config: reject bogus section names for --rename-section

You can feed junk to "git config --rename-section", which
will result in a config file that git will not even parse
(so you cannot fix it with git-config). We already have
syntactic sanity checks when setting a variable; let's do
the same for section names.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -1552,20 +1552,42 @@ static int section_name_match (const char *buf, const char *name)
 	return 0;
 }
 
+static int section_name_is_ok(const char *name)
+{
+	/* Empty section names are bogus. */
+	if (!*name)
+		return 0;
+
+	/*
+	 * Before a dot, we must be alphanumeric or dash. After the first dot,
+	 * anything goes, so we can stop checking.
+	 */
+	for (; *name && *name != '.'; name++)
+		if (*name != '-' && !isalnum(*name))
+			return 0;
+	return 1;
+}
+
 /* if new_name == NULL, the section is removed instead */
 int git_config_rename_section_in_file(const char *config_filename,
 				      const char *old_name, const char *new_name)
 {
 	int ret = 0, remove = 0;
 	char *filename_buf = NULL;
-	struct lock_file *lock = xcalloc(sizeof(struct lock_file), 1);
+	struct lock_file *lock;
 	int out_fd;
 	char buf[1024];
 	FILE *config_file;
 
+	if (new_name && !section_name_is_ok(new_name)) {
+		ret = error("invalid section name: %s", new_name);
+		goto out;
+	}
+
 	if (!config_filename)
 		config_filename = filename_buf = git_pathdup("config");
 
+	lock = xcalloc(sizeof(struct lock_file), 1);
 	out_fd = hold_lock_file_for_update(lock, config_filename, 0);
 	if (out_fd < 0) {
 		ret = error("could not lock config file %s", config_filename);

t/t1300-repo-config.sh

@@ -550,6 +550,14 @@ EOF
 
 test_expect_success "rename succeeded" "test_cmp expect .git/config"
 
+test_expect_success 'renaming empty section name is rejected' '
+	test_must_fail git config --rename-section branch.zwei ""
+'
+
+test_expect_success 'renaming to bogus section is rejected' '
+	test_must_fail git config --rename-section branch.zwei "bogus name"
+'
+
 cat >> .git/config << EOF
   [branch "zwei"] a = 1 [branch "vier"]
 EOF