Re: KMSAN: uninit-value in alauda_check_media

[Andrey Konovalov <andreyknvl@google.com>]

On Fri, Oct 11, 2019 at 5:06 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Fri, Oct 11, 2019 at 10:53:47AM -0400, Alan Stern wrote:
> > On Fri, 11 Oct 2019, Andrey Konovalov wrote:
> >
> > > On Fri, Oct 11, 2019 at 4:08 PM Alan Stern <stern@rowland.harvard.edu> wrote:
> >
> > > > Now yes, it's true that defining status as an array on the stack is
> > > > also a bug, since USB transfer buffers are not allowed to be stack
> > > > variables.
> > >
> > > Hi Alan,
> > >
> > > I'm curious, what is the reason for disallowing that? Should we try to
> > > somehow detect such cases automatically?
> >
> > Transfer buffers are read and written by DMA.  On systems that don't
> > have cache-coherent DMA controllers, it is essential that the CPU does
> > not access any cache line involved in a DMA transfer while the transfer
> > is in progress.  Otherwise the data in the cache would be different
> > from the data in the buffer, leading to corruption.
> >
> > (In theory it would be okay for the CPU to read (not write!) a cache
> > line assigned to a buffer for a DMA write (not read!) transfer.  But
> > even doing that isn't really a good idea.)
> >
> > (Also, this isn't an issue for x86 architectures, because x86 has
> > cache-coherent DMA.  But it is an issue on other architectures.)
> >
> > In practice, this means transfer buffers have to be allocated by
> > something like kmalloc, so that they occupies their own separate set of
> > cache lines.  Buffers on the stack obviously don't satisfy this
> > requirement.
> >
> > At some point there was a discussion about automatically detecting when
> > on-stack (or otherwise invalid) buffers are used for DMA transfers.  I
> > don't recall what the outcome was.
>
> A patchset from Kees was sent, but it needs a bit more work...

Hi Greg,

Could you send a link to the patchset?

Thanks!