Re: A question about PROT_NONE on PPC and PPC64

[Jamie Lokier]

Hi Paul,

Gabriel Paubert tells me that PROT_NONE _might_ have the bug described
below on the PPC 4xx and 8xx architectures, because their MMUs don't
implement "Ks = Kp = 1" segments.

Can you or someone else confirm whether the potential bug occurs on
PPC 4xx/8xx?

Thanks,
-- Jamie

Paul Mackerras wrote:
> > 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.
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