netdev - Re: [bpf, xdp] headroom - was: Re: Question about to KMSAN: uninit-value in can

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Message-ID: <993342e7-edd3-4c65-aaf9-673255ea11da@iogearbox.net>
Date: Mon, 5 Jan 2026 23:11:28 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Jakub Kicinski <kuba@...nel.org>, Prithvi <activprithvi@...il.com>
Cc: andrii@...nel.org, socketcan@...tkopp.net, mkl@...gutronix.de,
 linux-can@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzkaller-bugs@...glegroups.com, netdev@...r.kernel.org
Subject: Re: [bpf, xdp] headroom - was: Re: Question about to KMSAN:
 uninit-value in can_receive

On 1/2/26 9:04 PM, Jakub Kicinski wrote:
[...]
> 
> Without looking too deeply - XDP has historically left the new space
> uninitialized after push, expecting programs to immediately write the
> headers in that space.
Correct, and the same also holds true for tc BPF programs. This is an
optimization given zero'ing and then again immediately write of the
headers from the BPF program cannot be optimized by the compiler into
just the latter write. Additionally, load/attachments of these programs
require root capabilities (e.g. cap_bpf, cap_perfmon, cap_net_admin).