[PATCH net-next v4 1/4] net: phy: refuse a second attach before touching the PHY

[Aleksei Sviridkin <f@lex.la>]

phy_attach_direct() tests phydev->attached_dev only after it has taken
its references and, for a PHY with no driver, bound the generic one, and
then leaves through the error path that calls phy_detach_internal().
That call works on the PHY's current attachment, so a second attach of
a PHY in use tears down the first consumer: it clears that netdev's
phydev pointer and the PHY's attached_dev, removes the sysfs links,
suspends the PHY and, for the generic driver, unbinds it, while the
first consumer goes on using the PHY.

Found in review of a change that makes phy_detach() put only the
module the attach recorded: with it, this path leaves the first
consumer's reference pinned. On a Keenetic KN-1012, a test module
attaching a second netdev to the EN8811H that the lan4 DSA port holds
got -EBUSY and left lan4 with no PHY; with this change lan4 kept it.

Test whether the PHY is attached before anything is taken and return
from there. A PHY attached without a netdev, as DSA does for its CPU
and link ports, has no attached_dev, so test phy_link_change, which
every attach sets and every detach clears. A second attach of such a
PHY used to succeed and share it; with that change it would also leak
a module reference.

Fixes: a7dac9f9c169 ("phy: fix error case of phy_led_triggers_(un)register")
Assisted-by: LLM
Signed-off-by: Aleksei Sviridkin <f@lex.la>
---
Changes in v4:
- Rebased on net-next. The message names phy_detach_internal(), which
  the error path calls there.
- The test uses phy_link_change, so it also catches a PHY attached
  without a netdev.

 drivers/net/phy/phy_device.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
index 5b13a74e2fa9..0bdd2dc84d81 100644
--- a/drivers/net/phy/phy_device.c
+++ b/drivers/net/phy/phy_device.c
@@ -1794,6 +1794,12 @@ int phy_attach_direct(struct net_device *dev, struct phy_device *phydev,
 	struct module *ndev_owner = NULL;
 	int err;
 
+	/* Set by every attach, with or without a netdev */
+	if (phydev->phy_link_change) {
+		phydev_err(phydev, "PHY already attached\n");
+		return -EBUSY;
+	}
+
 	/* For Ethernet device drivers that register their own MDIO bus, we
 	 * will have bus->owner match ndev_mod, so we do not want to increment
 	 * our own module->refcnt here, otherwise we would not be able to
@@ -1835,12 +1841,6 @@ int phy_attach_direct(struct net_device *dev, struct phy_device *phydev,
 			goto error_module_put;
 	}
 
-	if (phydev->attached_dev) {
-		dev_err(&dev->dev, "PHY already attached\n");
-		err = -EBUSY;
-		goto error;
-	}
-
 	phydev->phy_link_change = phy_link_change;
 	if (dev) {
 		phydev->attached_dev = dev;
-- 
2.53.0