[PATCH] net: fix checksum with unaligned buffer

[Morten Brørup]

With this patch, the checksum can be calculated on an unligned part of
a packet buffer.
I.e. the buf parameter is no longer required to be 16 bit aligned.

The DPDK invariant that packet buffers must be 16 bit aligned remains
unchanged.
This invariant also defines how to calculate the 16 bit checksum on an
unaligned part of a packet buffer.

Bugzilla ID: 1035
Cc: stable at dpdk.org

Signed-off-by: Morten Brørup <mb at smartsharesystems.com>
---
 lib/net/rte_ip.h | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/lib/net/rte_ip.h b/lib/net/rte_ip.h
index b502481670..8e301d9c26 100644
--- a/lib/net/rte_ip.h
+++ b/lib/net/rte_ip.h
@@ -162,9 +162,22 @@ __rte_raw_cksum(const void *buf, size_t len, uint32_t sum)
 {
 	/* extend strict-aliasing rules */
 	typedef uint16_t __attribute__((__may_alias__)) u16_p;
-	const u16_p *u16_buf = (const u16_p *)buf;
-	const u16_p *end = u16_buf + len / sizeof(*u16_buf);
+	const u16_p *u16_buf;
+	const u16_p *end;
+
+	/* if buffer is unaligned, keeping it byte order independent */
+	if (unlikely((uintptr_t)buf & 1)) {
+		uint16_t first = 0;
+		if (unlikely(len == 0))
+			return 0;
+		((unsigned char *)&first)[1] = *(const unsigned char *)buf;
+		sum += first;
+		buf = (const void *)((uintptr_t)buf + 1);
+		len--;
+	}
 
+	u16_buf = (const u16_p *)buf;
+	end = u16_buf + len / sizeof(*u16_buf);
 	for (; u16_buf != end; ++u16_buf)
 		sum += *u16_buf;
 
-- 
2.17.1