diff --git a/builtin/grep.c b/builtin/grep.c
index 601f801158..7da8fef31a 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -421,11 +421,23 @@ static int grep_submodule(struct grep_opt *opt, struct repository *superproject,
 	struct repository submodule;
 	int hit;
 
-	if (!is_submodule_active(superproject, path))
-		return 0;
+	/*
+	 * NEEDSWORK: submodules functions need to be protected because they
+	 * access the object store via config_from_gitmodules(): the latter
+	 * uses get_oid() which, for now, relies on the global the_repository
+	 * object.
+	 */
+	grep_read_lock();
 
-	if (repo_submodule_init(&submodule, superproject, path))
+	if (!is_submodule_active(superproject, path)) {
+		grep_read_unlock();
 		return 0;
+	}
+
+	if (repo_submodule_init(&submodule, superproject, path)) {
+		grep_read_unlock();
+		return 0;
+	}
 
 	repo_read_gitmodules(&submodule);
 
@@ -439,7 +451,6 @@ static int grep_submodule(struct grep_opt *opt, struct repository *superproject,
 	 * store is no longer global and instead is a member of the repository
 	 * object.
 	 */
-	grep_read_lock();
 	add_to_alternates_memory(submodule.objects->objectdir);
 	grep_read_unlock();
 
diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 8699515e81..43d2a132a9 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -2032,8 +2032,12 @@ static int module_config(int argc, const char **argv, const char *prefix)
 		return print_config_from_gitmodules(the_repository, argv[1]);
 
 	/* Equivalent to ACTION_SET in builtin/config.c */
-	if (argc == 3)
+	if (argc == 3) {
+		if (!is_writing_gitmodules_ok())
+			die(_("please make sure that the .gitmodules file is in the working tree"));
+
 		return config_set_in_gitmodules_file_gently(argv[1], argv[2]);
+	}
 
 	usage_with_options(git_submodule_helper_usage, module_config_options);
 }
diff --git a/git-submodule.sh b/git-submodule.sh
index 25b9bc58cb..bff855f54a 100755
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -159,6 +159,11 @@ cmd_add()
 		shift
 	done
 
+	if ! git submodule--helper config --check-writeable >/dev/null 2>&1
+	then
+		 die "$(eval_gettext "please make sure that the .gitmodules file is in the working tree")"
+	fi
+
 	if test -n "$reference_path"
 	then
 		is_absolute_path "$reference_path" ||
diff --git a/submodule-config.c b/submodule-config.c
index 9faf141e47..6869394c32 100644
--- a/submodule-config.c
+++ b/submodule-config.c
@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "dir.h"
 #include "repository.h"
 #include "config.h"
 #include "submodule-config.h"
@@ -603,8 +604,34 @@ static void submodule_cache_check_init(struct repository *repo)
 static void config_from_gitmodules(config_fn_t fn, struct repository *repo, void *data)
 {
 	if (repo->worktree) {
-		char *file = repo_worktree_path(repo, GITMODULES_FILE);
-		git_config_from_file(fn, file, data);
+		struct git_config_source config_source = { 0 };
+		const struct config_options opts = { 0 };
+		struct object_id oid;
+		char *file;
+
+		file = repo_worktree_path(repo, GITMODULES_FILE);
+		if (file_exists(file)) {
+			config_source.file = file;
+		} else if (repo->submodule_prefix) {
+			/*
+			 * When get_oid and config_with_options, used below,
+			 * become able to work on a specific repository, this
+			 * warning branch can be removed.
+			 */
+			warning("nested submodules without %s in the working tree are not supported yet",
+				GITMODULES_FILE);
+			goto out;
+		} else if (get_oid(GITMODULES_INDEX, &oid) >= 0) {
+			config_source.blob = GITMODULES_INDEX;
+		} else if (get_oid(GITMODULES_HEAD, &oid) >= 0) {
+			config_source.blob = GITMODULES_HEAD;
+		} else {
+			goto out;
+		}
+
+		config_with_options(fn, data, &config_source, &opts);
+
+out:
 		free(file);
 	}
 }
diff --git a/t/t7411-submodule-config.sh b/t/t7411-submodule-config.sh
index 45953f9300..2cfabb18bc 100755
--- a/t/t7411-submodule-config.sh
+++ b/t/t7411-submodule-config.sh
@@ -134,7 +134,7 @@ test_expect_success 'error in history in fetchrecursesubmodule lets continue' '
 	)
 '
 
-test_expect_success 'reading submodules config with "submodule--helper config"' '
+test_expect_success 'reading submodules config from the working tree with "submodule--helper config"' '
 	(cd super &&
 		echo "../submodule" >expect &&
 		git submodule--helper config submodule.submodule.url >actual &&
@@ -192,4 +192,28 @@ test_expect_success 'non-writeable .gitmodules when it is in the current branch
 	)
 '
 
+test_expect_success 'reading submodules config from the index when .gitmodules is not in the working tree' '
+	ORIG=$(git -C super rev-parse HEAD) &&
+	test_when_finished "git -C super reset --hard $ORIG" &&
+	(cd super &&
+		git submodule--helper config submodule.submodule.url "staged_url" &&
+		git add .gitmodules &&
+		rm -f .gitmodules &&
+		echo "staged_url" >expect &&
+		git submodule--helper config submodule.submodule.url >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'reading submodules config from the current branch when .gitmodules is not in the index' '
+	ORIG=$(git -C super rev-parse HEAD) &&
+	test_when_finished "git -C super reset --hard $ORIG" &&
+	(cd super &&
+		git rm .gitmodules &&
+		echo "../submodule" >expect &&
+		git submodule--helper config submodule.submodule.url >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done
diff --git a/t/t7418-submodule-sparse-gitmodules.sh b/t/t7418-submodule-sparse-gitmodules.sh
new file mode 100755
index 0000000000..3f7f271883
--- /dev/null
+++ b/t/t7418-submodule-sparse-gitmodules.sh
@@ -0,0 +1,122 @@
+#!/bin/sh
+#
+# Copyright (C) 2018  Antonio Ospite <ao2@ao2.it>
+#
+
+test_description='Test reading/writing .gitmodules when not in the working tree
+
+This test verifies that, when .gitmodules is in the current branch but is not
+in the working tree reading from it still works but writing to it does not.
+
+The test setup uses a sparse checkout, however the same scenario can be set up
+also by committing .gitmodules and then just removing it from the filesystem.
+'
+
+. ./test-lib.sh
+
+test_expect_success 'sparse checkout setup which hides .gitmodules' '
+	git init upstream &&
+	git init submodule &&
+	(cd submodule &&
+		echo file >file &&
+		git add file &&
+		test_tick &&
+		git commit -m "Add file"
+	) &&
+	(cd upstream &&
+		git submodule add ../submodule &&
+		test_tick &&
+		git commit -m "Add submodule"
+	) &&
+	git clone upstream super &&
+	(cd super &&
+		cat >.git/info/sparse-checkout <<-\EOF &&
+		/*
+		!/.gitmodules
+		EOF
+		git config core.sparsecheckout true &&
+		git read-tree -m -u HEAD &&
+		test_path_is_missing .gitmodules
+	)
+'
+
+test_expect_success 'reading gitmodules config file when it is not checked out' '
+	echo "../submodule" >expect &&
+	git -C super submodule--helper config submodule.submodule.url >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'not writing gitmodules config file when it is not checked out' '
+	test_must_fail git -C super submodule--helper config submodule.submodule.url newurl &&
+	test_path_is_missing super/.gitmodules
+'
+
+test_expect_success 'initialising submodule when the gitmodules config is not checked out' '
+	test_must_fail git -C super config submodule.submodule.url &&
+	git -C super submodule init &&
+	git -C super config submodule.submodule.url >actual &&
+	echo "$(pwd)/submodule" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'updating submodule when the gitmodules config is not checked out' '
+	test_path_is_missing super/submodule/file &&
+	git -C super submodule update &&
+	test_cmp submodule/file super/submodule/file
+'
+
+ORIG_SUBMODULE=$(git -C submodule rev-parse HEAD)
+ORIG_UPSTREAM=$(git -C upstream rev-parse HEAD)
+ORIG_SUPER=$(git -C super rev-parse HEAD)
+
+test_expect_success 're-updating submodule when the gitmodules config is not checked out' '
+	test_when_finished "git -C submodule reset --hard $ORIG_SUBMODULE;
+			    git -C upstream reset --hard $ORIG_UPSTREAM;
+			    git -C super reset --hard $ORIG_SUPER;
+			    git -C upstream submodule update --remote;
+			    git -C super pull;
+			    git -C super submodule update --remote" &&
+	(cd submodule &&
+		echo file2 >file2 &&
+		git add file2 &&
+		test_tick &&
+		git commit -m "Add file2 to submodule"
+	) &&
+	(cd upstream &&
+		git submodule update --remote &&
+		git add submodule &&
+		test_tick &&
+		git commit -m "Update submodule"
+	) &&
+	git -C super pull &&
+	# The --for-status options reads the gitmodules config
+	git -C super submodule summary --for-status >actual &&
+	rev1=$(git -C submodule rev-parse --short HEAD) &&
+	rev2=$(git -C submodule rev-parse --short HEAD^) &&
+	cat >expect <<-EOF &&
+	* submodule ${rev1}...${rev2} (1):
+	  < Add file2 to submodule
+
+	EOF
+	test_cmp expect actual &&
+	# Test that the update actually succeeds
+	test_path_is_missing super/submodule/file2 &&
+	git -C super submodule update &&
+	test_cmp submodule/file2 super/submodule/file2 &&
+	git -C super status --short >output &&
+	test_must_be_empty output
+'
+
+test_expect_success 'not adding submodules when the gitmodules config is not checked out' '
+	git clone submodule new_submodule &&
+	test_must_fail git -C super submodule add ../new_submodule &&
+	test_path_is_missing .gitmodules
+'
+
+# This test checks that the previous "git submodule add" did not leave the
+# repository in a spurious state when it failed.
+test_expect_success 'init submodule still works even after the previous add failed' '
+	git -C super submodule init
+'
+
+test_done
diff --git a/t/t7814-grep-recurse-submodules.sh b/t/t7814-grep-recurse-submodules.sh
index 7184113b9b..fa475d52fa 100755
--- a/t/t7814-grep-recurse-submodules.sh
+++ b/t/t7814-grep-recurse-submodules.sh
@@ -380,4 +380,20 @@ test_expect_success 'grep --recurse-submodules should pass the pattern type alon
 	fi
 '
 
+# Recursing down into nested submodules which do not have .gitmodules in their
+# working tree does not work yet. This is because config_from_gitmodules()
+# uses get_oid() and the latter is still not able to get objects from an
+# arbitrary repository (the nested submodule, in this case).
+test_expect_failure 'grep --recurse-submodules with submodules without .gitmodules in the working tree' '
+	test_when_finished "git -C submodule checkout .gitmodules" &&
+	rm submodule/.gitmodules &&
+	git grep --recurse-submodules -e "(.|.)[\d]" >actual &&
+	cat >expect <<-\EOF &&
+	a:(1|2)d(3|4)
+	submodule/a:(1|2)d(3|4)
+	submodule/sub/a:(1|2)d(3|4)
+	EOF
+	test_cmp expect actual
+'
+
 test_done