netdev - Re: [PATCH net-next 2/2] net: ethtool: add a NO_CHANGE uAPI for new RXFH's input

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Date: Wed, 3 Jan 2024 14:16:54 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Ahmed Zaki <ahmed.zaki@...el.com>
Cc: <netdev@...r.kernel.org>, <corbet@....net>,
 <jesse.brandeburg@...el.com>, <anthony.l.nguyen@...el.com>,
 <davem@...emloft.net>, <edumazet@...gle.com>, <pabeni@...hat.com>,
 <vladimir.oltean@....com>, <andrew@...n.ch>, <horms@...nel.org>,
 <mkubecek@...e.cz>, <willemdebruijn.kernel@...il.com>, <gal@...dia.com>,
 <alexander.duyck@...il.com>, <ecree.xilinx@...il.com>, Jacob Keller
 <jacob.e.keller@...el.com>
Subject: Re: [PATCH net-next 2/2] net: ethtool: add a NO_CHANGE uAPI for new
 RXFH's input_xfrm

On Wed, 3 Jan 2024 08:40:47 -0700 Ahmed Zaki wrote:
> On 2024-01-02 17:05, Jakub Kicinski wrote:
> > On Thu, 21 Dec 2023 11:42:35 -0700 Ahmed Zaki wrote:  
> >> +	     rxfh.key_size == 0 && rxfh.hfunc == ETH_RSS_HASH_NO_CHANGE &&
> >> +	     rxfh.input_xfrm == RXH_XFRM_NO_CHANGE))  
> > 
> > This looks fine, but we also need a check to make sure input_xfrm
> > doesn't have bits other than RXH_XFRM_SYM_XOR set, right?  
> 
> I wrote the xfrm as a  bitmap/flags assuming we can have multiple 
> transformations set. Not sure what future transformations will look like 
> (other than RSS-sort,... and discussed before).

Ack, but right now the only valid inputs for the kernel are
0 (disable all), RXH_XFRM_NO_CHANGE and RXH_XFRM_SYM_XOR.
We need to reject all other inputs. If we accept random
inputs without validation we won't be able to use them later.
Refer to many LWN articles about how some syscall didn't check
that unused flags are 0 and now they can't allocate bits.
Because some user space was passing in garbage.

> Else, we can do the check you mentioned or may be better to have it as 
> enum (which would give us 256, not 8, allowable transformations).

Given that RXH_XFRM_SYM_XOR is 1 we can change the exact semantics
later. All that matters is that we reject unsupported for now.