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

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Message-ID: <0d53a3cc9f9448298bba04d06f51b23d@AcuMS.aculab.com>
Date:   Thu, 26 Jan 2023 09:27:02 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'David Howells' <dhowells@...hat.com>,
        Steve French <smfrench@...il.com>
CC:     Al Viro <viro@...iv.linux.org.uk>,
        Shyam Prasad N <nspmangalore@...il.com>,
        Rohith Surabattula <rohiths.msft@...il.com>,
        "Tom Talpey" <tom@...pey.com>, Stefan Metzmacher <metze@...ba.org>,
        "Christoph Hellwig" <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Jeff Layton" <jlayton@...nel.org>,
        "linux-cifs@...r.kernel.org" <linux-cifs@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Steve French <sfrench@...ba.org>
Subject: RE: [RFC 08/13] cifs: Add a function to read into an iter from a
 socket

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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