From c83b388a3f8f7a8c8d5a5cfddb6ab397005371a1 Mon Sep 17 00:00:00 2001
From: Laslo Hunhold <dev@frign.de>
Date: Thu, 17 May 2018 23:23:28 +0200
Subject: [PATCH] Properly handle *snprintf() errors

Posix guarantees that the resulting string is null-terminated, even if
we have an overflow. Instead of doing what has already been done,
properly warn when there has been an error or overflow, so the user can
do something about it.
---
 slstatus.c | 17 +++++++++++------
 util.c     | 12 ++++++------
 2 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/slstatus.c b/slstatus.c
index e8d367b..35bdcd6 100644
--- a/slstatus.c
+++ b/slstatus.c
@@ -1,4 +1,5 @@
 /* See LICENSE file for copyright and license details. */
+#include <errno.h>
 #include <locale.h>
 #include <signal.h>
 #include <stdio.h>
@@ -53,7 +54,7 @@ main(int argc, char *argv[])
 	struct sigaction act;
 	struct timespec start, current, diff, intspec, wait;
 	size_t i, len;
-	int sflag;
+	int sflag, ret;
 	char status[MAXLEN];
 
 	sflag = 0;
@@ -88,12 +89,16 @@ main(int argc, char *argv[])
 		for (i = len = 0; i < LEN(args); i++) {
 			const char * res = args[i].func(args[i].args);
 			res = (res == NULL) ? unknown_str : res;
-			len += snprintf(status + len, sizeof(status) - len,
-			                args[i].fmt, res);
-
-			if (len >= sizeof(status)) {
-				status[sizeof(status) - 1] = '\0';
+			if ((ret = snprintf(status + len, sizeof(status) - len,
+			                    args[i].fmt, res)) < 0) {
+				fprintf(stderr, "snprintf: %s\n",
+				        strerror(errno));
+				break;
+			} else if ((size_t)ret >= sizeof(status) - len) {
+				fprintf(stderr, "snprintf: Output truncated\n");
+				break;
 			}
+			len += ret;
 		}
 
 		if (sflag) {
diff --git a/util.c b/util.c
index 8808aba..6113049 100644
--- a/util.c
+++ b/util.c
@@ -10,15 +10,15 @@ const char *
 bprintf(const char *fmt, ...)
 {
 	va_list ap;
-	size_t len;
+	int ret;
 
 	va_start(ap, fmt);
-	len = vsnprintf(buf, sizeof(buf) - 1, fmt, ap);
-	va_end(ap);
-
-	if (len >= sizeof(buf)) {
-		buf[sizeof(buf)-1] = '\0';
+	if ((ret = vsnprintf(buf, sizeof(buf), fmt, ap)) < 0) {
+		fprintf(stderr, "vsnprintf: %s\n", strerror(errno));
+	} else if ((size_t)ret >= sizeof(buf)) {
+		fprintf(stderr, "vsnprintf: Output truncated\n");
 	}
+	va_end(ap);
 
 	return buf;
 }