Re: [PATCH net-next] netlink: provide an ability to set default extack message

[Jakub Kicinski <kuba@kernel.org>]

On Fri, 27 Jan 2023 00:32:13 +0200 Vladimir Oltean wrote:
> On Thu, Jan 26, 2023 at 09:15:03PM +0200, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@nvidia.com>
> > 
> > In netdev common pattern, xxtack pointer is forwarded to the drivers  
>                             ~~~~~~
>                             extack
> 
> > to be filled with error message. However, the caller can easily
> > overwrite the filled message.
> > 
> > Instead of adding multiple "if (!extack->_msg)" checks before any
> > NL_SET_ERR_MSG() call, which appears after call to the driver, let's
> > add this check to common code.
> > 
> > [1] https://lore.kernel.org/all/Y9Irgrgf3uxOjwUm@unreal
> > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> > ---  
> 
> I would somewhat prefer not doing this, and instead introducing a new
> NL_SET_ERR_MSG_WEAK() of sorts.

That'd be my preference too, FWIW. It's only the offload cases which
need this sort of fallback.

BTW Vladimir, I remember us discussing this. I was searching the
archive as you sent this, but can't find the thread. Mostly curious
whether I flip flipped on this or I'm not completely useless :)

> The reason has to do with the fact that an extack is sometimes also
> used to convey warnings rather than hard errors, for example right here
> in net/dsa/slave.c:
> 
> 	if (err == -EOPNOTSUPP) {
> 		if (extack && !extack->_msg)
> 			NL_SET_ERR_MSG_MOD(extack,
> 					   "Offloading not supported");
> 		NL_SET_ERR_MSG_MOD(extack,
> 				   "Offloading not supported");
> 		err = 0;
> 	}