Re: [PATCH v3] tcp: add support for SO_PEEK_OFF

On Tue, 2024-02-13 at 16:49 +0100, Eric Dumazet wrote:

On Tue, Feb 13, 2024 at 4:28 PM Paolo Abeni wrote:

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On Tue, 2024-02-13 at 14:34 +0100, Eric Dumazet wrote:

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This sk_peek_offset protocol, needing sk_peek_offset_bwd() in the non MSG_PEEK case is very strange IMO.

Ideally, we should read/write over sk_peek_offset only when MSG_PEEK is used by the caller.

That would only touch non fast paths.

Since the API is mono-threaded anyway, the caller should not rely on the fact that normal recvmsg() call would 'consume' sk_peek_offset.

Storing in sk_peek_seq the tcp next sequence number to be peeked should avoid changes in the non MSG_PEEK cases.

AFAICS that would need a new get_peek_off() sock_op and a bit somewhere (in sk_flags?) to discriminate when sk_peek_seq is actually set. Would that be acceptable?

We could have a parallel SO_PEEK_OFFSET option, reusing the same socket field.

The new semantic would be : Supported by TCP (so far), and tcp recvmsg() only reads/writes this field when MSG_PEEK is used. Applications would have to clear the values themselves.

I feel like there is some misunderstanding, or at least I can't follow. Let me be more verbose, to try to clarify my reasoning. Two consecutive recvmsg(MSG_PEEK) calls for TCP after SO_PEEK_OFF will return adjacent data. AFAICS this is the same semantic currently implemented by UDP and unix sockets. Currently 'sk_peek_off' maintains the next offset to be peeked into the current receive queue. To implement the above behaviour, tcp_recvmsg() has to update 'sk_peek_off' after MSG_PEEK, to move the offset to the next data, and after a plain read, to account for the data removed from the receive queue. I proposed to let introduce a tcp-specific set_peek_off doing something alike: WRTIE_ONCE(sk->sk_peek_off, tcp_sk(sk)->copied_seq + val); so that the recvmsg will need to update sk_peek_off only for MSG_PEEK, while retaining the semantic described above. To keep the userspace interface unchanged that will need a paired tcp_get_peek_off(), so that getsockopt(SO_PEEK_OFF) could return to the user a plain offset. An additional bit flag will be needed to store the information "the user-space enabled peek with offset". I don't understand how a setsockopt(PEEK_OFFSET) variant would help avoiding touching sk->sk_peek_offset? Thanks! Paolo