Re: [PATCH v2] iio: buffer: Ensure bounce buffer used for unaligned case is zeroed.

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 08 Jun 2026 21:00:30 +0100
Nuno Sá <noname.nuno@gmail.com> wrote:

> On Fri, 2026-06-05 at 22:18 +0300, Andy Shevchenko wrote:
> > On Fri, Jun 05, 2026 at 02:50:00PM +0100, Jonathan Cameron wrote:  
> > > On Thu, 4 Jun 2026 12:10:42 +0300
> > > Andy Shevchenko <andriy.shevchenko@intel.com> wrote:  
> > > > On Thu, Jun 04, 2026 at 09:43:07AM +0100, Jonathan Cameron wrote:  
> > 
> > ...
> >   
> > > > >  				   indio_dev->scan_bytes, GFP_KERNEL);
> > > > >  		if (!bb)
> > > > >  			return -ENOMEM;
> > > > > +		memset(bb, 0, indio_dev->scan_bytes);    
> > > > 
> > > > May I suggest different approach, id est use __GFP_ZERO instead of hunting
> > > > correct pointers?  
> > > 
> > > That's a weird beast when combined with a krealloc so I was a bit
> > > nervous about readability (and less so whether it was correct).
> > > It should be fine in that we will either get stale data or zeros
> > > because we always use this path to allocate the buffer so if you
> > > think it is obviously fine then I don't mind.
> > > 
> > > The oddities are that if an object grows within a slab but doesn't need
> > > a new one we are relying on those bits happening to be zero based on the
> > > original allocation doing __GFP_ZERO as well (as it's the same call)
> > > 
> > > I'm nervous though as that region off the end is sometimes used for
> > > debug objects and I really don't understand that bit of slab well enough.
> > > If it actually does this, then seems like we'd end up with a lot of
> > > nasty corner cases so I assume it doesn't.  
> > 
> > Such a bug will be a serious issue in mm. I don't think it exists.
> > But, of course, chances are not completely 0.
> > 
> > Current users (not a comprehensive list)
> > 
> > arch/arm64/kvm/nested.c:92
> > drivers/firmware/efi/capsule-loader.c:61
> > drivers/gpio/gpiolib.c:5198
> > drivers/media/platform/amphion/vdec.c:329
> > drivers/net/ethernet/intel/ice/ice_lib.c:3044
> > drivers/net/ethernet/mellanox/mlx5/core/steering/hws/bwc.c:785
> > drivers/net/wireless/ti/wl18xx/main.c:1582
> > drivers/nvme/target/pci-epf.c:708
> > drivers/spi/spi.c:1411
> > drivers/usb/gadget/function/uvc_configfs.c:880
> > fs/ceph/mdsmap.c:322
> > fs/smb/server/ksmbd_work.c:123
> > lib/tests/slub_kunit.c:273
> > sound/soc/meson/meson-card-utils.c:49
> > 
> > Also note the kernel-doc for krealloc_array(). It has some note WRT __GFP_ZERO
> > and it means the flag must be supported. Also there is a note in mm/slub.c.
> >   
> > > However, I couldn't convince myself enough not to just force a memset of the
> > > whole thing.  
> > 
> > I personally would go with __GFP_ZERO.  
> 
> If I'm not missing anything, this should also be proof that the flag should be fine:
> 
> https://elixir.bootlin.com/linux/v7.1-rc7/source/lib/tests/slub_kunit.c#L275
> 
Perfect!  Thanks. I'll hopefully spin an updated version in next few days

Jonathan

> - Nuno Sá