linux-kernel - Re: [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud

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Date:   Mon, 18 Feb 2019 11:49:18 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     Segher Boessenkool <segher@...nel.crashing.org>,
        erhard_f@...lbox.org, jack@...e.cz, linuxppc-dev@...abs.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud_present()

Balbir Singh <bsingharora@...il.com> writes:
> On Sun, Feb 17, 2019 at 07:34:20PM +1100, Michael Ellerman wrote:
>> Balbir Singh <bsingharora@...il.com> writes:
>> > On Sat, Feb 16, 2019 at 08:22:12AM -0600, Segher Boessenkool wrote:
>> >> 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.
>
> Fair enough, my point was that the compiler can help out. I'll see what
> -Wconversion finds on my local build :)

I get about 43MB of warnings here :)

cheers