On Sun, 2020-03-22 at 16:53 +0000, Jonathan Cameron wrote:
> On Sun, 22 Mar 2020 09:16:36 -0700
> Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxx/
>
> 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?
> >
