Messages in this thread | | | Date | Sat, 16 Feb 2019 08:22:12 -0600 | From | Segher Boessenkool <> | Subject | Re: [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud_present() |
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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
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