Re: [PATCH bpf-next v4 2/4] bpf: Reject TCP_NODELAY in bpf-tcp-cc

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 4/21/26 11:58 PM, KaFai Wan wrote:
> A BPF TCP congestion control program can call bpf_setsockopt() from
> its callbacks. In current kernels, if it calls
> bpf_setsockopt(TCP_NODELAY) from cwnd_event_tx_start(), the call can
> re-enter the TCP transmit path before the outer tcp_transmit_skb()
> has completed and advanced the send head.
>
> This can re-trigger CA_EVENT_TX_START and lead to unbounded recursion:
>
>    tcp_transmit_skb()
>      -> tcp_event_data_sent()
>        -> tcp_ca_event(sk, CA_EVENT_TX_START)
>          -> cwnd_event_tx_start()
>            -> bpf_setsockopt(TCP_NODELAY)
>              -> tcp_push_pending_frames()
>                -> tcp_write_xmit()
>                  -> tcp_transmit_skb()
>
> This leads to unbounded recursion and can overflow the kernel stack.
>
> Reject TCP_NODELAY with -EOPNOTSUPP for bpf-tcp-cc by introducing
> a dedicated setsockopt proto for BPF_PROG_TYPE_STRUCT_OPS TCP
> congestion control programs.
>
> Fixes: 7e41df5dbba2 ("bpf: Add a few optnames to bpf_setsockopt")
> Suggested-by: Martin KaFai Lau <martin.lau@linux.dev>
> Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>