[PATCH bpf] bpf: fix XDP FD-based attach/detach logic around XDP_FLAGS_UPDATE_IF_NOEXIST

[Andrii Nakryiko]

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 <s...@google.com>
Fixes: d4baa9368a5e ("bpf, xdp: Extract common XDP program attachment logic")
Signed-off-by: Andrii Nakryiko <andr...@fb.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