[PATCH bpf-next v2] net: Don't call XDP_SETUP_PROG when nothing is changed

From: Maxim Mikityanskiy <maximmi@mellanox.com>
To: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	"Saeed Mahameed" <saeedm@mellanox.com>,
	"Jesper Dangaard Brouer" <hawk@kernel.org>,
	"John Fastabend" <john.fastabend@gmail.com>,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"Maxim Mikityanskiy" <maximmi@mellanox.com>
Subject: [PATCH bpf-next v2] net: Don't call XDP_SETUP_PROG when nothing is changed
Date: Wed, 14 Aug 2019 14:34:06 +0000	[thread overview]
Message-ID: <20190814143352.3759-1-maximmi@mellanox.com> (raw)
In-Reply-To: <5b123e9a-095f-1db4-da6e-5af6552430e1@iogearbox.net>

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>
---
v2 changes: Cover the case when the program is set, but isn't changed.

 net/core/dev.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 49589ed2018d..b1afafee3e2a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -8126,12 +8126,15 @@ int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
 		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 *dev, struct netlink_ext_ack *extack,
 			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);
-- 
2.20.1

