[PATCH v2] ARM: Fix MAX_DMA_ADDRESS overflow

[Geert Uytterhoeven]

Hi Florian,

On Wed, Jul 6, 2022 at 10:27 PM Florian Fainelli <f.fainelli at gmail.com> wrote:
> On 7/6/22 12:44, Geert Uytterhoeven wrote:
> > On Wed, Jul 6, 2022 at 6:46 PM Florian Fainelli <f.fainelli at gmail.com> wrote:
> >> Commit 26f09e9b3a06 ("mm/memblock: add memblock memory allocation apis")
> >> added a check to determine whether arm_dma_zone_size is exceeding the
> >> amount of kernel virtual address space available between the upper 4GB
> >> virtual address limit and PAGE_OFFSET in order to provide a suitable
> >> definition of MAX_DMA_ADDRESS that should fit within the 32-bit virtual
> >> address space. The quantity used for comparison was off by a missing
> >> trailing 0, leading to MAX_DMA_ADDRESS to be overflowing a 32-bit
> >> quantity.
> >>
> >> This was caught with the bcm2711 platforms which defines a dma_zone_size
> >> of 1GB, and using a PAGE_OFFSET of 0xc000_0000 (CONFIG_VMSPLIT_3G) with
> >> CONFIG_DEBUG_VIRTUAL enabled would lead to MAX_DMA_ADDRESS being
> >> 0x1_0000_0000 which overflows the unsigned long type used throughout
> >> __pa() and __virt_addr_valid(). Because the virtual address passed to
> >> __virt_addr_valid() would now be 0, the function would loudly warn, thus
> >> making the platform unable to boot properly.
> >>
> >> Fixes: 26f09e9b3a06 ("mm/memblock: add memblock memory allocation apis")
> >> Signed-off-by: Florian Fainelli <f.fainelli at gmail.com>
> >> ---
> >> Changes in v2:
> >>
> >> - simplify the patch and drop the first patch that attempted to fix an
> >>    off by one in the calculation.
> >
> > Thanks for the update!
> >
> >> --- a/arch/arm/include/asm/dma.h
> >> +++ b/arch/arm/include/asm/dma.h
> >> @@ -10,7 +10,7 @@
> >>   #else
> >>   #define MAX_DMA_ADDRESS        ({ \
> >>          extern phys_addr_t arm_dma_zone_size; \
> >> -       arm_dma_zone_size && arm_dma_zone_size < (0x10000000 - PAGE_OFFSET) ? \
> >                                                    ^^^^^^^^^^
> > 0x10000000ULL, as the constant doesn't fit in 32-bit.
> > However, both gcc (9.4.0) and sparse don't seem to complain about
> > the missing suffix (anymore?).
>
> Thanks, I will the ULL suffix in v3.

I just remembered the suffix is not needed (but doesn't hurt), because
hexadecimal constants automatically have the right unsigned type.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at 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