From: Balbir Singh <bsingharora@gmail.com>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Segher Boessenkool <segher@kernel.crashing.org>,
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: Mon, 18 Feb 2019 08:55:56 +1100 [thread overview]
Message-ID: <20190217215556.GH31125@350D> (raw)
In-Reply-To: <87ef86dd9v.fsf@concordia.ellerman.id.au>
On Sun, Feb 17, 2019 at 07:34:20PM +1100, Michael Ellerman wrote:
> 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.
>
Fair enough, my point was that the compiler can help out. I'll see what
-Wconversion finds on my local build :)
Balbir Singh.
next prev parent reply other threads:[~2019-02-17 21:56 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-14 6:23 [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud_present() Michael Ellerman
2019-02-14 16:31 ` Jan Kara
2019-02-16 10:55 ` Balbir Singh
2019-02-16 14:22 ` Segher Boessenkool
2019-02-17 6:23 ` Balbir Singh
2019-02-17 8:34 ` Michael Ellerman
2019-02-17 21:55 ` Balbir Singh [this message]
2019-02-18 0:49 ` Michael Ellerman
2019-02-19 12:01 ` Balbir Singh
2019-02-19 20:15 ` Segher Boessenkool
2019-02-20 11:18 ` Michael Ellerman
2019-02-20 14:51 ` Segher Boessenkool
2019-02-17 8:34 ` Michael Ellerman
2019-02-16 13:15 ` Andreas Schwab
2019-02-17 8:26 ` Michael Ellerman
2019-02-17 8:21 ` Michael Ellerman
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