Re: [PATCH 1/2] ARM: Fix off by one in checking DMA zone size

[Arnd Bergmann]

On Fri, Apr 29, 2022 at 5:15 PM Russell King (Oracle)
<linux@xxxxxxxxxxxxxxx> wrote:
>
> On Thu, Mar 24, 2022 at 10:54:16AM -0700, Florian Fainelli wrote:
> > The maximum DMA address is PAGE_OFFSET + arm_dma_zone_size - 1, fix this
> > by doing the appropriate subtraction.
>
> The question is... is MAX_DMA_ADDRESS inclusive or exclusive. The answer
> is rather indeterminant, unfortunately.
>
> drivers/net/wan/cosa.c: if (b + len >= MAX_DMA_ADDRESS)
>
> So, if the buffer address + buffer length is equal to MAX_DMA_ADDRESS,
> then the buffer is not DMA-able. To me this looks completely wrong.
> It's also completely wrong because 'b' is a "unsigned long" which
> means on 32-bit, this can wrap.

This driver is on its way out and will be removed in 5.19.

> there's a bunch of memblock allocation functions that use
> __pa(MAX_DMA_ADDRESS) as the "min_addr" and if this is a minimum
> address, then surely this means that it is also an exclusive value.
>
> So, I came to the conclusion that MAX_DMA_ADDRESS is supposed to be
> exclusive. In other words, where PAGE_OFFSET + sizeof(ram) if
> sizeof(ram) is the size in bytes of the RAM, limited to the maximum
> addressable virtual address that RAM can be mapped to.
>
> I don't see an inclusive value being correct, at least not for the cases
> I've looekd at.

Looking at the other definitions, I see it's usually either ULONG_MAX (which
is inclusive) or an exclusive limit, same as the existing version on arm:

arch/alpha/include/asm/io.h:#define IDENT_ADDR     0xffff800000000000UL
arch/alpha/include/asm/dma.h:#define MAX_DMA_ADDRESS
(alpha_mv.mv_pci_tbi ?  \
arch/alpha/include/asm/dma.h-                            ~0UL :
IDENT_ADDR + 0x01000000)
arch/arc/include/asm/dma.h:#define MAX_DMA_ADDRESS 0xC0000000
arch/hexagon/include/asm/dma.h:#define MAX_DMA_ADDRESS  (PAGE_OFFSET)
arch/ia64/mm/init.c:unsigned long MAX_DMA_ADDRESS = PAGE_OFFSET + 0x100000000UL;
arch/m68k/include/asm/dma.h:#define MAX_DMA_ADDRESS PAGE_OFFSET
arch/mips/include/asm/dma.h:#define MAX_DMA_ADDRESS             PAGE_OFFSET
arch/mips/include/asm/dma.h:#define MAX_DMA_ADDRESS
(PAGE_OFFSET + 0x01000000)
arch/parisc/include/asm/dma.h:#define MAX_DMA_ADDRESS (~0UL)
arch/powerpc/include/asm/dma.h:#define MAX_DMA_ADDRESS          (~0UL)
arch/s390/include/asm/dma.h:#define MAX_DMA_ADDRESS         0x80000000
arch/sparc/include/asm/dma.h:#define MAX_DMA_ADDRESS  (~0UL)
arch/um/include/asm/dma.h:#define MAX_DMA_ADDRESS (uml_physmem)
arch/x86/include/asm/dma.h:#define MAX_DMA_ADDRESS      (PAGE_OFFSET +
0x1000000)
arch/x86/include/asm/dma.h:#define MAX_DMA_PFN   ((16UL * 1024 * 1024)
>> PAGE_SHIFT)
arch/x86/include/asm/dma.h:#define MAX_DMA_ADDRESS ((unsigned
long)__va(MAX_DMA_PFN << PAGE_SHIFT))

The exceptions are:
arch/xtensa/include/asm/kmem_layout.h:#define XCHAL_KIO_SIZE
         0x10000000
arch/xtensa/include/asm/dma.h:#define MAX_DMA_ADDRESS
(PAGE_OFFSET + XCHAL_KIO_SIZE - 1)
arch/microblaze/include/asm/dma.h:#define MAX_DMA_ADDRESS
(CONFIG_KERNEL_START + memory_size - 1)

        Arnd