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Date:   Tue, 11 Aug 2020 19:29:23 -0700
From:   Andrii Nakryiko <andriin@...com>
To:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <ast@...com>,
        <daniel@...earbox.net>
CC:     <andrii.nakryiko@...il.com>, <kernel-team@...com>,
        Andrii Nakryiko <andriin@...com>,
        Stanislav Fomichev <sdf@...gle.com>
Subject: [PATCH bpf] bpf: fix XDP FD-based attach/detach logic around XDP_FLAGS_UPDATE_IF_NOEXIST

Enforce XDP_FLAGS_UPDATE_IF_NOEXIST only if new BPF program to be attached is
non-NULL (i.e., we are not detaching a BPF program).

Reported-by: Stanislav Fomichev <sdf@...gle.com>
Fixes: d4baa9368a5e ("bpf, xdp: Extract common XDP program attachment logic")
Signed-off-by: Andrii Nakryiko <andriin@...com>
---
 net/core/dev.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 7df6c9617321..b5d1129d8310 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -8913,10 +8913,6 @@ static int dev_xdp_attach(struct net_device *dev, struct netlink_ext_ack *extack
 		NL_SET_ERR_MSG(extack, "Active program does not match expected");
 		return -EEXIST;
 	}
-	if ((flags & XDP_FLAGS_UPDATE_IF_NOEXIST) && cur_prog) {
-		NL_SET_ERR_MSG(extack, "XDP program already attached");
-		return -EBUSY;
-	}
 
 	/* put effective new program into new_prog */
 	if (link)
@@ -8927,6 +8923,10 @@ static int dev_xdp_attach(struct net_device *dev, struct netlink_ext_ack *extack
 		enum bpf_xdp_mode other_mode = mode == XDP_MODE_SKB
 					       ? XDP_MODE_DRV : XDP_MODE_SKB;
 
+		if ((flags & XDP_FLAGS_UPDATE_IF_NOEXIST) && cur_prog) {
+			NL_SET_ERR_MSG(extack, "XDP program already attached");
+			return -EBUSY;
+		}
 		if (!offload && dev_xdp_prog(dev, other_mode)) {
 			NL_SET_ERR_MSG(extack, "Native and generic XDP can't be active at the same time");
 			return -EEXIST;
-- 
2.24.1

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