netdev - Re: [PATCH net-next v5 03/15] bpf: introduce timestamp_used to allow UDP socket fetched in bpf prog

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Message-ID: <ba353503-bfd3-4de0-bb99-9c7e865e8a73@linux.dev>
Date: Wed, 15 Jan 2025 16:51:31 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, dsahern@...nel.org, willemdebruijn.kernel@...il.com,
 willemb@...gle.com, ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev,
 john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, horms@...nel.org, bpf@...r.kernel.org,
 netdev@...r.kernel.org
Subject: Re: [PATCH net-next v5 03/15] bpf: introduce timestamp_used to allow
 UDP socket fetched in bpf prog

On 1/14/25 6:54 PM, Jason Xing wrote:
> I construct my thoughts here according to our previous discussion:
> 1. not limiting the use of is_fullsock, so in patch 2, I will use the
> follow codes:
> +void bpf_skops_tx_timestamping(struct sock *sk, struct sk_buff *skb, int op)
> +{
> +       struct bpf_sock_ops_kern sock_ops;
> +
> +       memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
> +       sock_ops.op = op;
> +       sock_ops.is_fullsock = 1;
> +       sock_ops.sk = sk;

lgtm.

> +       BPF_CGROUP_RUN_PROG_SOCK_OPS(sk, &sock_ops, CGROUP_SOCK_OPS);

After looking through the set and looking again at how sk is used in 
__skb_tstamp_tx(), I think the sk must be fullsock here, so using 
__cgroup_bpf_run_filter_sock_ops() as in patch 2 is good. It will be useful to 
have a comment here to explain it must be a fullsock.

> +}
> 
> 2. introduce the allow_direct_access flag which is used to test if the
> socket is allowed to access tcp socket or not.

yeah, right now is only tcp_sock, but future will have UDP TS support.

May be the "allow_direct_access" naming is not obvious to mean the existing 
tcp_sock support. May be "allow_tcp_access"?

I was thinking to set the allow_direct_access for the "existing" sockops 
callback which must be tcp_sock and must have the sk locked.

> On the basis of the above bpf_skops_tx_timestamping() function, I
> would add one check there:
> + if (sk_is_tcp(sk))
> +       sock_ops. allow_direct_access = 1;

so don't set this in the new TS callback from bpf_skops_tx_timestamping 
regardless it is tcp or not.

> 
> Also, I need to set allow_direct_access to one as long as there is
> "sock_ops.is_fullsock = 1;" in the existing callbacks.

Only set allow_direct_access when the sk is fullsock in the "existing" sockops 
callback.

After thinking a bit more today, I think this should work. Please give it a try 
and check if some cases may be missed in sock_ops_convert_ctx_access().

> 
> 3. I will replace is_fullsock with allow_direct_access in
> SOCK_OPS_GET/SET_FIELD() instead of SOCK_OPS_GET_SK().

Yep.

> 
> Then the udp socket can freely access the socket with the helper
> SOCK_OPS_GET_SK() because it is a fullsock. And udp socket cannot
> access struct tcp_sock because in the timestamping callback, there is
> no place where setting allow_direct_access for udp use.

__sk_buff->sk? yes.