Subject: Re: [PATCH net-next v1 01/10] xfrm: extend add policy callback to set failure reason

From: Leon Romanovsky <[email-masked]>
Date: Thu, 26 Jan 2023 09:28:02 +0200

On Wed, Jan 25, 2023 at 11:02:26AM -0800, Jakub Kicinski wrote:
> On Tue, 24 Jan 2023 13:54:57 +0200 Leon Romanovsky wrote:
> > -	err = dev->xfrmdev_ops->xdo_dev_policy_add(xp);
> > +	err = dev->xfrmdev_ops->xdo_dev_policy_add(xp, extack);
> >  	if (err) {
> >  		xdo->dev = NULL;
> >  		xdo->real_dev = NULL;
> >  		xdo->type = XFRM_DEV_OFFLOAD_UNSPECIFIED;
> >  		xdo->dir = 0;
> >  		netdev_put(dev, &xdo->dev_tracker);
> > -		NL_SET_ERR_MSG(extack, "Device failed to offload this policy");
> 
> In a handful of places we do:
> 
> if (!extack->msg)
> 	NL_SET_ERR_MSG(extack, "Device failed to offload this policy");
> 
> in case the device did not provide the extack.
> Dunno if it's worth doing here.

Honestly, I followed devlink.c which didn't do that, but looked again
and found that devlink can potentially overwrite messages :)

For example in this case:
    997         err = ops->port_fn_state_get(port, &state, &opstate, extack);
    998         if (err) {
    999                 if (err == -EOPNOTSUPP)
   1000                         return 0;
   1001                 return err;
   1002         }
   1003         if (!devlink_port_fn_state_valid(state)) {
   1004                 WARN_ON_ONCE(1);
   1005                 NL_SET_ERR_MSG_MOD(extack, "Invalid state read from driver");
   1006                 return -EINVAL;
   1007         }

So what do you think about the following change, so we can leave
NL_SET_ERR_MSG_MOD() in devlink and xfrm intact? 

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 38f6334f408c..d6f3a958e30b 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -101,7 +101,7 @@ struct netlink_ext_ack {
                                                        \
        do_trace_netlink_extack(__msg);                 \
                                                        \
-       if (__extack)                                   \
+       if (__extack && !__extack->msg)                 \
                __extack->_msg = __msg;                 \
 } while (0)

@@ -111,7 +111,7 @@ struct netlink_ext_ack {
 #define NL_SET_ERR_MSG_FMT(extack, fmt, args...) do {                         \
        struct netlink_ext_ack *__extack = (extack);                           \
                                                                               \
-       if (!__extack)                                                         \
+       if (!__extack || __extack->msg)                                        \
                break;                                                         \
        if (snprintf(__extack->_msg_buf, NETLINK_MAX_FMTMSG_LEN,               \
                     "%s" fmt "%s", "", ##args, "") >=                         \