linux-kernel - Re: [PATCH v2 2/5] block nbd: send handle in network order

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Message-ID: <ZBjqQckL7d5EJPlh@ovpn-8-29.pek2.redhat.com>
Date:   Tue, 21 Mar 2023 07:20:33 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Eric Blake <eblake@...hat.com>
Cc:     josef@...icpanda.com, linux-block@...r.kernel.org,
        nbd@...er.debian.org, philipp.reisner@...bit.com,
        lars.ellenberg@...bit.com, christoph.boehmwalder@...bit.com,
        corbet@....net, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, ming.lei@...hat.com
Subject: Re: [PATCH v2 2/5] block nbd: send handle in network order

On Fri, Mar 17, 2023 at 03:27:46PM -0500, Eric Blake wrote:
> The NBD spec says the client handle (or cookie) is opaque on the
> server, and therefore it really doesn't matter what endianness we use;
> to date, the use of memcpy() between u64 and a char[8] has exposed
> native endianness when treating the handle as a 64-bit number.

No, memcpy() works fine for char[8], which doesn't break endianness.

> However, since NBD protocol documents that everything else is in
> network order, and tools like Wireshark will dump even the contents of
> the handle as seen over the network, it's worth using a consistent
> ordering regardless of the native endianness.
> 
> Plus, using a consistent endianness now allows an upcoming patch to
> simplify this to directly use integer assignment instead of memcpy().

It isn't necessary, given ->handle is actually u64, which is handled by
nbd client only.

> 
> Signed-off-by: Eric Blake <eblake@...hat.com>
> 
> ---
> v2: new patch
> ---
>  drivers/block/nbd.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 592cfa8b765a..8a9487e79f1c 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -560,6 +560,7 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
>  	unsigned long size = blk_rq_bytes(req);
>  	struct bio *bio;
>  	u64 handle;
> +	__be64 tmp;
>  	u32 type;
>  	u32 nbd_cmd_flags = 0;
>  	int sent = nsock->sent, skip = 0;
> @@ -606,7 +607,8 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
>  		request.len = htonl(size);
>  	}
>  	handle = nbd_cmd_handle(cmd);
> -	memcpy(request.handle, &handle, sizeof(handle));
> +	tmp = cpu_to_be64(handle);
> +	memcpy(request.handle, &tmp, sizeof(tmp));

This way copies handle two times, really not fun.


thanks,
Ming