On Tue, Feb 11, 2025 at 08:50:51PM +0100, Stefano
Brivio wrote:
It looks like a detail, but it's critical if
we're dealing with
somebody, such as near-future self, using TCP_REPAIR to migrate TCP
connections in the guest or container.
The last packet sent from the 'source' process/guest/container
typically reports a small window, or zero, because the guest/container
hadn't been draining it for a while.
The next packet, appearing as the target sets TCP_REPAIR_OFF on the
migrated socket, is a keep-alive (also called "window probe" in CRIU
or TCP_REPAIR-related code), and it comes with an updated window
value, reflecting the pre-migration "regular" value.
If we ignore it, it might take a while/forever before we realise we
can actually restart sending.
Fixes: 238c69f9af45 ("tcp: Acknowledge keep-alive segments, ignore them for the
rest")
Signed-off-by: Stefano Brivio <sbrivio(a)redhat.com>
Reviewed-by: David Gibson <david(a)gibson.dropbear.id.au>
Although...
---
tcp.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/tcp.c b/tcp.c
index af6bd95..2addf4a 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1664,8 +1664,10 @@ static int tcp_data_from_tap(const struct ctx *c, struct
tcp_tap_conn *conn,
tcp_send_flag(c, conn, ACK);
tcp_timer_ctl(c, conn);
- if (p->count == 1)
+ if (p->count == 1) {
... not really this patch, but this condition seems wrong to me. IIUC
it's attempting to detect the last packet in the batch, which isn't
necessarily the same thing as the _only_ packet in the batch.
Admittedly, it probably will be for a keep-alive, but
I'm having a
hard time convincing myself it absolutely has to be.
It is, because it makes no sense to batch keep-alives...