netdev - Re: [PATCH v3] tcp: add support for SO_PEEK

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Message-ID: <CANn89iKXOZdT7_ww_Jytm4wMoXAe0=pqX+M_iVpNGaHqe_9o4Q@mail.gmail.com>
Date: Thu, 15 Feb 2024 18:46:18 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Jon Maloy <jmaloy@...hat.com>, kuba@...nel.org, passt-dev@...st.top, 
	sbrivio@...hat.com, lvivier@...hat.com, dgibson@...hat.com, 
	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH v3] tcp: add support for SO_PEEK_OFF

On Thu, Feb 15, 2024 at 6:41 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> Note: please send text-only email to netdev.
>
> On Thu, 2024-02-15 at 10:11 -0500, Jon Maloy wrote:
> > 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.
>
> I agree the above should avoid touching cold cachelines in the
> fastpath, and looks functionally correct to me.
>
> The last word is up to Eric :)
>

An actual patch seems needed.

In the current form, local variable peek_offset is 0 when !MSG_PEEK.

So the "else if (peek_offset > 0)" would always be false.