[PATCH bpf-next] net: Don't uninstall an XDP program when none is installed

[Maxim Mikityanskiy <maximmi@mellanox.com>]

dev_change_xdp_fd doesn't perform any checks in case it uninstalls an
XDP program. It means that the driver's ndo_bpf can be called with
XDP_SETUP_PROG asking to set it to NULL even if it's already NULL. This
case happens if the user runs `ip link set eth0 xdp off` when there is
no XDP program attached.

The drivers typically perform some heavy operations on XDP_SETUP_PROG,
so they all have to handle this case internally to return early if it
happens. This patch puts this check into the kernel code, so that all
drivers will benefit from it.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
---
Björn, please take a look at this, Saeed told me you were doing
something related, but I couldn't find it. If this fix is already
covered by your work, please tell about that.

 net/core/dev.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index 66f7508825bd..68b3e3320ceb 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -8089,6 +8089,9 @@ int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
 			bpf_prog_put(prog);
 			return -EINVAL;
 		}
+	} else {
+		if (!__dev_xdp_query(dev, bpf_op, query))
+			return 0;
 	}
 
 	err = dev_xdp_install(dev, bpf_op, extack, flags, prog);
-- 
2.19.1