RE: [RFC 08/13] cifs: Add a function to read into an iter from a socket

From: David Laight <David.Laight@ACULAB.COM>
To: 'David Howells' <dhowells@redhat.com>, Steve French <smfrench@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Shyam Prasad N <nspmangalore@gmail.com>,
	Rohith Surabattula <rohiths.msft@gmail.com>,
	"Tom Talpey" <tom@talpey.com>,
	Stefan Metzmacher <metze@samba.org>,
	"Christoph Hellwig" <hch@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	"Jeff Layton" <jlayton@kernel.org>,
	"linux-cifs@vger.kernel.org" <linux-cifs@vger.kernel.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Steve French <sfrench@samba.org>
Subject: RE: [RFC 08/13] cifs: Add a function to read into an iter from a socket
Date: Thu, 26 Jan 2023 09:27:02 +0000	[thread overview]
Message-ID: <0d53a3cc9f9448298bba04d06f51b23d@AcuMS.aculab.com> (raw)
In-Reply-To: <20230125214543.2337639-9-dhowells@redhat.com>

From: David Howells
> Sent: 25 January 2023 21:46

> Add a helper function to read data from a socket into the given iterator.
...
> +int
> +cifs_read_iter_from_socket(struct TCP_Server_Info *server, struct iov_iter *iter,
> +			   unsigned int to_read)
> +{
> +	struct msghdr smb_msg;
> +	int ret;
> +
> +	smb_msg.msg_iter = *iter;
> +	if (smb_msg.msg_iter.count > to_read)
> +		smb_msg.msg_iter.count = to_read;
> +	ret = cifs_readv_from_socket(server, &smb_msg);
> +	if (ret > 0)
> +		iov_iter_advance(iter, ret);
> +	return ret;
> +}

On the face of it that passes a largely uninitialised 'struct msghdr'
to cifs_readv_from_socket() in order to pass an iov_iter.
That seems to be asking for trouble.

I'm also not 100% sure that taking a copy of an iov_iter is a good idea.

If cifs_readv_from_socket() only needs the iov_iter then wouldn't
it be better to do the wrapper the other way around?
(Probably as an inline function)
Something like:

int
cifs_readv_from_socket(struct TCP_Server_Info *server, struct msghdr *smb_msg)
{
	return cifs_read_iter_from_socket(server, &smb_msg->msg_iter, smb_msg->msg_iter.count);
}

and then changing cifs_readv_from_socket() to just use the iov_iter.

	David

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