[PATCHv2 net] sctp: fix an issue that plpmtu can never go to complete state

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When doing plpmtu probe, the probe size is growing every time when it
receives the ACK during the Search state until the probe fails. When
the failure occurs, pl.probe_high is set and it goes to the Complete
state.

However, if the link pmtu is huge, like 65535 in loopback_dev, the probe
eventually keeps using SCTP_MAX_PLPMTU as the probe size and never fails.
Because of that, pl.probe_high can not be set, and the plpmtu probe can
never go to the Complete state.

Fix it by setting pl.probe_high to SCTP_MAX_PLPMTU when the probe size
grows to SCTP_MAX_PLPMTU in sctp_transport_pl_recv(). Also, not allow
the probe size greater than SCTP_MAX_PLPMTU in the Complete state.

Fixes: b87641aff9e7 ("sctp: do state transition when a probe succeeds on HB ACK recv path")
Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
---
v2:
- fix the probe size can't reach SCTP_MAX_PLPMTU, as Paolo suggested.
---
 net/sctp/transport.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/sctp/transport.c b/net/sctp/transport.c
index 2f66a2006517..2abe45af98e7 100644
--- a/net/sctp/transport.c
+++ b/net/sctp/transport.c
@@ -324,9 +324,12 @@ bool sctp_transport_pl_recv(struct sctp_transport *t)
 		t->pl.probe_size += SCTP_PL_BIG_STEP;
 	} else if (t->pl.state == SCTP_PL_SEARCH) {
 		if (!t->pl.probe_high) {
-			t->pl.probe_size = min(t->pl.probe_size + SCTP_PL_BIG_STEP,
-					       SCTP_MAX_PLPMTU);
-			return false;
+			if (t->pl.probe_size < SCTP_MAX_PLPMTU) {
+				t->pl.probe_size = min(t->pl.probe_size + SCTP_PL_BIG_STEP,
+						       SCTP_MAX_PLPMTU);
+				return false;
+			}
+			t->pl.probe_high = SCTP_MAX_PLPMTU;
 		}
 		t->pl.probe_size += SCTP_PL_MIN_STEP;
 		if (t->pl.probe_size >= t->pl.probe_high) {
@@ -341,7 +344,7 @@ bool sctp_transport_pl_recv(struct sctp_transport *t)
 	} else if (t->pl.state == SCTP_PL_COMPLETE) {
 		/* Raise probe_size again after 30 * interval in Search Complete */
 		t->pl.state = SCTP_PL_SEARCH; /* Search Complete -> Search */
-		t->pl.probe_size += SCTP_PL_MIN_STEP;
+		t->pl.probe_size = min(t->pl.probe_size + SCTP_PL_MIN_STEP, SCTP_MAX_PLPMTU);
 	}
 
 	return t->pl.state == SCTP_PL_COMPLETE;
-- 
2.39.1




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