[PATCH v2] ARM: dma-mapping: fix potential uninitialized return

[linux@armlinux.org.uk (Russell King - ARM Linux)]

On Thu, Nov 29, 2018 at 10:11:45AM +0000, Vladimir Murzin wrote:
> On 11/29/18 9:50 AM, Vladimir Murzin wrote:
> > On 11/28/18 6:59 PM, Nathan Jones wrote:
> >> If neither of the if() statements fire then the return value is
> >> uninitialized. In the worst case it returns 0 which means the caller
> >> will think the function succeeded.
> > 
> > "ret" is updated indirectly via:
> > 
> >         if (dma_mmap_from_dev_coherent(dev, vma, cpu_addr, size, &ret))
> >                 return ret;
> 
> Ok. I've had a look how dma_mmap_from_dev_coherent() is implemented and it
> looks like "ret" is not updated if dev doesn't have reserved memory.
> It looks like arm64 also might be affected by this as well.
> 
> So, would it be better to update dma_mmap_from_dev_coherent() with

I don't think so - if we were to remove the call to
dma_mmap_from_dev_coherent(), it reintroduces the bug.

Quite why we have dma_mmap_from_dev_coherent() returning a 0/1 and
error code via pointer I'm really not sure.

	ret = dma_mmap_from_dev_coherent(...);
	if (ret)
		return ret > 0 ? 0 : ret;

and have dma_mmap_from_dev_coherent() return -ve for errnos, 1 if
mapped via the coherent pool mechanism, or 0 otherwise.

The down-side is that 'ret' would be zero for the follow-on code,
which would need explicit initialisation - but at least it's then
obvious what is going on.

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