netdev - Re: [PATCH net-next v2 1/1] ppp: Fix KMSAN warning by initializing 2-byte header

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Message-ID: <20250224142644.7c084e9e@kernel.org>
Date: Mon, 24 Feb 2025 14:26:44 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, andrew+netdev@...n.ch,
 davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
 horms@...nel.org, ricardo@...liere.net, viro@...iv.linux.org.uk,
 dmantipov@...dex.ru, aleksander.lobakin@...el.com,
 linux-ppp@...r.kernel.org, linux-kernel@...r.kernel.org, mrpre@....com,
 syzbot+853242d9c9917165d791@...kaller.appspotmail.com
Subject: Re: [PATCH net-next v2 1/1] ppp: Fix KMSAN warning by initializing
 2-byte header

On Fri, 21 Feb 2025 14:12:19 +0800 Jiayuan Chen wrote:
> +		/* Check if we should pass this packet.
> +		 * BPF filter instructions assume each PPP packet has a 4-byte
> +		 * header (e.g., those generated by libpcap), and then default
> +		 * to skipping the first 2 bytes at the beginning of the
> +		 * instruction. However, we still need to initialize these
> +		 * 2-byte new headers to prevent crafted BPF programs from
> +		 * reading them which would cause reading of uninitialized
> +		 * data. Here, we set the headers according to the RFC 1662.
> +		 */
> +		*(u16 *)skb_push(skb, 2) = htons(0xff03);

The constant from the RFC deserves a #define or enum.
Looks like we may already need it in one other place:

drivers/net/wan/fsl_ucc_hdlc.h:#define DEFAULT_PPP_HEAD    0xff03
-- 
pw-bot: cr