Date:   Sun, 22 Mar 2020 17:40:30 +0000
From:   "Ardelean, Alexandru" <alexandru.Ardelean@...log.com>
To:     "keescook@...omium.org" <keescook@...omium.org>,
        "jic23@...nel.org" <jic23@...nel.org>
CC:     "Costina, Adrian" <Adrian.Costina@...log.com>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "Grozav, Andrei" <Andrei.Grozav@...log.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "lars@...afoo.de" <lars@...afoo.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Hennerich, Michael" <Michael.Hennerich@...log.com>,
        "Nagy, Laszlo" <Laszlo.Nagy@...log.com>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "Bogdan, Dragos" <Dragos.Bogdan@...log.com>,
        "Csomortani, Istvan" <Istvan.Csomortani@...log.com>
Subject: Re: [PATCH v11 5/8] iio: adc: adi-axi-adc: add support for AXI ADC IP
 core

On Sun, 2020-03-22 at 16:53 +0000, Jonathan Cameron wrote:
> On Sun, 22 Mar 2020 09:16:36 -0700
> Kees Cook <keescook@...omium.org> wrote:
> 
> > On Sun, Mar 22, 2020 at 12:45:39PM +0200, Andy Shevchenko wrote:
> > > +Cc Kees (see below about allocation size checks)
> > > 
> > > On Sun, Mar 22, 2020 at 11:36 AM Ardelean, Alexandru
> > > <alexandru.Ardelean@...log.com> wrote:  
> > > > On Sat, 2020-03-21 at 23:38 +0200, Andy Shevchenko wrote:  
> > > > > On Sat, Mar 21, 2020 at 10:55 AM Alexandru Ardelean
> > > > > <alexandru.ardelean@...log.com> wrote:  
> > > 
> > > ...
> > >   
> > > > > > +static struct adi_axi_adc_conv *adi_axi_adc_conv_register(struct
> > > > > > device
> > > > > > *dev,
> > > > > > +                                                         int
> > > > > > sizeof_priv)
> > > > > > +{
> > > > > > +       struct adi_axi_adc_client *cl;
> > > > > > +       size_t alloc_size;
> > > > > > +
> > > > > > +       alloc_size = sizeof(struct adi_axi_adc_client);
> > > > > > +       if (sizeof_priv) {
> > > > > > +               alloc_size = ALIGN(alloc_size, IIO_ALIGN);
> > > > > > +               alloc_size += sizeof_priv;
> > > > > > +       }
> > > > > > +       alloc_size += IIO_ALIGN - 1;  
> > > > > 
> > > > > Have you looked at linux/overflow.h?  
> > > > 
> > > > i did now;
> > > > any hints where i should look closer?  
> > > 
> > > It seems it lacks of this kind of allocation size checks... Perhaps add
> > > one?
> > > Kees, what do you think?
> > >   
> > > > > > +       cl = kzalloc(alloc_size, GFP_KERNEL);
> > > > > > +       if (!cl)
> > > > > > +               return ERR_PTR(-ENOMEM);  
> > 
> > My head hurts trying to read this! ;) Okay, so the base size is
> > sizeof(struct adi_axi_adc_client). But if sizeof_priv is non-zero
> > (this arg should be size_t not int), then we need to make the struct
> > size ALIGNed? And then what is the "+= IIO_ALIGN - 1" for?
> 
> I'm a bit embarrassed.  I can't remember what the += IIO_ALIGN - 1
> was for in the first place and I can't work it out now.
> 
> The purpose of the fun here was to end up with a structure that
> was either
> a) sizeof(struct iio_dev) long,
> b) sizeof(struct iio_dev) + padding + sizeof_priv 
> where the padding ensured that any __cacheline_aligned elements
> in the private structure were cacheline aligned within resulting
> allocation.
> 
> So why the extra IIO_ALIGN - 1....
> 
> The original patch doesn't help much either given it's got a question
> in there for why this bit is needed.
> 
> https://lore.kernel.org/linux-iio/1302890160-8823-5-git-send-email-jic23@cam.ac.uk/
> 
> However, it rang a slight bell.  Seems I lifted the code from netdev.
> https://elixir.bootlin.com/linux/latest/source/net/core/dev.c#L9718
> 
> I'm fairly sure we don't need that padding here..  What can I say,
> I was young and stupid :)
> 
> I did add a question mark so clearly meant to come back and
> take another look ;)
> 
> One vague thought is that it's about ensuring we are big enough to
> ensure we are cacheline aligned.  That's obviously not a problem with
> current struct iio_dev which is far from small,
> but in theory it could have been.  Also, thinking about it we only
> need the struct iio_dev to be cacheline aligned if we have
> an iio_priv structure.  If we have one of those it will definitely
> be big enough anyway.
> 
> At somepoint I'd like to look at cleaning it up for iio_device_alloc
> but with a lot of testing as who knows what is relying on this behaviour
> or if I've missed something.  Crashes around this alignment are
> infrequent and nasty to trace at the best of times.

In the meantime, are there any objections if I leave the allocation as-is for
this driver as well?
I've tested the driver a bit more with this form.

> 
> Jonathan
> 
> > It's not clear to me what the expect alignment/padding is here.
> > 
> > I would probably construct this as:
> > 
> > 	sizeof_self = sizeof(struct adi_axi_adc_client);
> > 	if (sizeof_priv)
> > 		sizeof_self = ALIGN(sizeof_self, IIO_ALIGN);
> > 	if (check_add_overflow(sizeof_self, sizeof_priv, &sizeof_alloc))
> > 		return ERR_PTR(-ENOMEM);
> > 	if (check_add_overflow(sizeof_alloc, IIO_ALIGN - 1, &sizeof_alloc))
> > 		return ERR_PTR(-ENOMEM);
> > 
> > But I don't understand the "IIO_ALIGN - 1" part, so I assume this could
> > be shortened with better use of ALIGN()?
> > 
> > Also, this feels like a weird driver allocation overall:
> > 
> > +	struct adi_axi_adc_conv **ptr, *conv;
> > +
> > +	ptr = devres_alloc(devm_adi_axi_adc_conv_release, sizeof(*ptr),
> > +			   GFP_KERNEL);
> > +	if (!ptr)
> > +		return ERR_PTR(-ENOMEM);
> > +
> > +	conv = adi_axi_adc_conv_register(dev, sizeof_priv);
> > 
> > devres_alloc() allocates storage for a _single pointer_. :P That's not
> > useful for resource tracking. Why is devres_alloc() being called here
> > and not down in adi_axi_adc_conv_register() and just passing the pointer
> > back up?
> > 

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