cifs: Add a function to read into an iter from a socket

Add a helper function to read data from a socket into the given iterator.
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org

Link: https://lore.kernel.org/r/164928617874.457102.10021662143234315566.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/165211419563.3154751.18431990381145195050.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/165348879662.2106726.16881134187242702351.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/165364826398.3334034.12541600783145647319.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/166126395495.708021.12328677373159554478.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/166697258876.61150.3530237818849429372.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166732031039.3186319.10691316510079412635.stgit@warthog.procyon.org.uk/ # rfc
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifsproto.h

@@ -244,6 +244,9 @@ extern int cifs_read_page_from_socket(struct TCP_Server_Info *server,
 					struct page *page,
 					unsigned int page_offset,
 					unsigned int to_read);
+int cifs_read_iter_from_socket(struct TCP_Server_Info *server,
+			       struct iov_iter *iter,
+			       unsigned int to_read);
 extern int cifs_setup_cifs_sb(struct cifs_sb_info *cifs_sb);
 void cifs_mount_put_conns(struct cifs_mount_ctx *mnt_ctx);
 int cifs_mount_get_session(struct cifs_mount_ctx *mnt_ctx);

fs/cifs/connect.c

@@ -733,6 +733,20 @@ cifs_read_page_from_socket(struct TCP_Server_Info *server, struct page *page,
 	return cifs_readv_from_socket(server, &smb_msg);
 }
 
+int
+cifs_read_iter_from_socket(struct TCP_Server_Info *server, struct iov_iter *iter,
+			   unsigned int to_read)
+{
+	struct msghdr smb_msg = { .msg_iter = *iter };
+	int ret;
+
+	iov_iter_truncate(&smb_msg.msg_iter, to_read);
+	ret = cifs_readv_from_socket(server, &smb_msg);
+	if (ret > 0)
+		iov_iter_advance(iter, ret);
+	return ret;
+}
+
 static bool
 is_smb_response(struct TCP_Server_Info *server, unsigned char type)
 {