From: Maxim Mikityanskiy <maximmi@mellanox.com>
Date: Wed, 14 Aug 2019 14:34:06 +0000
Subject: net: Don't call XDP_SETUP_PROG when nothing is changed
Patch-mainline: v5.4-rc1
Git-commit: c14a9f633d9eb5c0fdd0cb4135b6be978fd538f3
References: bsc#1154353

Don't uninstall an XDP program when none is installed, and don't install
an XDP program that has the same ID as the one already installed.

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 symmetrical case is possible when the user tries to set the program
that is already set.

The drivers typically perform some heavy operations on XDP_SETUP_PROG,
so they all have to handle these cases internally to return early if
they happen. 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>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 net/core/dev.c |   15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -8126,12 +8126,15 @@ int dev_change_xdp_fd(struct net_device
 		bpf_chk = generic_xdp_install;
 
 	if (fd >= 0) {
+		u32 prog_id;
+
 		if (!offload && __dev_xdp_query(dev, bpf_chk, XDP_QUERY_PROG)) {
 			NL_SET_ERR_MSG(extack, "native and generic XDP can't be active at the same time");
 			return -EEXIST;
 		}
-		if ((flags & XDP_FLAGS_UPDATE_IF_NOEXIST) &&
-		    __dev_xdp_query(dev, bpf_op, query)) {
+
+		prog_id = __dev_xdp_query(dev, bpf_op, query);
+		if ((flags & XDP_FLAGS_UPDATE_IF_NOEXIST) && prog_id) {
 			NL_SET_ERR_MSG(extack, "XDP program already attached");
 			return -EBUSY;
 		}
@@ -8146,6 +8149,14 @@ int dev_change_xdp_fd(struct net_device
 			bpf_prog_put(prog);
 			return -EINVAL;
 		}
+
+		if (prog->aux->id == prog_id) {
+			bpf_prog_put(prog);
+			return 0;
+		}
+	} else {
+		if (!__dev_xdp_query(dev, bpf_op, query))
+			return 0;
 	}
 
 	err = dev_xdp_install(dev, bpf_op, extack, flags, prog);