Re: [PATCH net-next] net: revert to lockless TC_SETUP_BLOCK and TC_SETUP_FT

[Simon Horman <horms@kernel.org>]

On Fri, Mar 07, 2025 at 08:47:26PM -0800, Stanislav Fomichev wrote:
> There is a couple of places from which we can arrive to ndo_setup_tc
> with TC_SETUP_BLOCK/TC_SETUP_FT:
> - netlink
> - netlink notifier
> - netdev notifier
> 
> Locking netdev too deep in this call chain seems to be problematic
> (especially assuming some/all of the call_netdevice_notifiers
> NETDEV_UNREGISTER) might soon be running with the instance lock).
> Revert to lockless ndo_setup_tc for TC_SETUP_BLOCK/TC_SETUP_FT. NFT
> framework already takes care of most of the locking. Document
> the assumptions.
> 
> ndo_setup_tc TC_SETUP_BLOCK
>   nft_block_offload_cmd
>     nft_chain_offload_cmd
>       nft_flow_block_chain
>         nft_flow_offload_chain
> 	  nft_flow_rule_offload_abort
> 	    nft_flow_rule_offload_commit
> 	  nft_flow_rule_offload_commit
> 	    nf_tables_commit
> 	      nfnetlink_rcv_batch
> 	        nfnetlink_rcv_skb_batch
> 		  nfnetlink_rcv
> 	nft_offload_netdev_event
> 	  NETDEV_UNREGISTER notifier
> 
> ndo_setup_tc TC_SETUP_FT
>   nf_flow_table_offload_cmd
>     nf_flow_table_offload_setup
>       nft_unregister_flowtable_hook
>         nft_register_flowtable_net_hooks
> 	  nft_flowtable_update
> 	  nf_tables_newflowtable
> 	    nfnetlink_rcv_batch (.call NFNL_CB_BATCH)
> 	nft_flowtable_update
> 	  nf_tables_newflowtable
> 	nft_flowtable_event
> 	  nf_tables_flowtable_event
> 	    NETDEV_UNREGISTER notifier
>       __nft_unregister_flowtable_net_hooks
>         nft_unregister_flowtable_net_hooks
> 	  nf_tables_commit
> 	    nfnetlink_rcv_batch (.call NFNL_CB_BATCH)
> 	  __nf_tables_abort
> 	    nf_tables_abort
> 	      nfnetlink_rcv_batch
> 	__nft_release_hook
> 	  __nft_release_hooks
> 	    nf_tables_pre_exit_net -> module unload
> 	  nft_rcv_nl_event
> 	    netlink_register_notifier (oh boy)
>       nft_register_flowtable_net_hooks
>       	nft_flowtable_update
> 	  nf_tables_newflowtable
>         nf_tables_newflowtable
> 
> Fixes: c4f0f30b424e ("net: hold netdev instance lock during nft ndo_setup_tc")
> Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>

Thanks Stan,

Thinking aloud: the dev_setup_tc() helper checked if ndo_setup_tc is
non-NULL, but that is not the case here. But that seems ok because it was
also the case prior to the cited commit.

Reviewed-by: Simon Horman <horms@kernel.org>

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