linux-kernel - Re: [PATCH v2] ARM: Fix MAX_DMA

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Date:   Wed, 6 Jul 2022 13:26:59 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Russell King <linux@...linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Grygorii Strashko <grygorii.strashko@...com>,
        open list <linux-kernel@...r.kernel.org>,
        Santosh Shilimkar <ssantosh@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>
Subject: Re: [PATCH v2] ARM: Fix MAX_DMA_ADDRESS overflow

On 7/6/22 12:44, Geert Uytterhoeven wrote:
> Hi Florian,
> 
> On Wed, Jul 6, 2022 at 6:46 PM Florian Fainelli <f.fainelli@...il.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@...il.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.
-- 
Florian