fsck: detect gitmodules URLs with embedded newlines

The credential protocol can't handle values with newlines. We already
detect and block any such URLs from being used with credential helpers,
but let's also add an fsck check to detect and block gitmodules files
with such URLs. That will let us notice the problem earlier when
transfer.fsckObjects is turned on. And in particular it will prevent bad
objects from spreading, which may protect downstream users running older
versions of Git.

We'll file this under the existing gitmodulesUrl flag, which covers URLs
with option injection. There's really no need to distinguish the exact
flaw in the URL in this context. Likewise, I've expanded the description
of t7416 to cover all types of bogus URLs.

fsck.c

@@ -14,6 +14,7 @@
 #include "packfile.h"
 #include "submodule-config.h"
 #include "config.h"
+#include "credential.h"
 
 static struct oidset gitmodules_found = OIDSET_INIT;
 static struct oidset gitmodules_done = OIDSET_INIT;
@@ -941,6 +942,19 @@ static int fsck_tag(struct tag *tag, const char *data,
 	return fsck_tag_buffer(tag, data, size, options);
 }
 
+static int check_submodule_url(const char *url)
+{
+	struct credential c = CREDENTIAL_INIT;
+	int ret;
+
+	if (looks_like_command_line_option(url))
+		return -1;
+
+	ret = credential_from_url_gently(&c, url, 1);
+	credential_clear(&c);
+	return ret;
+}
+
 struct fsck_gitmodules_data {
 	struct object *obj;
 	struct fsck_options *options;
@@ -965,7 +979,7 @@ static int fsck_gitmodules_fn(const char *var, const char *value, void *vdata)
 				    "disallowed submodule name: %s",
 				    name);
 	if (!strcmp(key, "url") && value &&
-	    looks_like_command_line_option(value))
+	    check_submodule_url(value) < 0)
 		data->ret |= report(data->options, data->obj,
 				    FSCK_MSG_GITMODULES_URL,
 				    "disallowed submodule url: %s",

t/t7416-submodule-dash-url.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-test_description='check handling of .gitmodule url with dash'
+test_description='check handling of disallowed .gitmodule urls'
 . ./test-lib.sh
 
 test_expect_success 'create submodule with protected dash in url' '
@@ -60,4 +60,20 @@ test_expect_success 'trailing backslash is handled correctly' '
 	test_i18ngrep ! "unknown option" err
 '
 
+test_expect_success 'fsck rejects embedded newline in url' '
+	# create an orphan branch to avoid existing .gitmodules objects
+	git checkout --orphan newline &&
+	cat >.gitmodules <<-\EOF &&
+	[submodule "foo"]
+	url = "https://one.example.com?%0ahost=two.example.com/foo.git"
+	EOF
+	git add .gitmodules &&
+	git commit -m "gitmodules with newline" &&
+	test_when_finished "rm -rf dst" &&
+	git init --bare dst &&
+	git -C dst config transfer.fsckObjects true &&
+	test_must_fail git push dst HEAD 2>err &&
+	grep gitmodulesUrl err
+'
+
 test_done