netdev - Re: [REGRESSION][BISECTED][PATCH] net: ipv4: fix regression in broadcast routes

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Message-ID: <aKmzB57MKbpXh-_Z@karahi.gladserv.com>
Date: Sat, 23 Aug 2025 14:24:39 +0200
From: Brett Sheffield <brett@...recast.net>
To: Jakub Kicinski <kuba@...nel.org>
Cc: regressions@...ts.linux.dev, netdev@...r.kernel.org,
	stable@...r.kernel.org, davem@...emloft.net, dsahern@...nel.org,
	oscmaes92@...il.com
Subject: Re: [REGRESSION][BISECTED][PATCH] net: ipv4: fix regression in
 broadcast routes

On 2025-08-22 18:32, Jakub Kicinski wrote:
> Thanks for bisecting and fixing!
> 
> > The broadcast_pmtu.sh selftest provided with the original patch still
> > passes with this patch applied.
> 
> Hm, yes, AFACT we're losing PMTU discovery but perhaps original commit
> wasn't concerned with that. Hopefully Oscar can comment.

Indeed. This takes it back to the previous behaviour.

> On Fri, 22 Aug 2025 16:50:51 +0000 Brett A C Sheffield wrote:
> > +		if (type == RTN_BROADCAST) {
> > +			/* ensure MTU value for broadcast routes is retained */
> > +			ip_dst_init_metrics(&rth->dst, res->fi->fib_metrics);
> 
> You need to check if res->fi is actually set before using it

Ah, yes.  Fixed.

> Could you add a selftest / test case for the scenario we broke?
> selftests can be in C / bash / Python. If bash hopefully socat
> can be used to repro, cause it looks like wakeonlan is not very
> widely packaged.

Self-test added using socat as requested. If you want this wrapped in namespaces
etc. let me know. I started doing that, but decided a simpler test without
requiring root was better and cleaner.

Thanks for the review Jakub.  v2 patches sent.

Cheers,


Brett
-- 
Brett Sheffield (he/him)
Librecast - Decentralising the Internet with Multicast
https://librecast.net/
https://blog.brettsheffield.com/