linux-kernel - Re: [PATCH] staging: lustre: Fix unneeded byte-ordering cast

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Message-ID: <87lgena72y.fsf@notabene.neil.brown.name>
Date:   Tue, 20 Mar 2018 12:06:45 +1100
From:   NeilBrown <neilb@...e.com>
To:     Justin Skists <j.skists@...il.com>, devel@...verdev.osuosl.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        James Simmons <jsimmons@...radead.org>,
        Patrick Farrell <paf@...y.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: lustre: Fix unneeded byte-ordering cast

On Sat, Mar 17 2018, Justin Skists wrote:

> Fix sparse warning:
>
>   CHECK   drivers/staging//lustre/lnet/lnet/acceptor.c
> drivers/staging//lustre/lnet/lnet/acceptor.c:243:30: warning: cast to
> restricted __le32
>
> LNET_PROTO_TCP_MAGIC, as a define, is already CPU byte-ordered when
> compared to 'magic', so no need for a cast.
>
> Signed-off-by: Justin Skists <j.skists@...il.com>
> ---
>  drivers/staging/lustre/lnet/lnet/acceptor.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/staging/lustre/lnet/lnet/acceptor.c b/drivers/staging/lustre/lnet/lnet/acceptor.c
> index fb478e20e204..13e981781b9a 100644
> --- a/drivers/staging/lustre/lnet/lnet/acceptor.c
> +++ b/drivers/staging/lustre/lnet/lnet/acceptor.c
> @@ -240,7 +240,7 @@ lnet_accept(struct socket *sock, __u32 magic)
>  			return -EPROTO;
>  		}
>  
> -		if (magic == le32_to_cpu(LNET_PROTO_TCP_MAGIC))
> +		if (magic == LNET_PROTO_TCP_MAGIC)
>  			str = "'old' socknal/tcpnal";
>  		else
>  			str = "unrecognised";

This code is almost completely irrelevant (it just choose which error
message to use when failing), but we may as well get it right and I
cannot see why your change is a fix.

"magic" was passed as an argument from lnet_acceptor() to lnet_accept(),
and lnet_acceptor() got it by reading bytes off the network with
lnet_sock_read().
My knowledge is far from complete, but from what I've seen, lustre sends
data in host-byte-order on the sender, and expects the receiver to
determine which byte-order that is (often by looking at a "magic" word
like this) and do any byte-swap that is necessary.

While I agree that LNET_PROTO_TCP_MAGIC is in host-byte-order so calling
le32_to_cpu() on it makes no sense, I don't agree that "magic" is also
host byte-ordered.

I suspect a more correct fix would be to use
  lnet_accept_magic(magic, LNET_PROTO_TCP_MAGIC)
as the condition of the if().  This is consistent with other code that
tests magic, and it is consistent with the general understanding that
"magic" should be in host-byte-order for the peer which sent the
message.

Could you resubmit with that change?

Thanks,
NeilBrown

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