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

On Fri, Jan 27, 2023 at 12:32:13AM +0200, Vladimir Oltean wrote:

On Thu, Jan 26, 2023 at 09:15:03PM +0200, Leon Romanovsky wrote:

quoted

From: Leon Romanovsky <leonro@nvidia.com>

In netdev common pattern, xxtack pointer is forwarded to the drivers

                            ~~~~~~
                            extack

quoted

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 (local)
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.

It means changing ALL error unwind places where extack was forwarded
before to subfunctions.

Places like this:
 ret = func(..., extack)
 if (ret) {
   NL_SET_ERR_MSG_MOD...
   return ret;
 }

will need to be changed to something like this:
 ret = func(..., extack)
 if (ret) {
   NL_SET_ERR_MSG_WEAK...
   return ret;
 }

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;
	}

Imagine (not the case here) that below such a "warning extack" lies
something like this:

	if (arg > range) {
		NL_SET_ERR_MSG_MOD(extack, "Argument outside expected range");
		return -ERANGE;
	}

What you'll get is:

Error: Offloading not supported (error code -ERANGE).

whereas before, we relied on any NL_SET_ERR_MSG_MOD() call to overwrite
the "warning" extack, and that to only be shown on error code 0.

Can we please discuss current code and not over-engineered case which
doesn't exist in the reality?

Even for your case, I would like to see NL_SET_ERR_MSG_FORCE() to
explicitly say that message will be overwritten.

Thanks