linux-kernel - Re: [PATCH net v2 1/2] net: ipv4: fix incorrect MTU in broadcast routes

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Message-ID: <20250710141648-oscmaes92@gmail.com>
Date: Thu, 10 Jul 2025 16:16:48 +0200
From: Oscar Maes <oscmaes92@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: netdev@...r.kernel.org, davem@...emloft.net, dsahern@...nel.org,
	edumazet@...gle.com, pabeni@...hat.com, horms@...nel.org,
	stable@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2 1/2] net: ipv4: fix incorrect MTU in broadcast
 routes

On Tue, Jul 08, 2025 at 06:54:30PM -0700, Jakub Kicinski wrote:
> On Thu,  3 Jul 2025 17:28:37 +0200 Oscar Maes wrote:
> >  	if (type == RTN_BROADCAST) {
> >  		flags |= RTCF_BROADCAST | RTCF_LOCAL;
> > -		fi = NULL;
> >  	} else if (type == RTN_MULTICAST) {
> >  		flags |= RTCF_MULTICAST | RTCF_LOCAL;
> >  		if (!ip_check_mc_rcu(in_dev, fl4->daddr, fl4->saddr,
> 
> Not super familiar with this code, but do we not need to set 
> do_cache = false; ? I'm guessing cache interactions may have
> been the reason fib_info was originally cleared, not sure if
> that's still relevant..
> 
> I'd also target this at net-next, unless you can pinpoint
> some kernel version where MTU on bcast routes worked..
> -- 
> pw-bot: cr

The caching mechanism was introduced after this line, back when nhc was embedded in fib_info.
(see https://lore.kernel.org/netdev/20120720.142612.691540831359186107.davem@davemloft.net/)

I'll resend to net-next.