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

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Message-ID: <1f1c04902562c58736862ce24316f5bc85757bcb.camel@redhat.com>
Date: Tue, 05 Dec 2023 09:52:34 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Vladimir Oltean <olteanv@...il.com>, Jakub Kicinski <kuba@...nel.org>
Cc: "Austin, Alex (DCCG)" <alexaust@....com>, 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)

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