netdev - Re: [PATCH net 1/2] net: ipv4: fix regression in local-broadcast routes

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Message-ID: <20250826121126-oscmaes92@gmail.com>
Date: Tue, 26 Aug 2025 14:11:26 +0200
From: Oscar Maes <oscmaes92@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: bacs@...recast.net, brett@...recast.net, davem@...emloft.net,
	dsahern@...nel.org, netdev@...r.kernel.org,
	regressions@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH net 1/2] net: ipv4: fix regression in local-broadcast
 routes

On Mon, Aug 25, 2025 at 03:56:30PM -0700, Jakub Kicinski wrote:
> On Mon, 25 Aug 2025 08:09:17 +0200 Oscar Maes wrote:
> > Commit 9e30ecf23b1b ("net: ipv4: fix incorrect MTU in broadcast routes")
> > introduced a regression where local-broadcast packets would have their
> > gateway set in __mkroute_output, which was caused by fi = NULL being
> > removed.
> > 
> > Fix this by resetting the fib_info for local-broadcast packets.
> 
> Meaning that 9e30ecf23b1b would still change behavior for the subnet
> broadcast address?
> 

Yes. This is OK because subnet broadcast addresses have separate entries
in the local fib table.

> > diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> > index f639a2ae881a..98d237e3ec04 100644
> > --- a/net/ipv4/route.c
> > +++ b/net/ipv4/route.c
> > @@ -2575,9 +2575,12 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
> >  		    !netif_is_l3_master(dev_out))
> >  			return ERR_PTR(-EINVAL);
> >  
> > -	if (ipv4_is_lbcast(fl4->daddr))
> > +	if (ipv4_is_lbcast(fl4->daddr)) {
> >  		type = RTN_BROADCAST;
> > -	else if (ipv4_is_multicast(fl4->daddr))
> > +
> > +		/* reset fi to prevent gateway resolution */
> > +		fi = NULL;
> > +	} else if (ipv4_is_multicast(fl4->daddr))
> >  		type = RTN_MULTICAST;
> >  	else if (ipv4_is_zeronet(fl4->daddr))
> >  		return ERR_PTR(-EINVAL);
> 
> nit: please add curly braces around all branches of this if / else if /
> else ladder, per kernel coding style guide.
> -- 
> pw-bot: cr