Re: [RFC PATCH] cw1200: use kmalloc() allocation instead of stack

[Ulf Hansson]

On Tue, 22 Jun 2021 at 22:33, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Tue, Jun 22, 2021 at 10:24 PM Jernej Skrabec
> <jernej.skrabec@xxxxxxxxx> wrote:
> >
> > It turns out that if CONFIG_VMAP_STACK is enabled and src or dst is
> > memory allocated on stack, SDIO operations fail due to invalid memory
> > address conversion:
>
> Thank you for sending this!
>
> It's worth pointing out that even without CONFIG_VMAP_STACK, using
> dma_map_sg() on a stack variable is broken, though it will appear to
> work most of the time but rarely cause a stack data corruption when
> the cache management goes wrong.
>
> This clearly needs to be fixed somewhere, if not with your patch, then
> a similar one.
>
> > diff --git a/drivers/net/wireless/st/cw1200/hwio.c b/drivers/net/wireless/st/cw1200/hwio.c
> > index 3ba462de8e91..5521cb7f2233 100644
> > --- a/drivers/net/wireless/st/cw1200/hwio.c
> > +++ b/drivers/net/wireless/st/cw1200/hwio.c
> > @@ -66,33 +66,65 @@ static int __cw1200_reg_write(struct cw1200_common *priv, u16 addr,
> >  static inline int __cw1200_reg_read_32(struct cw1200_common *priv,
> >                                         u16 addr, u32 *val)
> >  {
> > -       __le32 tmp;
> > -       int i = __cw1200_reg_read(priv, addr, &tmp, sizeof(tmp), 0);
> > -       *val = le32_to_cpu(tmp);
> > +       __le32 *tmp;
> > +       int i;
> > +
> > +       tmp = kmalloc(sizeof(*tmp), GFP_KERNEL);
> > +       if (!tmp)
> > +               return -ENOMEM;
> > +
> > +       i = __cw1200_reg_read(priv, addr, tmp, sizeof(*tmp), 0);
> > +       *val = le32_to_cpu(*tmp);
> > +       kfree(tmp);
> >         return i;
> >  }
>
> There is a possible problem here when the function gets called from
> atomic context, so it might need to use GFP_ATOMIC instead of
> GFP_KERNEL. If it's never called from atomic context, then this patch
> looks correct to me.

I would be surprised if this is called from atomic context (when IRQs
are turned off), because in most cases, to complete the read/write
request the mmc controller driver relies on IRQs being delivered.

>
> The alternative would be to add a bounce buffer check based on
> is_vmalloc_or_module_addr() in sdio_io_rw_ext_helper(), which would
> add a small bit of complexity there but solve the problem for
> all drivers at once. In this case, it would probably have to use
> GFP_ATOMIC regardless of whether __cw1200_reg_read_32()
> is allowed to sleep, since other callers might not.

I like the idea, but...

I don't think we should see this as an alternative, but rather as a
complement which would have performance issues. A warning should be
printed, if the buffer isn't properly allocated.

Additionally, I don't think GFT_ATOMIC should be needed.

Kind regards
Uffe