[Cluster-devel] [PATCH dlm-tool 2/2] dlm_controld: fix rare off by one

[Alexander Aring <aahringo@redhat.com>]

While debugging I came across a rare off by one when the snprintf()
filled string _exactly_ matched the size (with '\0') and we return the
bytes written without \0. We will then write a "\n\0" pattern at the
end but when the string exactly matched there is missing byte in the
calculation of the "\n\0" pattern because the return value only reduced
the size by one. To fix that we substract -1 from the return value of
snprintf() to have at the end two bytes for the "\n\0" pattern. We just
need to be careful we do that in cases where snprintf() doesn't return
zero to not hit a negative offset calculation.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
---
 dlm_controld/logging.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/dlm_controld/logging.c b/dlm_controld/logging.c
index 2c57138c..42712d8b 100644
--- a/dlm_controld/logging.c
+++ b/dlm_controld/logging.c
@@ -181,10 +181,12 @@ void log_level(char *name_in, uint32_t level_in, const char *fmt, ...)
 	ret = vsnprintf(log_str + pos, len - pos, fmt, ap);
 	va_end(ap);
 
-	if (ret >= len - pos)
+	if (ret >= len - pos) {
 		pos = len - 1;
-	else
-		pos += ret;
+	} else {
+		if (ret != 0)
+			pos += ret - 1;
+	}
 
 	log_str[pos++] = '\n';
 	log_str[pos++] = '\0';
-- 
2.31.1