net: ethtool: optionally skip rtnl_lock on Netlink path for GET ops

ethnl_default_doit() and ethnl_default_dump_one() are both used
exclusively for GET callbacks (former to get info for a single
device or get global strings). ops-locked devices don't need
rtnl_lock for GET callbacks, stop taking it.

Introduce an opt-out mechanism for devices which use phylink (fbnic)
since phylink currently depends on rtnl_lock protection. Subsequent
patches will add more exceptions, anyway. Practically the new helpers
for judging if command needs rtnl_lock could also call
netdev_need_ops_lock() but I find that it makes the code in the callers
slightly less obvious.

Add a helper for IOCTLs already, even tho it's unused so that
we can keep them in sync as the series progresses.

This is the first user-visible step of moving ethtool ops out
from under rtnl. Subsequent patches do the same for SET ops,
as well as the ioctl path.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20260605002912.3456868-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 17 insertions, 1 deletions

diff --git a/include/linux/ethtool.h b/include/linux/ethtool.h
index f51346a6a686..1da49161d36f 100644
--- a/include/linux/ethtool.h
+++ b/include/linux/ethtool.h
@@ -930,6 +930,13 @@ struct kernel_ethtool_ts_info {
 	u32 rx_filters;
 };
 
+/* Bits for ethtool_ops::op_needs_rtnl
+ * LINKSETTINGS cover a number of commands, but in most cases we want to keep
+ * these bits separate, per GET and SET. GET is much easier to "unlock".
+ */
+#define ETHTOOL_OP_NEEDS_RTNL_LINKSETTINGS	BIT(0)
+#define ETHTOOL_OP_NEEDS_RTNL_GPAUSEPARAM	BIT(1)
+
 /**
  * struct ethtool_ops - optional netdev operations
  * @supported_input_xfrm: supported types of input xfrm from %RXH_XFRM_*.
@@ -956,6 +963,14 @@ struct kernel_ethtool_ts_info {
  * @supported_coalesce_params: supported types of interrupt coalescing.
  * @supported_ring_params: supported ring params.
  * @supported_hwtstamp_qualifiers: bitfield of supported hwtstamp qualifier.
+ * @op_needs_rtnl: mask of %ETHTOOL_OP_NEEDS_RTNL_* bits.
+ *	For use with ops-locked drivers (ignored otherwise). Selects which
+ *	ethtool callbacks driver needs to still be executed under rtnl_lock
+ *	(in addition to the netdev instance lock).
+ *	The following commonly used core APIs currently require rtnl_lock
+ *	(this list may not be exhaustive):
+ *	 - phylink helpers (note that phydev is currently unsupported!)
+ *
  * @get_drvinfo: Report driver/device information. Modern drivers no
  *	longer have to implement this callback. Most fields are
  *	correctly filled in by the core using system information, or
@@ -1155,7 +1170,7 @@ struct kernel_ethtool_ts_info {
  *
  * All operations are optional (i.e. the function pointer may be set
  * to %NULL) and callers must take this into account.  Callers must
- * hold the RTNL lock.
+ * hold the RTNL lock or netdev instance lock (see @op_needs_rtnl).
  *
  * See the structures used by these operations for further documentation.
  * Note that for all operations using a structure ending with a zero-
@@ -1178,6 +1193,7 @@ struct ethtool_ops {
 	u32	supported_coalesce_params;
 	u32	supported_ring_params;
 	u32	supported_hwtstamp_qualifiers;
+	u32	op_needs_rtnl;
 	void	(*get_drvinfo)(struct net_device *, struct ethtool_drvinfo *);
 	int	(*get_regs_len)(struct net_device *);
 	void	(*get_regs)(struct net_device *, struct ethtool_regs *, void *);