	}
+
+	/*
+	 * Verify we can set an v3 fscap for real root this was regressed at
+	 * some point. Make sure this doesn't happen again!
+	 */
+	pid = fork();
+	if (pid < 0) {
+		log_stderr("failure: fork");
+		goto out;
+	}
+	if (pid == 0) {
+		if (!switch_userns(attr.userns_fd, 0, 0, false))
+			die("failure: switch_userns");
+
+		if (expected_dummy_vfs_caps_uid(file1_fd2, -1))
+			die("failure: expected_dummy_vfs_caps_uid");
+		if (errno != ENODATA)
+			die("failure: errno");
+
+		if (set_dummy_vfs_caps(file1_fd2, 0, 0))
+			die("failure: set_dummy_vfs_caps");
+
+		if (!expected_dummy_vfs_caps_uid(file1_fd2, 0))
+			die("failure: expected_dummy_vfs_caps_uid");
+
+		if (!expected_dummy_vfs_caps_uid(file1_fd, 0) && errno != EOVERFLOW)
+			die("failure: expected_dummy_vfs_caps_uid");
+
+		exit(EXIT_SUCCESS);
+	}
+
+	if (wait_for_pid(pid))
+		goto out;
+
+	if (!expected_dummy_vfs_caps_uid(file1_fd2, 10000)) {
+		log_stderr("failure: expected_dummy_vfs_caps_uid");
+		goto out;
+	}
+
+	if (!expected_dummy_vfs_caps_uid(file1_fd, 0)) {
+		log_stderr("failure: expected_dummy_vfs_caps_uid");
+		goto out;
+	}
+
+	fret = 0;
+	log_debug("Ran test");
+out:
+	safe_close(attr.userns_fd);
+	safe_close(file1_fd);
+	safe_close(file1_fd2);
+	safe_close(open_tree_fd);
+
+	return fret;
+}
+
 static int fscaps_idmapped_mounts_in_userns_separate_userns(void)
 {
 	int fret = -1;
@@ -8745,7 +8860,7 @@ struct t_idmapped_mounts {
 	{ fscaps,							"fscaps on regular mounts",									},
 	{ fscaps_idmapped_mounts,					"fscaps on idmapped mounts",									},
 	{ fscaps_idmapped_mounts_in_userns,				"fscaps on idmapped mounts in user namespace",							},
-	{ fscaps_idmapped_mounts_in_userns_separate_userns,		"fscaps on idmapped mounts in user namespace with different id mappings ",			},
+	{ fscaps_idmapped_mounts_in_userns_separate_userns,		"fscaps on idmapped mounts in user namespace with different id mappings",			},
 	{ fsids_mapped,							"mapped fsids",											},
 	{ fsids_unmapped,						"unmapped fsids",										},
 	{ hardlink_crossing_mounts,					"cross mount hardlink",										},
@@ -8789,6 +8904,10 @@ struct t_idmapped_mounts {
 	{ threaded_idmapped_mount_interactions,				"threaded operations on idmapped mounts",							},
 };
 
+struct t_idmapped_mounts fscaps_in_ancestor_userns[] = {
+	{ fscaps_idmapped_mounts_in_userns_valid_in_ancestor_userns,	"fscaps on idmapped mounts in user namespace writing fscap valid in ancestor userns",		},
+};
+
 static bool run_test(struct t_idmapped_mounts suite[], size_t suite_size)
 {
 	int i;
@@ -8826,6 +8945,7 @@ static bool run_test(struct t_idmapped_mounts suite[], size_t suite_size)
 
 int main(int argc, char *argv[])
 {
+	const char *invocation_name;
 	int fret, ret;
 	int index = 0;
 	bool supported = false;
@@ -8913,8 +9033,17 @@ int main(int argc, char *argv[])
 
 	fret = EXIT_FAILURE;
 
-	if (!run_test(basic_suite, ARRAY_SIZE(basic_suite)))
-		goto out;
+	invocation_name = basename(argv[0]);
+	if (strcmp(invocation_name, "idmapped-mounts") == 0) {
+		if (!run_test(basic_suite, ARRAY_SIZE(basic_suite)))
+			goto out;
+	} else if (strcmp(invocation_name, "fscaps-in-ancestor-userns") == 0) {
+		if (!run_test(fscaps_in_ancestor_userns,
+			      ARRAY_SIZE(fscaps_in_ancestor_userns)))
+			goto out;
+	} else {
+		die("idmapped mount test suite \"%s\" unknown", invocation_name);
+	}
 
 	fret = EXIT_SUCCESS;
 
diff --git a/tests/generic/637 b/tests/generic/637
new file mode 100755
index 00000000..25c601b5
--- /dev/null
+++ b/tests/generic/637
@@ -0,0 +1,42 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2021 Christian Brauner.  All Rights Reserved.
+#
+# FS QA Test 637
+#
+# Test that fscaps on idmapped mounts behave correctly.
+#
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+_cleanup()
+{
+	cd /
+	rm -f $tmp.*
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+
+# remove previous $seqres.full before test
+rm -f $seqres.full
+
+# real QA test starts here
+
+_supported_fs generic
+_require_idmapped_mounts
+_require_test
+
+echo "Silence is golden"
+
+$here/src/idmapped-mounts/fscaps-in-ancestor-userns --device "$TEST_DEV" --mount "$TEST_DIR" --fstype "$FSTYP"
+
+status=$?
+exit
diff --git a/tests/generic/637.out b/tests/generic/637.out
new file mode 100644
index 00000000..55a3d825
--- /dev/null
+++ b/tests/generic/637.out
@@ -0,0 +1,2 @@
+QA output created by 637
+Silence is golden
diff --git a/tests/generic/group b/tests/generic/group
index 105763c4..9dfefdf4 100644
--- a/tests/generic/group
+++ b/tests/generic/group
@@ -639,3 +639,4 @@
 634 auto quick atime bigtime
 635 auto quick atime bigtime shutdown
 636 auto quick swap
+637 auto quick idmapped
