netdev - Re: [RFC bpf-next v1 3/7] bpf: Support pulling non-linear xdp data

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Message-ID: <aKz73WdkzhOmLhjJ@mini-arch>
Date: Mon, 25 Aug 2025 17:12:13 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Amery Hung <ameryhung@...il.com>, bpf@...r.kernel.org,
	netdev@...r.kernel.org, alexei.starovoitov@...il.com,
	andrii@...nel.org, daniel@...earbox.net, martin.lau@...nel.org,
	mohsin.bashr@...il.com, saeedm@...dia.com, tariqt@...dia.com,
	mbloch@...dia.com, maciej.fijalkowski@...el.com,
	kernel-team@...a.com
Subject: Re: [RFC bpf-next v1 3/7] bpf: Support pulling non-linear xdp data

On 08/25, Jakub Kicinski wrote:
> On Mon, 25 Aug 2025 15:46:02 -0700 Stanislav Fomichev wrote:
> > > > skb_frag_address can return NULL for unreadable frags.  
> > > 
> > > Is it safe to assume that drivers will ensure frags to be readable? It
> > > seems at least mlx5 does.
> > > 
> > > I did a quick check and found other xdp kfuncs using
> > > skb_frag_address() without checking the return.  
> > 
> > The unreadable frags will always be unredabale to the host. This is TCP
> > device memory, the memory on the accelerators that is not mapped onto
> > the CPU. Any attempts to read that memory should gracefully error out.
> > 
> > Can you also pls fix that other one? (not as part of the series should
> > be ok)
> 
> But we don't support mixing XDP with unreadable mem today.
> Is the concern just for future proofing?

Good point. I though we did add that proofing during the initial rx
patch series, but I don't see any. Ignore me!