Subject: [PATCH v4 4/4] submodule: fix case-folding gitdir filesystem colisions

From: Adrian Ratiu <[email-masked]>
Date: Fri, 7 Nov 2025 17:05:47 +0200

Add a new check in validate_submodule_git_dir() to detect and
prevent case-folding filesystem colisions. When this new check
is triggered, a stricter casefolding aware URI encoding is used
to percent-encode uppercase characters, e.g. Foo becomes %46oo.

By using this check/retry mechanism the uppercase encoding is
only applied when necessary, so case-sensitive filesystems are
not affected.

Signed-off-by: Adrian Ratiu <[email-masked]>
---
 submodule.c                   | 47 +++++++++++++++++++++++++++++++++--
 t/t7425-submodule-encoding.sh | 15 +++++++++++
 url.c                         | 12 +++++++++
 url.h                         |  1 +
 4 files changed, 73 insertions(+), 2 deletions(-)

diff --git a/submodule.c b/submodule.c
index ceaff0c1aa..ecbffac2c6 100644
--- a/submodule.c
+++ b/submodule.c
@@ -2280,7 +2280,7 @@ 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 = git_dir + len - suffix_len;
 	bool suffixes_match = !strcmp(p, submodule_name);
-	int ret = 0;
+	int ret = 0, config_ignorecase = 0;
 
 	/*
 	 * We prevent the contents of sibling submodules' git directories to
@@ -2318,6 +2318,42 @@ int validate_submodule_git_dir(char *git_dir, const char *submodule_name)
 	if (p && strchr(p, '/') != NULL)
 		return error("submodule gitdir name '%s' contains unexpected '/'", p);
 
+	/* Prevent conflicts on case-folding filesystems */
+	repo_config_get_bool(the_repository, "core.ignorecase", &config_ignorecase);
+	if (ignore_case || config_ignorecase) {
+		char *lower_gitdir = xstrdup(git_dir);
+		char *module_name = find_last_submodule_name(lower_gitdir);
+
+		if (module_name) {
+			for (p = module_name; *p; p++)
+				*p = tolower(*p);
+
+			/*
+			 * If lower path is different and already exists, check for collision.
+			 * Intentionally double-check to eliminate false-positives.
+			 */
+			if (strcmp(lower_gitdir, git_dir) && is_git_directory(lower_gitdir)) {
+				char *canonical = real_pathdup(git_dir, 0);
+				if (canonical) {
+					struct strbuf norm_git_dir = STRBUF_INIT;
+					strbuf_addstr(&norm_git_dir, git_dir);
+					strbuf_normalize_path(&norm_git_dir);
+
+					if (strcmp(canonical, norm_git_dir.buf))
+						ret = error(_("submodule git dir '%s' "
+							      "collides with '%s'"),
+							    canonical, norm_git_dir.buf);
+
+					strbuf_release(&norm_git_dir);
+					FREE_AND_NULL(canonical);
+				}
+			}
+		}
+
+		FREE_AND_NULL(lower_gitdir);
+		return ret;
+	}
+
 	return 0;
 }
 
@@ -2653,13 +2689,20 @@ void submodule_name_to_gitdir(struct strbuf *buf, struct repository *r,
 	if (!validate_and_set_submodule_gitdir(buf, submodule_name))
 		return;
 
-	/* Case 2: Try URI-safe (RFC3986) encoding first, this fixes nested gitdirs */
+	/* Case 2.1: Try URI-safe (RFC3986) encoding first, this fixes nested gitdirs */
 	strbuf_reset(buf);
 	repo_git_path_append(r, buf, "modules/");
 	strbuf_addstr_urlencode(buf, submodule_name, is_rfc3986_unreserved);
 	if (!validate_and_set_submodule_gitdir(buf, submodule_name))
 		return;
 
+	/* Case 2.2: Try extended uppercase URI (RFC3986) encoding, to fix case-folding */
+	strbuf_reset(buf);
+	repo_git_path_append(r, buf, "modules/");
+	strbuf_addstr_urlencode(buf, submodule_name, is_casefolding_rfc3986_unreserved);
+	if (!validate_and_set_submodule_gitdir(buf, submodule_name))
+		return;
+
 	/* Case 3: error out */
 	die(_("Cannot construct a valid gitdir path for submodule '%s': "
 	      "please set a unique git config for 'submodule.%s.gitdir'."),
diff --git a/t/t7425-submodule-encoding.sh b/t/t7425-submodule-encoding.sh
index a42d358f5b..f92b3e6338 100755
--- a/t/t7425-submodule-encoding.sh
+++ b/t/t7425-submodule-encoding.sh
@@ -143,4 +143,19 @@ test_expect_success 'submodule git dir nesting detection must work with parallel
 	verify_submodule_gitdir_path clone_parallel hippo/hooks modules/hippo%2fhooks
 '
 
+test_expect_success 'verify case-folding conflict is correctly encoded' '
+	git clone -c extensions.submoduleEncoding=true -c core.ignoreCase=true main cloned-folding &&
+	(
+		cd cloned-folding &&
+
+		git submodule add ../new-sub "folding" &&
+		test_commit lowercase &&
+
+		git submodule add ../new-sub "FoldinG" &&
+		test_commit uppercase
+	) &&
+	verify_submodule_gitdir_path cloned-folding "folding" "modules/folding" &&
+	verify_submodule_gitdir_path cloned-folding "FoldinG" "modules/%46oldin%47"
+'
+
 test_done
diff --git a/url.c b/url.c
index 0fb1859b28..057e6e5c6e 100644
--- a/url.c
+++ b/url.c
@@ -14,6 +14,18 @@ int is_rfc3986_unreserved(char ch)
 		ch == '-' || ch == '_' || ch == '.' || ch == '~';
 }
 
+/*
+ * This is a variant of is_rfc3986_unreserved() that treats uppercase
+ * letters as "reserved". This forces them to be percent-encoded, allowing
+ * 'Foo' (%46oo) and 'foo' (foo) to be distinct on case-folding filesystems.
+ */
+int is_casefolding_rfc3986_unreserved(char c)
+{
+	return (c >= 'a' && c <= 'z') ||
+	       (c >= '0' && c <= '9') ||
+	       c == '-' || c == '.' || c == '_' || c == '~';
+}
+
 int is_urlschemechar(int first_flag, int ch)
 {
 	/*
diff --git a/url.h b/url.h
index 131a262066..92e3c63514 100644
--- a/url.h
+++ b/url.h
@@ -22,5 +22,6 @@ void end_url_with_slash(struct strbuf *buf, const char *url);
 void str_end_url_with_slash(const char *url, char **dest);
 
 int is_rfc3986_unreserved(char ch);
+int is_casefolding_rfc3986_unreserved(char c);
 
 #endif /* URL_H */
-- 
2.51.0