Re: [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud_present()
From: Segher Boessenkool
Date: Sat Feb 16 2019 - 09:25:49 EST
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