Re: A question about PROT_NONE on PPC and PPC64

[Paul Mackerras <paulus@samba.org>]

Jamie Lokier writes:

> PPC32
> =====
> 
> In include/asm-ppc/pgtable.h, there's:
> 
> #define PAGE_NONE	__pgprot(_PAGE_BASE)
> #define PAGE_READONLY	__pgprot(_PAGE_BASE | _PAGE_USER)
> 
> It appears the only difference betwen PROT_READ and PROT_NONE is
> whether _PAGE_USER is set.
> 
> Thus PROT_NONE pages aren't readable from userspace, but it appears
> they _are_ readable from kernel space.  Is this correct?

No.  Kernel accesses to pages in the user portion of the address space
(0 .. TASK_SIZE-1) are done using the user permissions.  On classic
PPC this is implemented (in part) by setting Ks = Kp = 1 in the
segment descriptors for the user segments, which tells the hardware to
check the access as if it was a user access even in supervisor mode.

We do the same on ppc64 as well.

> So does this mean that PROT_NONE pages, although they aren't readable
> from userspace, are readable from kernel space?  I.e. that write()
> with a PROT_NONE region would succeed, instead of returning EFAULT as
> it should?

If you do find a ppc32 or ppc64 machine where the write succeeds
instead of returning EFAULT, let me know please.

Paul.