Re: [RFC PATCH v6 4/5] mmc: tmio: Use dma_max_mapping_size() instead of a workaround

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Shimoda-san,

On Mon, Jun 17, 2019 at 6:54 AM Yoshihiro Shimoda
<yoshihiro.shimoda.uh@renesas.com> wrote:
> > From: Geert Uytterhoeven, Sent: Friday, June 14, 2019 4:27 PM
> > On Fri, Jun 14, 2019 at 9:18 AM Christoph Hellwig wrote:
> > > On Thu, Jun 13, 2019 at 10:35:44PM +0200, Geert Uytterhoeven wrote:
> > > > I'm always triggered by the use of min_t() and other casts:
> > > > mmc->max_blk_size and mmc->max_blk_count are both unsigned int.
> > > > dma_max_mapping_size() returns size_t, which can be 64-bit.
> > > >
> > > >  1) Can the multiplication overflow?
> > > >     Probably not, as per commit 2a55c1eac7882232 ("mmc: renesas_sdhi:
> > > >     prevent overflow for max_req_size"), but I thought I'd better ask.
>
> Geert-san:
>
> I agree.
>
> > > >  2) In theory, dma_max_mapping_size() can return a number that doesn't
> > > >     fit in 32-bit, and will be truncated (to e.g. 0), leading to max_req_size
> > > >     is zero?
>
> Geert-san:
>
> I agree. If dma_max_mapping_size() return 0x1_0000_0000, it will be truncated to 0
> and then max_req_size is set to zero. It is a problem. Also, the second argument
> "mmc->max_blk_size * mmc->max_blk_count" will not be overflow and then the value is
> 0xffff_ffff or less. So, I also think this should use size_t instead of unsigned int.
>
> > > This really should use a min_t on size_t.  Otherwise the patch looks
> > > fine:
> >
> > Followed by another min() to make it fit in mmc->max_req_size, which is
> > unsigned int.
>
> Geert-san:
>
> I'm afraid, but I cannot understand this means.
> Is this patch is possible to be upstream? Or, do you have any concern?

Please disregard my last comment: as the value of "mmc->max_blk_size *
mmc->max_blk_count" is always 0xffff_ffff or less, "min_t(size_t,
mmc->max_blk_size * mmc->max_blk_count, dma_max_mapping_size(&pdev->dev))"
will always be 0xffff_ffff or less, too, so there is no extra step needed
to make it fit in mmc->max_req_size.


Sorry for the confusion.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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