netdev - Re: [PATCH net-next] bonding: Remove support for use

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Message-ID: <aFGqZGvSSEfR-sDi@mini-arch>
Date: Tue, 17 Jun 2025 10:48:20 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Jay Vosburgh <jv@...sburgh.net>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, Jonathan Corbet <corbet@....net>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Hangbin Liu <liuhangbin@...il.com>, linux-doc@...r.kernel.org
Subject: Re: [PATCH net-next] bonding: Remove support for use_carrier = 0

On 06/16, Jay Vosburgh wrote:
> Stanislav Fomichev <stfomichev@...il.com> wrote:
> 
> >On 06/16, Jay Vosburgh wrote:
> >> 	 Remove the ability to disable use_carrier in bonding, and remove
> >> all code related to the old link state check that utilizes ethtool or
> >> ioctl to determine the link state of an interface in a bond.
> >> 
> >> 	To avoid acquiring RTNL many times per second, bonding's miimon
> >> link monitor inspects link state under RCU, but not under RTNL.  However,
> >> ethtool implementations in drivers may sleep, and therefore the ethtool or
> >> ioctl strategy is unsuitable for use with calls into driver ethtool
> >> functions.
> >> 
> >> 	The use_carrier option was introduced in 2003, to provide
> >> backwards compatibility for network device drivers that did not support
> >> the then-new netif_carrier_ok/on/off system.  Today, device drivers are
> >> expected to support netif_carrier_*, and the use_carrier backwards
> >> compatibility logic is no longer necessary.
> >> 
> >> 	Bonding now always behaves as if use_carrier=1, which relies on
> >> netif_carrier_ok() to determine the link state of interfaces.  This has
> >> been the default setting for use_carrier since its introduction.  For
> >> backwards compatibility, the option itself remains, but may only be set to
> >> 1, and queries will always return 1.
> >> 
> >> Reported-by: syzbot+b8c48ea38ca27d150063@...kaller.appspotmail.com
> >> Closes: https://syzkaller.appspot.com/bug?extid=b8c48ea38ca27d150063
> >> Link: https://lore.kernel.org/lkml/000000000000eb54bf061cfd666a@google.com/
> >> Link: https://lore.kernel.org/netdev/20240718122017.d2e33aaac43a.I10ab9c9ded97163aef4e4de10985cd8f7de60d28@changeid/
> >> Link: http://lore.kernel.org/netdev/aEt6LvBMwUMxmUyx@mini-arch
> >> Signed-off-by: Jay Vosburgh <jv@...sburgh.net>
> >
> >Acked-by: Stanislav Fomichev <sdf@...ichev.me>
> >
> >Maybe better to target 'net' with the following?
> >Fixes: f7a11cba0ed7 ("bonding: hold ops lock around get_link")
> 
> 	I targeted net-next and left the Fixes: tag off on purpose.
> 
> 	First, the bug this nominally fixes is many years old, and
> wasn't introduced by f7a11cba0ed7.
> 
> 	More importantly, though, this patch is removing functionality
> that someone theoretically could be relying on, and I don't think such
> removals should happen in the middle of a stable series.  The default
> setting for use_carrier (i.e., using netif_carrier) will never hit the
> issue in practice, so the exposure seems to be minimal for common use.

SG, especially assuming that use_carrier has a default of 1 (so the
issue should not appear in most/default setups).