netdev - Re: [RFC PATCH net-next 1/3] netlink: add support for formatted extack messages

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Date:   Fri, 07 Oct 2022 15:35:29 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     ecree@...inx.com, netdev@...r.kernel.org, linux-net-drivers@....com
Cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        edumazet@...gle.com, habetsm.xilinx@...il.com,
        marcelo.leitner@...il.com, Edward Cree <ecree.xilinx@...il.com>
Subject: Re: [RFC PATCH net-next 1/3] netlink: add support for formatted
 extack messages

> +#define NL_SET_ERR_MSG_FMT(extack, fmt, args...) do {		\
> +	struct netlink_ext_ack *__extack = (extack);		\
> +								\
> +	scnprintf(__extack->_msg_buf, NETLINK_MAX_FMTMSG_LEN,	\
> +		  (fmt), ##args);				\

Maybe that should print some kind of warning if the string was longer
than the buffer? OTOH, I guess the user would notice anyway, and until
you run the code nobody can possibly notice ... too bad then?

Maybe we could at least _statically_ make sure that the *format* string
(fmt) is shorter than say 60 chars or something to give some wiggle room
for the print expansion?

	/* allow 20 chars for format expansion */
	BUILD_BUG_ON(strlen(fmt) > NETLINK_MAX_FMTMSG_LEN - 20);

might even work? Just as a sanity check.

> +	do_trace_netlink_extack(__extack->_msg_buf);		\
> +								\
> +	if (__extack)						\
> +		__extack->_msg = __extack->_msg_buf;		\

That "if (__extack)" check seems a bit strange, you've long crashed with
a NPD if it was really NULL?

johannes