[PATCH V2 5/5] Tools: hv: kvp: Fix a warning of buffer overflow with gcc 8.0.1

[kys@linuxonhyperv.com]

From: Dexuan Cui <decui@microsoft.com>

The patch fixes:

hv_kvp_daemon.c: In function 'kvp_set_ip_info':
hv_kvp_daemon.c:1305:2: note: 'snprintf' output between 41 and 4136 bytes
into a destination of size 4096

The "(unsigned int)str_len" is to avoid:

hv_kvp_daemon.c:1309:30: warning: comparison of integer expressions of
different signedness: 'int' and 'long unsigned int' [-Wsign-compare]

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
---
 tools/hv/hv_kvp_daemon.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/tools/hv/hv_kvp_daemon.c b/tools/hv/hv_kvp_daemon.c
index bbb2a8ef367c..d7e06fe0270e 100644
--- a/tools/hv/hv_kvp_daemon.c
+++ b/tools/hv/hv_kvp_daemon.c
@@ -1178,6 +1178,7 @@ static int kvp_set_ip_info(char *if_name, struct hv_kvp_ipaddr_value *new_val)
 	FILE *file;
 	char cmd[PATH_MAX];
 	char *mac_addr;
+	int str_len;
 
 	/*
 	 * Set the configuration for the specified interface with
@@ -1301,8 +1302,18 @@ static int kvp_set_ip_info(char *if_name, struct hv_kvp_ipaddr_value *new_val)
 	 * invoke the external script to do its magic.
 	 */
 
-	snprintf(cmd, sizeof(cmd), KVP_SCRIPTS_PATH "%s %s",
-		 "hv_set_ifconfig", if_file);
+	str_len = snprintf(cmd, sizeof(cmd), KVP_SCRIPTS_PATH "%s %s",
+			   "hv_set_ifconfig", if_file);
+	/*
+	 * This is a little overcautious, but it's necessary to suppress some
+	 * false warnings from gcc 8.0.1.
+	 */
+	if (str_len <= 0 || (unsigned int)str_len >= sizeof(cmd)) {
+		syslog(LOG_ERR, "Cmd '%s' (len=%d) may be too long",
+		       cmd, str_len);
+		return HV_E_FAIL;
+	}
+
 	if (system(cmd)) {
 		syslog(LOG_ERR, "Failed to execute cmd '%s'; error: %d %s",
 				cmd, errno, strerror(errno));
-- 
2.18.0