diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c

index cc79d059f2..29adde60bd 100644

--- a/builtin/submodule--helper.c

+++ b/builtin/submodule--helper.c

@@ -25,11 +25,32 @@ static int check_name(int argc, const char **argv, const char *prefix)

 	return 0;

 }

+/*

+ * Exit non-zero if the proposed submodule repository path is inside

+ * another submodules' git dir.

+ */

+static int validate_git_dir(int argc, const char **argv, const char *prefix)

+{

+	char *sm_gitdir;

+

+	if (argc != 3)

+		usage("git submodule--helper validate-git-dir <path> <name>");

+	sm_gitdir = xstrdup(argv[1]);

+	if (validate_submodule_git_dir(sm_gitdir, argv[2]) < 0) {

+		free(sm_gitdir);

+		return 1;

+	}

+	free(sm_gitdir);

+	return 0;

+}

+

 int cmd_submodule__helper(int argc, const char **argv, const char *prefix)

 {

 	if (argc < 2)

 		usage("git submodule--helper <command>");

 	if (!strcmp(argv[1], "check-name"))

 		return check_name(argc - 1, argv + 1, prefix);

+	if (!strcmp(argv[1], "validate-git-dir"))

+		return validate_git_dir(argc - 1, argv + 1, prefix);

 	die(_("'%s' is not a valid submodule--helper subcommand"), argv[1]);

 }

diff -ruNp a/git-submodule.sh b/git-submodule.sh

--- a/git-submodule.sh	2020-01-09 20:01:48.885647299 +0100

+++ b/git-submodule.sh	2020-01-10 08:42:05.107514269 +0100

@@ -253,6 +253,11 @@ module_clone()

 	gitdir_base="$gitdir/modules/$base_name"

 	gitdir="$gitdir/modules/$name"

+        if ! git submodule--helper validate-git-dir "$gitdir" "$name"

+        then

+	        die "$(eval_gettextln "refusing to create/use '\$gitdir' in another submodule's git dir")"

+        fi

+

 	if test -d "$gitdir"

 	then

 		mkdir -p "$sm_path"

diff --git a/git-submodule.sh b/git-submodule.sh

index ca16579c3c..bf5ffdb1f6 100755

--- a/git-submodule.sh

+++ b/git-submodule.sh

@@ -419,6 +419,11 @@ Use -f if you really want to add it." >&2

 		fi

 	else

+		sm_gitdir=".git/modules/$sm_name"

+		if ! git submodule--helper validate-git-dir "$sm_gitdir" "$sm_name"

+		then

+			die "$(eval_gettextln "refusing to create/use '\$sm_gitdir' in another submodule's git dir")"

+		fi

 		if test -d ".git/modules/$sm_name"

 		then

 			if test -z "$force"

diff --git a/submodule.c b/submodule.c

index 6337cab091..9927f56a33 100644

--- a/submodule.c

+++ b/submodule.c

@@ -1034,3 +1034,43 @@ int merge_submodule(unsigned char result[20], const char *path,

 	free(merges.objects);

 	return 0;

 }

+int validate_submodule_git_dir(char *git_dir, const char *submodule_name)

+{

+	size_t len = strlen(git_dir), suffix_len = strlen(submodule_name);

+	char *p;

+	int ret = 0;

+

+	if (len <= suffix_len || (p = git_dir + len - suffix_len)[-1] != '/' ||

+	    strcmp(p, submodule_name))

+		die("BUG: submodule name '%s' not a suffix of git dir '%s'",

+		    submodule_name, git_dir);

+

+	/*

+	 * We prevent the contents of sibling submodules' git directories to

+	 * clash.

+	 *

+	 * Example: having a submodule named `hippo` and another one named

+	 * `hippo/hooks` would result in the git directories

+	 * `.git/modules/hippo/` and `.git/modules/hippo/hooks/`, respectively,

+	 * but the latter directory is already designated to contain the hooks

+	 * of the former.

+	 */

+	for (; *p; p++) {

+		if (is_dir_sep(*p)) {

+			char c = *p;

+

+			*p = '\0';

+			if (is_git_directory(git_dir))

+				ret = -1;

+			*p = c;

+

+			if (ret < 0)

+				return error(_("submodule git dir '%s' is "

+					       "inside git dir '%.*s'"),

+					     git_dir,

+					     (int)(p - git_dir), git_dir);

+		}

+	}

+

+	return 0;

+}

diff --git a/submodule.h b/submodule.h

index 59dbdfbd17..9c99457a3f 100644

--- a/submodule.h

+++ b/submodule.h

@@ -37,6 +37,11 @@ int find_unpushed_submodules(unsigned char new_sha1[20], const char *remotes_nam

 		struct string_list *needs_pushing);

 int push_unpushed_submodules(unsigned char new_sha1[20], const char *remotes_name);

+/*

+ * Make sure that no submodule's git dir is nested in a sibling submodule's.

+ */

+int validate_submodule_git_dir(char *git_dir, const char *submodule_name);

+

 /*

  * Returns 0 if the name is syntactically acceptable as a submodule "name"

  * (e.g., that may be found in the subsection of a .gitmodules file) and -1

diff --git a/t/t7415-submodule-names.sh b/t/t7415-submodule-names.sh

index 6456d5ae43..29393de617 100755

--- a/t/t7415-submodule-names.sh

+++ b/t/t7415-submodule-names.sh

@@ -151,4 +151,27 @@ test_expect_success 'fsck detects symlinked .gitmodules file' '

 	)

 '

+test_expect_success 'git dirs of sibling submodules must not be nested' '

+	git init nested &&

+	(

+		cd nested &&

+		test_commit nested &&

+		cat >.gitmodules <<-EOF &&

+		[submodule "hippo"]

+			url = .

+			path = thing1

+		[submodule "hippo/hooks"]

+			url = .

+			path = thing2

+		EOF

+		git clone . thing1 &&

+		git clone . thing2 &&

+		git add .gitmodules thing1 thing2 &&

+		test_tick &&

+		git commit -m nested

+	) &&

+	test_must_fail git clone --recurse-submodules nested clone 2>err &&

+	test_i18ngrep "is inside git dir" err

+'

+

 test_done