[PATCH v2] ARM: Fix MAX_DMA_ADDRESS overflow

[Geert Uytterhoeven]

Hi Florian,

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?).

> +       arm_dma_zone_size && arm_dma_zone_size < (0x100000000 - PAGE_OFFSET) ? \
>                 (PAGE_OFFSET + arm_dma_zone_size) : 0xffffffffUL; })
>  #endif

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