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Subject: Re: [Intel-wired-lan] [PATCH][next] virtchnl: Replace one-element
 array in struct virtchnl_vsi_queue_config_info
To:     "Nguyen, Anthony L" <anthony.l.nguyen@intel.com>,
        "Brandeburg, Jesse" <jesse.brandeburg@intel.com>,
        "gustavoars@kernel.org" <gustavoars@kernel.org>
Cc:     "intel-wired-lan@lists.osuosl.org" <intel-wired-lan@lists.osuosl.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-hardening@vger.kernel.org" <linux-hardening@vger.kernel.org>
References: <20210525231658.GA176466@embeddedor>
 <f3674339c0390ced22b365101f2d3e3a2bf26845.camel@intel.com>
From:   "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Message-ID: <85e62583-5db9-ac5c-fdf9-8e3f6a50f3fa@embeddedor.com>
Date:   Fri, 28 May 2021 18:04:51 -0500
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On 5/28/21 16:56, Nguyen, Anthony L wrote:
> On Tue, 2021-05-25 at 18:16 -0500, Gustavo A. R. Silva wrote:
>> There is a regular need in the kernel to provide a way to declare
>> having a
>> dynamically sized set of trailing elements in a structure. Kernel
>> code
>> should always use “flexible array members”[1] for these cases. The
>> older
>> style of one-element or zero-length arrays should no longer be
>> used[2].
>>
>> Refactor the code according to the use of a flexible-array member in
>> struct
>> virtchnl_vsi_queue_config_info instead of one-element array, and use
>> the
>> flex_array_size() helper.
>>
>> [1] https://en.wikipedia.org/wiki/Flexible_array_member
>> [2] 
>> https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays
>>
>> Link: https://github.com/KSPP/linux/issues/79
>> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>> ---
>>  include/linux/avf/virtchnl.h | 9 ++++-----
>>  1 file changed, 4 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/avf/virtchnl.h
>> b/include/linux/avf/virtchnl.h
>> index b554913804bd..ed9c4998f8ac 100644
>> --- a/include/linux/avf/virtchnl.h
>> +++ b/include/linux/avf/virtchnl.h
>> @@ -338,10 +338,10 @@ struct virtchnl_vsi_queue_config_info {
>>  	u16 vsi_id;
>>  	u16 num_queue_pairs;
>>  	u32 pad;
>> -	struct virtchnl_queue_pair_info qpair[1];
>> +	struct virtchnl_queue_pair_info qpair[];
>>  };
>>  
>> -VIRTCHNL_CHECK_STRUCT_LEN(72, virtchnl_vsi_queue_config_info);
>> +VIRTCHNL_CHECK_STRUCT_LEN(8, virtchnl_vsi_queue_config_info);
>>  
>>  /* VIRTCHNL_OP_REQUEST_QUEUES
>>   * VF sends this message to request the PF to allocate additional
>> queues to
>> @@ -997,9 +997,8 @@ virtchnl_vc_validate_vf_msg(struct
>> virtchnl_version_info *ver, u32 v_opcode,
>>  		if (msglen >= valid_len) {
>>  			struct virtchnl_vsi_queue_config_info *vqc =
>>  			    (struct virtchnl_vsi_queue_config_info
>> *)msg;
>> -			valid_len += (vqc->num_queue_pairs *
>> -				      sizeof(struct
>> -					     virtchnl_queue_pair_info))
>> ;
>> +			valid_len += flex_array_size(vqc, qpair,
>> +						     vqc-
>>> num_queue_pairs);
> 
> The virtchnl file acts as a binary interface between physical and
> virtual functions. There's no guaruntee that the PF and VF will both
> have the newest version. Thus changing this will break compatibility.
> Specifically, the way the size was validated for this op code
> incorrectly expects an extra queue pair structure. Some other
> structures have similar length calculation flaws. We agree that fixing
> this is important, but the fix needs to account that old drivers will
> send an off by 1 size. 
> 
> To properly handle compatibility we need to introduce a feature flag to
> indicate the new behavior. If the feature flag is not set, we acccept
> messages with the old format (with the extra size). If both the PF and
> VF support the feature flag, we'll use the correct size calculations.
> We're looking to add this and would like to do all the virtchnl
> structure fixes in one series.
> 

Oh OK, I see. In this case, I think something like this might work just
fine:

https://git.kernel.org/linus/c0a744dcaa29e9537e8607ae9c965ad936124a4d

What do you think?

Thanks
--
Gustavo