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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Balbir Singh <bsingharora@gmail.com>,
        Segher Boessenkool <segher@kernel.crashing.org>
Cc:     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()
In-Reply-To: <20190217062333.GC31125@350D>
References: <20190214062339.7139-1-mpe@ellerman.id.au> <20190216105511.GA31125@350D> <20190216142206.GE14180@gate.crashing.org> <20190217062333.GC31125@350D>
Date:   Sun, 17 Feb 2019 19:34:20 +1100
Message-ID: <87ef86dd9v.fsf@concordia.ellerman.id.au>
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Balbir Singh <bsingharora@gmail.com> writes:
> On Sat, Feb 16, 2019 at 08:22:12AM -0600, Segher Boessenkool wrote:
>> 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.
>>
>
> Thanks, I am surprised the compiler does not complain about the truncation
> of bits. I wonder if we are missing -Wconversion

Good luck with that :)

What I should start doing is building with it enabled and then comparing
the output before and after commits to make sure we're not introducing
new cases.

cheers