conf: Fix size checking of -I interface name

Network interface names must fit in a buffer of IFNAMSIZ bytes, including
the terminating \0.  IFNAMSIZ is 16 on Linux, so interface names can be
up to (and including) 15 characters long.

We validate this for the -I option, but we have an off by one error.  We
pass (IFNAMSIZ - 1) as the buffer size to snprintf(), but that buffer size
already includes the terminating \0, so this actually truncates the value
to 14 characters.  The return value returned from snprintf() however, is
the number of characters that would have been printed *excluding* the
terminating \0, so by comparing it >= IFNAMSIZ - 1 we are giving an error
on names >= 15 characters rather than strictly > 15 characters.

Link: https://bugs.passt.top/show_bug.cgi?id=61
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/conf.c b/conf.c
index 68487a4..1906436 100644
--- a/conf.c
+++ b/conf.c
@@ -1439,9 +1439,9 @@ void conf(struct ctx *c, int argc, char **argv)
 			if (*c->pasta_ifn)
 				die("Multiple --ns-ifname options given");
 
-			ret = snprintf(c->pasta_ifn, IFNAMSIZ - 1, "%s",
+			ret = snprintf(c->pasta_ifn, IFNAMSIZ, "%s",
 				       optarg);
-			if (ret <= 0 || ret >= IFNAMSIZ - 1)
+			if (ret <= 0 || ret >= IFNAMSIZ)
 				die("Invalid interface name: %s", optarg);
 
 			break;