netdev - RE: [PATCH] net/rds: fix unaligned memory access

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Date:	Thu, 30 Apr 2015 15:23:39 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'David Ahern' <david.ahern@...cle.com>,
	"rds-devel@....oracle.com" <rds-devel@....oracle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	shamir rabinovitch <shamir.rabinovitch@...cle.com>
Subject: RE: [PATCH] net/rds: fix unaligned memory access

From: David Ahern
> Sent: 30 April 2015 15:28
> rdma_conn_param private data is copied using memcpy after headers such
> as cma_hdr (see cma_resolve_ib_udp as example). so the start of the
> private data is aligned to the end of the structure that come before. if
> this structure end with u32 the meaning is that the start of the private
> data will be 4 bytes aligned. structures that use u8/u16/u32/u64 are
> naturally aligned but in case the structure start is not 8 bytes aligned,
> all u64 members of this structure will not be aligned. to solve this issue
> we must use special macros that allow unaligned access to those
> unaligned members.
> 
> Addresses the following kernel log seen when attempting to use RDMA:
> 
> Kernel unaligned access at TPC[10507a88] rds_ib_cm_connect_complete+0x1bc/0x1e0 [rds_rdma]
...
> diff --git a/net/rds/ib_cm.c b/net/rds/ib_cm.c
> index 31b74f5..e0bbbac 100644
> --- a/net/rds/ib_cm.c
> +++ b/net/rds/ib_cm.c
> @@ -183,8 +183,17 @@ void rds_ib_cm_connect_complete(struct rds_connection *conn, struct rdma_cm_even
> 
>  	/* If the peer gave us the last packet it saw, process this as if
>  	 * we had received a regular ACK. */
> -	if (dp && dp->dp_ack_seq)
> -		rds_send_drop_acked(conn, be64_to_cpu(dp->dp_ack_seq), NULL);

Or just mark 'dp_ack_seq' with __attribute((aligned(4))).
Then the compiler will generate two 32bit loads.
(provided be64_to_cpu() doesn't do anything stupid.)

	David

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