[openggsn] [PATCH 1/2] Fix possible buffer overflow for gsn_restart file path

[Neels Hofmeyr]

For strncat, to obtain n, one must not subtract the length of what is appended,
but the length of what is already written from the buffer size.

Verified with this little test program:

 #include <stdio.h>
 #include <string.h>

 int main() {
   char buf[20];
   strncpy(buf, "123", 10);
   strncat(buf, "456789012345", 10 - strlen(buf));
   printf("%s\n", buf);
   return 0;
 }

It prints "1234567890".
---
 gtp/gtp.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/gtp/gtp.c b/gtp/gtp.c
index 12cb492..34e1dc6 100644
--- a/gtp/gtp.c
+++ b/gtp/gtp.c
@@ -648,9 +648,10 @@ static void log_restart(struct gsn_t *gsn)
 	int counter = 0;
 	char filename[NAMESIZE];
 
-	filename[NAMESIZE - 1] = 0;	/* No null term. guarantee by strncpy */
+	/* guarantee nul term, strncpy might omit if too long */
+	filename[NAMESIZE - 1] = 0;
 	strncpy(filename, gsn->statedir, NAMESIZE - 1);
-	strncat(filename, RESTART_FILE, NAMESIZE - 1 - sizeof(RESTART_FILE));
+	strncat(filename, RESTART_FILE, NAMESIZE - 1 - strlen(filename));
 
 	i = umask(022);
 
-- 
2.1.4