The code in udp_invert_portmap() is written based on an incorrect
understanding of C's (arcane) integer promotion rules.  We calculate
'(in_port_t)i + delta' expecting the result to be of type in_port_t (16
bits).  However "small integer types" (those narrower than 'int') are
always promoted to int for expressions, meaning this calculation can
overrun the rdelta[] array.

Fix this, and use a new intermediate for the index, to make it very clear
what it's type is.  We also change i to unsigned, to avoid any possible
confusion from mixing signed and unsigned types.

Link: https://bugs.passt.top/show_bug.cgi?id=80
Reported-by: Laurent Jacquot <jk(a)lutty.net>
Suggested-by: Laurent Jacquot <jk(a)lutty.net>

Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
---
 udp.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/udp.c b/udp.c
index c031a053..a3961bfd 100644
--- a/udp.c
+++ b/udp.c
@@ -258,15 +258,16 @@ void udp_portmap_clear(void)
  */
 static void udp_invert_portmap(struct udp_port_fwd *fwd)
 {
-	int i;
+	unsigned int i;
 
 	static_assert(ARRAY_SIZE(fwd->f.delta) == ARRAY_SIZE(fwd->rdelta),
 		      "Forward and reverse delta arrays must have same size");
 	for (i = 0; i < ARRAY_SIZE(fwd->f.delta); i++) {
 		in_port_t delta = fwd->f.delta[i];
+		in_port_t rport = i + delta;
 
 		if (delta)
-			fwd->rdelta[(in_port_t)i + delta] = NUM_PORTS - delta;
+			fwd->rdelta[rport] = NUM_PORTS - delta;
 	}
 }
 
-- 
2.43.2