Re: [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud_present()

From: Segher Boessenkool <segher@kernel.crashing.org>
To: Balbir Singh <bsingharora@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
	erhard_f@mailbox.org, jack@suse.cz, linuxppc-dev@ozlabs.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	aneesh.kumar@linux.vnet.ibm.com
Subject: Re: [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud_present()
Date: Sat, 16 Feb 2019 08:22:12 -0600	[thread overview]
Message-ID: <20190216142206.GE14180@gate.crashing.org> (raw)
In-Reply-To: <20190216105511.GA31125@350D>

Hi all,

On Sat, Feb 16, 2019 at 09:55:11PM +1100, Balbir Singh wrote:
> On Thu, Feb 14, 2019 at 05:23:39PM +1100, Michael Ellerman wrote:
> > In v4.20 we changed our pgd/pud_present() to check for _PAGE_PRESENT
> > rather than just checking that the value is non-zero, e.g.:
> > 
> >   static inline int pgd_present(pgd_t pgd)
> >   {
> >  -       return !pgd_none(pgd);
> >  +       return (pgd_raw(pgd) & cpu_to_be64(_PAGE_PRESENT));
> >   }
> > 
> > Unfortunately this is broken on big endian, as the result of the
> > bitwise && is truncated to int, which is always zero because

(Bitwise "&" of course).

> Not sure why that should happen, why is the result an int? What
> causes the casting of pgd_t & be64 to be truncated to an int.

Yes, it's not obvious as written...  It's simply that the return type of
pgd_present is int.  So it is truncated _after_ the bitwise and.

Segher