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

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Message-ID: <20250822183250.2a9cb92c@kernel.org>
Date: Fri, 22 Aug 2025 18:32:50 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Brett A C Sheffield <bacs@...recast.net>
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

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.

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

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.