netdev - Re: [PATCH bpf-next v2 3/7] bpf: Support pulling non-linear xdp data

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Message-ID: <CAMB2axO8rWWuSAo6pfuU5LRL=v43TJjhs66iCk8K4FALKdwRVQ@mail.gmail.com>
Date: Wed, 10 Sep 2025 15:11:14 -0400
From: Amery Hung <ameryhung@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, alexei.starovoitov@...il.com, 
	andrii@...nel.org, daniel@...earbox.net, stfomichev@...il.com, 
	martin.lau@...nel.org, mohsin.bashr@...il.com, noren@...dia.com, 
	dtatulea@...dia.com, saeedm@...dia.com, tariqt@...dia.com, mbloch@...dia.com, 
	maciej.fijalkowski@...el.com, kernel-team@...a.com
Subject: Re: [PATCH bpf-next v2 3/7] bpf: Support pulling non-linear xdp data

On Wed, Sep 10, 2025 at 2:04 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed, 10 Sep 2025 11:17:52 -0400 Amery Hung wrote:
> > > Larger note: I wonder if we should support "shifting the buffer down"
> > > if there's insufficient tailroom. XDP has rather copious headroom,
> > > but tailroom may be pretty tight, and it may depend on the length of
> > > the headers. So if there's not enough tailroom but there's enough
> > > headroom -- should we try to memmove the existing headers?
> >
> > I think it should. If users want to reserve space for metadata, they
> > can check the headroom before pulling data.
> >
> > If the kfunc does not do memmove(), users are still able to do so in
> > XDP programs through bpf_xdp_adjust_head() and memmove(), but it feels
> > less easy to use IMO.
>
> Actually, I don't think adjust_head() would even work. The program can
> adjust head and memmove() the header, but there's no way to "punch out"
> the end of the head buffer. We can only grow and shrink start of packet
> and end of packet. After adjust_head + memmove in the prog buffer would
> look something like:

Ahh. You are right.

>
>   _ _ _ _ __________ _____ _ _ _ _      ________
>    hroom |  headers | old | troom      |  frag0 |
>   - - - - ---------- ----- - - - -      --------
>
> and the program has no way to "free" the "old" to let pull grab data
> from frag0 in its place...
>
> skb pull helper can allocate a completely fresh buffer, but IDK if
> drivers are ready to have the head buffer swapped under their feet.
> So I think that best we can do is have the pull() helper aromatically
> memmove the headers.

Agree. Will make the kfunc memmove headers if more spaces are needed.