ipv6: fix error handling in ignore_routes_with_linkdown sysctl

[ Upstream commit cf4f2b1440 ]

When writing to the ignore_routes_with_linkdown sysctl, if
proc_dointvec() fails to parse the input, it returns a negative error
code. The current implementation is overwriting that error for write
operations.

This results in a silent failure, it returns a successful write although
the configuration was not modified at all. When modifying the "all"
variant it can also modify the configuration of existing interfaces to
the wrong value.

Fix this by checking the return value of proc_dointvec() and returning
early on failure.

Fixes: 35103d1117 ("net: ipv6 sysctl option to ignore routes when nexthop link is down")
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260622130857.5115-3-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/ipv6/addrconf.c

@@ -6639,6 +6639,8 @@ int addrconf_sysctl_ignore_routes_with_linkdown(struct ctl_table *ctl,
 	lctl.data = &val;
 
 	ret = proc_dointvec(&lctl, write, buffer, lenp, ppos);
+	if (ret)
+		return ret;
 
 	if (write)
 		ret = addrconf_fixup_linkdown(ctl, valp, val);