Re: [PATCH net] net/packet: preserve TX_RING progress on a later frame error

From: Mark Amirkan <redacted>

tpacket_snd() can transmit one or more frames before a later frame fails
validation.  The failing frame is marked TP_STATUS_WRONG_FORMAT, but its
error replaces len_sum, so send() reports failure despite the earlier
transmission.

Return the completed byte count when it is nonzero, as the allocation
failure path already does.  Keep TP_STATUS_WRONG_FORMAT on the bad frame
so userspace can identify it.

In a two-frame TPACKET_V2 test, a valid 60-byte frame followed by an
oversized frame sends the first frame but returns -EMSGSIZE.  With this
change, send() returns 60 and the second frame remains marked
TP_STATUS_WRONG_FORMAT.

Fixes: 69e3c75f4d54 ("net: TX_RING and packet mmap")
Cc: stable@vger.kernel.org
Assisted-by: Symbolic
Signed-off-by: Mark Amirkan <redacted>
---
 net/packet/af_packet.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 76bde7906d..b7e1848b61 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c

@@ -2893,7 +2893,7 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 				continue;
 			} else {
 				status = TP_STATUS_WRONG_FORMAT;
-				err = tp_len;
+				err = len_sum ? : tp_len;
 				goto out_status;
 			}