linux-kernel - Re: [PATCH v8 1/3] devres: provide devm

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Message-ID: <20200821110403.GP1891694@smile.fi.intel.com>
Date:   Fri, 21 Aug 2020 14:04:03 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Michal Simek <michal.simek@...inx.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-iio <linux-iio@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH v8 1/3] devres: provide devm_krealloc()

On Fri, Aug 21, 2020 at 01:51:19PM +0300, Andy Shevchenko wrote:
> On Fri, Aug 21, 2020 at 10:59:19AM +0200, Bartosz Golaszewski wrote:
> > On Fri, Aug 21, 2020 at 10:35 AM Andy Shevchenko
> > <andriy.shevchenko@...ux.intel.com> wrote:
> > > On Thu, Aug 20, 2020 at 08:51:08PM +0200, Bartosz Golaszewski wrote:

...

> > > > +static struct devres *to_devres(void *data)
> > > > +{
> > > > +     return data - ALIGN(sizeof(struct devres), ARCH_KMALLOC_MINALIGN);
> > > > +}
> > > > +
> > > > +static size_t devres_data_size(size_t total_size)
> > > > +{
> > > > +     return total_size - ALIGN(sizeof(struct devres), ARCH_KMALLOC_MINALIGN);
> > > > +}

> > The data pointer in struct devres is defined as:
> > 
> >     u8 __aligned(ARCH_KMALLOC_MINALIGN) data[];
> > 
> > And this value (assigned the value of ARCH_DMA_MINALIGN) varies from
> > one arch to another. I wasn't really sure if offsetof() would work for
> > every case so I went with something very explicit.
> 
> I have checked with a small program simulating to_devres() with your variant,
> offsetof() and container_of().
> 
> The result is this: if MINALIGN < sizeof(long) and since struct is unpacked the
> offsetof(), and thus container_of(), gives correct result, while ALIGN()
> approach mistakenly moves pointer too back.

...

> I think you need to change this to use container_of() and offsetof().

To be clear, there is probably no real problem, except unlikely possible
MINALIGN=4 on 64-bit arch, but for sake of the correctness.


-- 
With Best Regards,
Andy Shevchenko