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Date: Thu, 23 Oct 2025 17:51:19 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Jakub Sitnicki <jakub@cloudflare.com>
Cc: bpf@vger.kernel.org, "David S. Miller" <davem@davemloft.net>, Eric
 Dumazet <edumazet@google.com>, Paolo Abeni <pabeni@redhat.com>, Simon
 Horman <horms@kernel.org>, Martin KaFai Lau <martin.lau@linux.dev>, Daniel
 Borkmann <daniel@iogearbox.net>, John Fastabend <john.fastabend@gmail.com>,
 Stanislav Fomichev <sdf@fomichev.me>, Alexei Starovoitov <ast@kernel.org>,
 Andrii Nakryiko <andrii@kernel.org>, Eduard Zingerman <eddyz87@gmail.com>,
 Song Liu <song@kernel.org>, Yonghong Song <yonghong.song@linux.dev>, KP
 Singh <kpsingh@kernel.org>, Hao Luo <haoluo@google.com>, Jiri Olsa
 <jolsa@kernel.org>, Arthur Fabre <arthur@arthurfabre.com>,
 netdev@vger.kernel.org, kernel-team@cloudflare.com
Subject: Re: [PATCH bpf-next v2 01/15] net: Preserve metadata on
 pskb_expand_head
Message-ID: <20251023175119.62785270@kernel.org>
In-Reply-To: <20251019-skb-meta-rx-path-v2-1-f9a58f3eb6d6@cloudflare.com>
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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?