netdev - Re: [PATCH net-next 1/2] sfc: Implement ndo_hwtstamp

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Date: Tue, 5 Dec 2023 13:45:32 +0000
From: "Austin, Alex (DCCG)" <alexaust@....com>
To: Paolo Abeni <pabeni@...hat.com>, Vladimir Oltean <olteanv@...il.com>,
 Jakub Kicinski <kuba@...nel.org>
Cc: Alex Austin <alex.austin@....com>, netdev@...r.kernel.org,
 linux-net-drivers@....com, ecree.xilinx@...il.com, habetsm.xilinx@...il.com,
 davem@...emloft.net, edumazet@...gle.com, richardcochran@...il.com,
 lorenzo@...nel.org, memxor@...il.com, alardam@...il.com, bhelgaas@...gle.com
Subject: Re: [PATCH net-next 1/2] sfc: Implement ndo_hwtstamp_(get|set)

Based on comments above, my preference is to keep these patches as they are.

Thanks,

Alex

On 05/12/2023 08:52, Paolo Abeni wrote:
> Caution: This message originated from an External Source. Use proper caution when opening attachments, clicking links, or responding.
>
>
> On Mon, 2023-12-04 at 20:45 +0200, Vladimir Oltean wrote:
>> On Mon, Dec 04, 2023 at 10:17:05AM -0800, Jakub Kicinski wrote:
>>> On Mon, 4 Dec 2023 13:00:35 +0200 Vladimir Oltean wrote:
>>>> If I may intervene. The "request state" will ultimately go away once all
>>>> drivers are converted. I know it's more fragile and not all fields are
>>>> valid, but I think I would like drivers to store the kernel_ variant of
>>>> the structure, because more stuff will be added to the kernel_ variant
>>>> in the future (the hwtstamp provider + qualifier), and doing this from
>>>> the beginning will avoid reworking them again.
>>> Okay, you know the direction of this work better, so:
>>>
>>> pw-bot: under-review
>> I mean your observation is in principle fair. If drivers save the struct
>> kernel_hwtstamp_config in the set() method and give it back in the get()
>> method (this is very widespread BTW), it's reasonable to question what
>> happens with the temporary fields, ifr and copied_to_user. Won't we
>> corrupt the teporary fields of the kernel_hwtstamp_config structure from
>> the set() with the previous ones from the get()?
>>
>> The answer, I think, is that we do, but in a safe way. Because we implement
>> ndo_hwtstamp_set(), the copied_to_user that we save is false (aka "the
>> driver implementation didn't call copy_to_user()"). And when we give
>> this structure back in ndo_hwtstamp_get(), we overwrite false with false,
>> and a good ifr pointer with a bad one.
>>
>> But the only reason we transport the ifr along with the
>> kernel_hwtstamp_config is for generic_hwtstamp_ioctl_lower() to work,
>> aka a new API upper driver on top of an old API real driver. Which is
>> not the case here, and no one looks at the stale ifr pointer.
>>
>> It's a lot to think about to make sure that something bad won't happen,
>> I agree. I still don't believe it will break in subtle ways, but nonetheless
>> I do recognize the tradeoff. One approach is more straightforward
>> code-wise but more subtle behavior-wise, and the other is the opposite.
> I tried to dig into the relevant code as far as I can, and I tend to
> agree with Vladimir: the current approach looks reasonably safe, and
> forward looking.
>
> I think any eventual bugs (I could not find any) would be pre-existent
> to this patch, rooted in dev_ioctl.c and to be addressed there.
>
> I think this patches should go in the current form.
>
> Cheers,
>
> Paolo
>