src: fix strncpy -Wstringop-truncation warnings

-Wstringop-truncation warning was introduced in GCC-8 as truncation
checker for strncpy and strncat.

Systems using gcc version >= 8 would receive the following warnings:

read_config_yy.c: In function ‘yyparse’:
read_config_yy.y:1594:2: warning: ‘strncpy’ specified bound 16 equals destination size [-Wstringop-truncation]
 1594 |  strncpy(policy->name, $2, CTD_HELPER_NAME_LEN);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
read_config_yy.y:1384:2: warning: ‘strncpy’ specified bound 256 equals destination size [-Wstringop-truncation]
 1384 |  strncpy(conf.stats.logfile, $2, FILENAME_MAXLEN);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
read_config_yy.y:692:2: warning: ‘strncpy’ specified bound 108 equals destination size [-Wstringop-truncation]
  692 |  strncpy(conf.local.path, $2, UNIX_PATH_MAX);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
read_config_yy.y:169:2: warning: ‘strncpy’ specified bound 256 equals destination size [-Wstringop-truncation]
  169 |  strncpy(conf.lockfile, $2, FILENAME_MAXLEN);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
read_config_yy.y:119:2: warning: ‘strncpy’ specified bound 256 equals destination size [-Wstringop-truncation]
  119 |  strncpy(conf.logfile, $2, FILENAME_MAXLEN);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

main.c: In function ‘main’:
main.c:168:5: warning: ‘strncpy’ specified bound 4096 equals destination size [-Wstringop-truncation]
  168 |     strncpy(config_file, argv[i], PATH_MAX);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix the issue by checking for string length first. Also using
snprintf instead.

In addition, correct an off-by-one when warning about maximum config
file path length.

Signed-off-by: Jose M. Guisado Gomez <guigom@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 3 insertions, 4 deletions

diff --git a/src/main.c b/src/main.c
index 7062e12..31e0eed 100644
--- a/src/main.c
+++ b/src/main.c
@@ -120,8 +120,8 @@ do_chdir(const char *d)
 
 int main(int argc, char *argv[])
 {
+	char config_file[PATH_MAX + 1] = {};
 	int ret, i, action = -1;
-	char config_file[PATH_MAX] = {};
 	int type = 0;
 	struct utsname u;
 	int version, major, minor;
@@ -165,13 +165,12 @@ int main(int argc, char *argv[])
 			break;
 		case 'C':
 			if (++i < argc) {
-				strncpy(config_file, argv[i], PATH_MAX);
-				if (strlen(argv[i]) >= PATH_MAX){
-					config_file[PATH_MAX-1]='\0';
+				if (strlen(argv[i]) > PATH_MAX) {
 					dlog(LOG_WARNING, "Path to config file"
 					     " to long. Cutting it down to %d"
 					     " characters", PATH_MAX);
 				}
+				snprintf(config_file, PATH_MAX, "%s", argv[i]);
 				break;
 			}
 			show_usage(argv[0]);