Re: [PATCH] powerpc/mm: Fix virt_addr_valid() etc. on 64-bit hash

[Breno Leitao <leitao@debian.org>]

On Thu, May 18, 2017 at 08:37:31PM +1000, Michael Ellerman wrote:
> virt_addr_valid() is supposed to tell you if it's OK to call virt_to_page() on
> an address. What this means in practice is that it should only return true for
> addresses in the linear mapping which are backed by a valid PFN.
> 
> We are failing to properly check that the address is in the linear mapping,
> because virt_to_pfn() will return a valid looking PFN for more or less any
> address. That bug is actually caused by __pa(), used in virt_to_pfn().
> 
> eg: __pa(0xc000000000010000) = 0x10000  # Good
>     __pa(0xd000000000010000) = 0x10000  # Bad!
>     __pa(0x0000000000010000) = 0x10000  # Bad!
> 
> This started happening after commit bdbc29c19b26 ("powerpc: Work around gcc
> miscompilation of __pa() on 64-bit") (Aug 2013), where we changed the definition
> of __pa() to work around a GCC bug. Prior to that we subtracted PAGE_OFFSET from
> the value passed to __pa(), meaning __pa() of a 0xd or 0x0 address would give
> you something bogus back.
> 
> Until we can verify if that GCC bug is no longer an issue, or come up with
> another solution, this commit does the minimal fix to make virt_addr_valid()
> work, by explicitly checking that the address is in the linear mapping region.
> 
> Fixes: bdbc29c19b26 ("powerpc: Work around gcc miscompilation of __pa() on 64-bit")
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Tested-by: Breno Leitao <breno.leitao@gmail.com>