Re: [PATCH net-next v4 3/5] net: hdlc_fr: Improve the initial checks when we receive an skb
From: Xie He <hidden>
Date: 2020-10-30 19:22:33
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On Fri, Oct 30, 2020 at 9:31 AM Willem de Bruijn [off-list ref] wrote:
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Add an fh->ea2 check to the initial checks in fr_rx. fh->ea2 == 1 means the second address byte is the final address byte. We only support the case where the address length is 2 bytes.Can you elaborate a bit for readers not intimately familiar with the codebase? Is there something in the following code that has this implicit assumption on 2-byte address lengths?
Yes, the address length must be 2 bytes, otherwise the 3rd and 4th bytes would not be the control and protocol fields as we assumed in the code. The frame format is specified in RFC 2427 (https://tools.ietf.org/html/rfc2427). We can see the overall frame format on Page 3. If the address length is longer than 2 bytes, all the following fields will be shifted behind.