Re: [PATCH bpf-next 1/2] bpf: tcp: Reject TCP_NODELAY from BPF hdr opt callbacks

[Martin KaFai Lau <martin.lau@linux.dev>]

On Tue, Apr 14, 2026 at 07:23:09PM +0800, KaFai Wan wrote:
> A BPF_SOCK_OPS program can enable
> BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG and then call
> bpf_setsockopt(TCP_NODELAY) from BPF_SOCK_OPS_HDR_OPT_LEN_CB.
> 
> That reaches __tcp_sock_set_nodelay(), which may call
> tcp_push_pending_frames(). The transmit path then computes TCP
> options again, re-enters bpf_skops_hdr_opt_len(), and invokes the
> same BPF callback recursively. This can loop until the kernel
> stack overflows.
> 
> TCP_NODELAY is not safe from the header option callback context.
> Reject it with -EOPNOTSUPP when TCP header option callbacks are
> enabled on the socket, so the callback cannot recurse back into
> tcp_push_pending_frames() through do_tcp_setsockopt().
> 
> Reported-by: Quan Sun <2022090917019@std.uestc.edu.cn>
> Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
> Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
> Closes: https://lore.kernel.org/bpf/d1d523c9-6901-4454-a183-94462b8f3e4e@std.uestc.edu.cn/
> Fixes: 7e41df5dbba2 ("bpf: Add a few optnames to bpf_setsockopt")
> Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
> ---
>  net/ipv4/tcp.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 202a4e57a218..7ac4c98be19d 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -4004,7 +4004,10 @@ int do_tcp_setsockopt(struct sock *sk, int level, int optname,
>  
>  	switch (optname) {
>  	case TCP_NODELAY:
> -		__tcp_sock_set_nodelay(sk, val);
> +		if (val && BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG))

It will break the syscall setsockopt and also break the existing bpf prog
that calls bpf_setsockopt(TCP_NODELAY) in CB other than the
BPF_SOCK_OPS_HDR_OPT_LEN_CB/BPF_SOCK_OPS_WRITE_HDR_OPT_CB.

Lets brainstorm other options suggested on the list that have smaller
blast radius.

pw-bot: cr

> +			err = -EOPNOTSUPP;
> +		else
> +			__tcp_sock_set_nodelay(sk, val);
>  		break;
>  
>  	case TCP_THIN_LINEAR_TIMEOUTS:
> -- 
> 2.43.0
>