linux: clean up the SocketCAN header for classic CAN frames.

We already clean it up for CAN FD frames; clean it up for classic CAN
frames as well, to allow heuristics to be used to try to guess whether
the len8_dlc field was filled in.

(cherry picked from commit e9eaad85f3888c72f79de705fae7e0768d8a622d)

1 files changed, 30 insertions, 9 deletions

diff --git a/pcap-linux.c b/pcap-linux.c
index e931f84f..52decc31 100644
--- a/pcap-linux.c
+++ b/pcap-linux.c
@@ -3936,20 +3936,41 @@ static int pcap_handle_packet_mmap(
 				 * does happen.)
 				 *
 				 * Update this if Linux adds more flag
-				 * bits to the fd_flags field or uses
-				 * either of the reserved fields for
-				 * FD frames.
+				 * bits to the fd_flags field, uses the
+				 * len8_dlc field for CAN FD frames, or
+				 * uses the reserved frame for CAN FD
+				 * frames.
 				 */
 				canhdr->fd_flags &= ~(CANFD_FDF|CANFD_ESI|CANFD_BRS);
-				canhdr->reserved1 = 0;
-				canhdr->reserved2 = 0;
+				canhdr->len8_dlc = 0;
+				canhdr->reserved = 0;
 			} else {
 				/*
-				 * Clear CANFD_FDF if it's set (probably
-				 * again meaning that that field is
-				 * uninitialized junk).
+				 * The FD flags field is CAN FD-only,
+				 * so it should be zero.
+				 *
+				 * If it's not zero, or the reserved
+				 * field is not zero, or the len8_dlc
+				 * field does not have a value between
+				 * 8 and 15, assume the header was not
+				 * initialized past the payload_length
+				 * field, and zero out the len8_dlc field.
+				 *
+				 * Then zero out the fd_flags and reserved
+				 * fields.
+				 *
+				 * Update this if Linux uses the fd_flags
+				 * field or the reserved field, or puts
+				 * values not between 8 and 15 in the
+				 * len8_dlc field, for classic CAN frames.
 				 */
-				canhdr->fd_flags &= ~CANFD_FDF;
+				if (canhdr->fd_flags != 0 ||
+				    canhdr->reserved != 0 ||
+				    (canhdr->len8_dlc < 8 ||
+				     canhdr->len8_dlc > 15))
+					canhdr->len8_dlc = 0;
+				canhdr->fd_flags = 0;
+				canhdr->reserved = 0;
 			}
 		}
 	}