[PATCH net] net/smc: do not credit bytes that splice() did not take

smc_rx_recvmsg() offers a chunk of the RMB to splice_to_pipe() and then
credits the whole chunk regardless of what was taken.

splice_to_pipe() takes only what the pipe has room for. A short count and
a zero count are both non-negative, so both fall through as success: the
call returns more than it delivered, and the consumer cursor and
bytes_to_rcv are advanced by copylen rather than by what the reader
received. The skipped bytes are never handed to anyone.

A pipe that is not empty is enough to hit this; SPLICE_F_NONBLOCK on a
full pipe produces the zero case.

Account what splice_to_pipe() actually took, and stop there rather than
continuing to the second chunk of a wrapped read. When it took nothing,
return -EAGAIN instead of reporting a length that was not delivered.

Fixes: 9014db202cb7 ("smc: add support for splice()")
Cc: [email protected]
Reviewed-by: Sidraya Jayagond <[email protected]>
Signed-off-by: Hidayath Khan <[email protected]>
---
 net/smc/smc_rx.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/net/smc/smc_rx.c b/net/smc/smc_rx.c
index 5c9e4d8b57de..d951dbe8eec3 100644
--- a/net/smc/smc_rx.c
+++ b/net/smc/smc_rx.c
@@ -488,6 +488,24 @@ int smc_rx_recvmsg(struct smc_sock *smc, struct msghdr *msg,
 						read_done = -EFAULT;
 					goto out;
 				}
+				/* splice_to_pipe() takes only what the pipe
+				 * has room for, which may be less than was
+				 * offered, or nothing. Account what it took
+				 * and stop: crediting the whole chunk would
+				 * advance the consumer past data the reader
+				 * never received.
+				 */
+				if (!msg && rc < chunk_len) {
+					if (!rc) {
+						if (!read_done)
+							read_done = -EAGAIN;
+						goto out;
+					}
+					copylen = chunk_len_sum - chunk_len + rc;
+					read_remaining -= rc;
+					read_done += rc;
+					break;
+				}
 			}
 			read_remaining -= chunk_len;
 			read_done += chunk_len;

base-commit: d3083202a78cd0040be6a88953f728b0d3db1990
-- 
2.52.0