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

On 2024-02-13 14:31, Eric Dumazet wrote:

On Tue, Feb 13, 2024 at 7:39 PM Paolo Abeni wrote:

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

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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?

I was trying to avoid using an extra storage, I was not trying to implement the alternative myself :0)

If the recvmsg( MSG_PEEK) is supposed to auto-advance the peek_offset, we probably need more than a mere 32bit field.

I wonder if the following could be acceptable: if (flags & MSG_PEEK)        sk_peek_offset_fwd(sk, used); else if (peek_offset > 0)       sk_peek_offset_bwd(sk, used); peek_offset is already present in the data cache, and if it has the value zero it means either that that sk->sk_peek_off is unused (-1) or actually is zero. Either way, no rewind is needed in that case. ///jon

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Thanks!

Paolo