[B.A.T.M.A.N.] [PATCH RFC] batman-adv: always assume 2-byte packet alignment

Matthias Schiffer
2018-01-24T11:21:37Z

NIC drivers generally try to ensure that the "network header" is aligned
to a 4-byte boundary. This is not always possible: When Ethernet frames are
encapsulated in other packets with 4-byte aligned headers, the inner
Ethernet header will have 4-byte alignment, and in consequence, the inner
network header is aligned to 2, but not to 4 bytes.

Most parts of batman-adv only care about 2-byte alignment; in particular,
no unaligned accesses occur in performance-critical paths that handle
actual payload data. This is not true for OGM handling: the seqno and crc
fields are accessed as 32-bit values. To avoid these unaligned accesses,
this patch reduces the expected packet alignment to 2 bytes for all of
batadv's packet types.

As no unaligned accesses existed on the performance-critical paths anyways,
this chance does have any (positive or negative) effect on performance, but
it still makes sense to avoid these accesses to prevent log noise when
examining other unaligned accesses in the kernel while batman-adv is
active.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
---
include/uapi/linux/batadv_packet.h | 13 ++-----------
1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/include/uapi/linux/batadv_packet.h b/include/uapi/linux/batadv_packet.h
index daefd728..894d8d2f 100644
--- a/include/uapi/linux/batadv_packet.h
+++ b/include/uapi/linux/batadv_packet.h
@@ -196,8 +196,6 @@ struct batadv_bla_claim_dst {
__be16 group; /* group id */
};

-#pragma pack()
-
/**
* struct batadv_ogm_packet - ogm (routing protocol) packet
* @packet_type: batman-adv packet type, part of the general header
@@ -222,9 +220,6 @@ struct batadv_ogm_packet {
__u8 reserved;
__u8 tq;
__be16 tvlv_len;
- /* __packed is not needed as the struct size is divisible by 4,
- * and the largest data type in this struct has a size of 4.
- */
};

#define BATADV_OGM_HLEN sizeof(struct batadv_ogm_packet)
@@ -249,9 +244,6 @@ struct batadv_ogm2_packet {
__u8 orig[ETH_ALEN];
__be16 tvlv_len;
__be32 throughput;
- /* __packed is not needed as the struct size is divisible by 4,
- * and the largest data type in this struct has a size of 4.
- */
};

#define BATADV_OGM2_HLEN sizeof(struct batadv_ogm2_packet)
@@ -405,7 +397,6 @@ struct batadv_icmp_packet_rr {
* misalignment of the payload after the ethernet header. It may also lead to
* leakage of information when the padding it not initialized before sending.
*/
-#pragma pack(2)

/**
* struct batadv_unicast_packet - unicast packet for network payload
@@ -533,8 +524,6 @@ struct batadv_coded_packet {
__be16 coded_len;
};

-#pragma pack()
-
/**
* struct batadv_unicast_tvlv_packet - generic unicast packet with tvlv payload
* @packet_type: batman-adv packet type, part of the general header
@@ -641,4 +630,6 @@ struct batadv_tvlv_mcast_data {
__u8 reserved[3];
};

+#pragma pack()
+
#endif /* _UAPI_LINUX_BATADV_PACKET_H_ */
--
2.16.1