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

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Message-ID: <20251023175119.62785270@kernel.org>
Date: Thu, 23 Oct 2025 17:51:19 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Jakub Sitnicki <jakub@...udflare.com>
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 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?