Re: [PATCH net-next v7 1/3] net: ethtool: Introduce per-PHY DUMP operations

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

On Fri, 25 Apr 2025 17:10:44 -0700
Jakub Kicinski <kuba@kernel.org> wrote:

> On Fri, 25 Apr 2025 09:01:53 +0200 Maxime Chevallier wrote:
> > > > +/* perphy ->dumpit() handler for GET requests. */
> > > > +static int ethnl_perphy_dumpit(struct sk_buff *skb,
> > > > +			       struct netlink_callback *cb)
> > > > +{
> > > > +	struct ethnl_perphy_dump_ctx *ctx = ethnl_perphy_dump_context(cb);
> > > > +	struct ethnl_dump_ctx *ethnl_ctx = &ctx->ethnl_ctx;
> > > > +	int ret = 0;
> > > > +
> > > > +	if (ethnl_ctx->req_info->dev) {
> > > > +		ret = ethnl_perphy_dump_one_dev(skb, ethnl_ctx->req_info->dev,
> > > > +						ctx, genl_info_dump(cb));
> > > > +		if (ret < 0 && ret != -EOPNOTSUPP && likely(skb->len))
> > > > +			ret = skb->len;
> > > > +
> > > > +		netdev_put(ethnl_ctx->req_info->dev,
> > > > +			   &ethnl_ctx->req_info->dev_tracker);      
> > > 
> > > You have to release this in .done
> > > dumpit gets called multiple times until we run out of objects to dump.
> > > OTOH user may close the socket without finishing the dump operation.
> > > So all .dumpit implementations must be "balanced". The only state we
> > > should touch in them is the dump context to know where to pick up from
> > > next time.    
> > 
> > Thanks for poiting it out.
> > 
> > Now that you say that, I guess that I should also move the reftracker
> > I'm using for the netdev_hold in ethnl_perphy_dump_one_dev() call to
> > struct ethnl_perphy_dump_ctx ? That way we make sure the netdev doesn't
> > go away in-between the multiple .dumpit() calls then...
> > 
> > Is that correct ?  
> 
> Probably not. That'd allow an unprivileged user space program to take 
> a ref on a netdev, and never call recv() to make progress on the dump.
> 
> The possibility that the netdev may disappear half way thru is inherent
> to the netlink dump model. We will pick back up within that netdev
> based on its ifindex, no need to hold it.

OK I'm good with that then ! Thanks for the explanations, makes a lot
of sense. I'll followup with a new version soon :)

Maxime