netdev - Re: [PATCH bpf-next v2 01/15] net: Preserve metadata on pskb_expand

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Message-ID: <87ikg4v6h7.fsf@cloudflare.com>
Date: Fri, 24 Oct 2025 14:17:56 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: bpf@...r.kernel.org,  "David S. Miller" <davem@...emloft.net>,  Eric
 Dumazet <edumazet@...gle.com>,  Paolo Abeni <pabeni@...hat.com>,  Simon
 Horman <horms@...nel.org>,  Martin KaFai Lau <martin.lau@...ux.dev>,
  Daniel Borkmann <daniel@...earbox.net>,  John Fastabend
 <john.fastabend@...il.com>,  Stanislav Fomichev <sdf@...ichev.me>,  Alexei
 Starovoitov <ast@...nel.org>,  Andrii Nakryiko <andrii@...nel.org>,
  Eduard Zingerman <eddyz87@...il.com>,  Song Liu <song@...nel.org>,
  Yonghong Song <yonghong.song@...ux.dev>,  KP Singh <kpsingh@...nel.org>,
  Hao Luo <haoluo@...gle.com>,  Jiri Olsa <jolsa@...nel.org>,  Arthur Fabre
 <arthur@...hurfabre.com>,  netdev@...r.kernel.org,
  kernel-team@...udflare.com
Subject: Re: [PATCH bpf-next v2 01/15] net: Preserve metadata on
 pskb_expand_head

On Thu, Oct 23, 2025 at 05:51 PM -07, Jakub Kicinski wrote:
> On Sun, 19 Oct 2025 14:45:25 +0200 Jakub Sitnicki wrote:
>> pskb_expand_head() copies headroom, including skb metadata, into the newly
>> allocated head, but then clears the metadata. As a result, metadata is lost
>> when BPF helpers trigger an skb head reallocation.
>
> True, then again if someone is reallocating headroom they may very well
> push a header after, shifting metadata into an uninitialized part of
> the headroom. Not sure we can do much about that, but perhaps worth
> being more explicit in the commit msg?

This is where the skb_data_move() helper, proposed by the next patch,
comes in. We will try to move the metadata out of the way if possible,
and clear it if don't have enough headroom left. That approach relies on
all pskb_expand_head users adopting the helper, which is no simple task.

I can add guidance for pskb_expand_head users to the commit description,
or maybe better yet, add a note in pskb_expand_head docs.