git: bbda52e814e0 - stable/13 - if_wg: fix erroneous calculation in calculate_padding() for p_mtu == 0

The branch stable/13 has been updated by kevans:

URL: https://cgit.FreeBSD.org/src/commit/?id=bbda52e814e0c760b2beaeae40a2c76ff43d1975

commit bbda52e814e0c760b2beaeae40a2c76ff43d1975
Author:     Aaron LI <aly@aaronly.me>
AuthorDate: 2024-01-17 23:29:23 +0000
Commit:     Kyle Evans <kevans@FreeBSD.org>
CommitDate: 2024-01-30 05:37:46 +0000

    if_wg: fix erroneous calculation in calculate_padding() for p_mtu == 0
    
    In practice this is harmless; only keepalive packets may realistically have
    p_mtu == 0, and they'll also have no payload so the math works out the same
    either way.  Still, let's prefer technical accuracy and calculate the amount
    of padding needed rather than the padded length...
    
    PR:             276363
    
    (cherry picked from commit b891f61ef538a4e9b4658b4b756635c8036a5788)
---
 sys/dev/wg/if_wg.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/sys/dev/wg/if_wg.c b/sys/dev/wg/if_wg.c
index 6cd5c985a89f..e961c9f24d94 100644
--- a/sys/dev/wg/if_wg.c
+++ b/sys/dev/wg/if_wg.c
@@ -1465,8 +1465,12 @@ calculate_padding(struct wg_packet *pkt)
 {
 	unsigned int padded_size, last_unit = pkt->p_mbuf->m_pkthdr.len;
 
-	if (__predict_false(!pkt->p_mtu))
-		return (last_unit + (WG_PKT_PADDING - 1)) & ~(WG_PKT_PADDING - 1);
+	/* Keepalive packets don't set p_mtu, but also have a length of zero. */
+	if (__predict_false(pkt->p_mtu == 0)) {
+		padded_size = (last_unit + (WG_PKT_PADDING - 1)) &
+		    ~(WG_PKT_PADDING - 1);
+		return (padded_size - last_unit);
+	}
 
 	if (__predict_false(last_unit > pkt->p_mtu))
 		last_unit %= pkt->p_mtu;
@@ -1474,7 +1478,7 @@ calculate_padding(struct wg_packet *pkt)
 	padded_size = (last_unit + (WG_PKT_PADDING - 1)) & ~(WG_PKT_PADDING - 1);
 	if (pkt->p_mtu < padded_size)
 		padded_size = pkt->p_mtu;
-	return padded_size - last_unit;
+	return (padded_size - last_unit);
 }
 
 static void