Re: Table of mmap PROT_* implementations by architecture

[Kyle Moffett]

On Jun 30, 2004, at 23:36, Jamie Lokier wrote:
> From a study of Linux 2.6.5 source code, and some patches.
> This is based on studying the source, not running tests, so there
> may be errors.
> ======================+================================================ 
> ========
> Requested PROT flags  | ---    R--    -W-    RW-    --X    R-X    -WX   
>   RWX
> ======================+================================================ 
> ========
> [...]
> ppc                   | ---(1) r-x    rwx(5) rwx    r-x(5) r-x     
> rwx(5) rwx
> ppc64                 | ---(1) r-x    rwx(5) rwx    r-x(5) r-x     
> rwx(5) rwx
> ppc (PaX)	      | ---(1) r--    rw-    rw-    r-x    r-x    rw-(2)  
> rw-(2)
> ppc64 (PaX for 2.6)   | ---(1) r--    rw-    rw-    r-x    r-x     
> rw-(2) rw-(2)
> [...]
>
> (1) - In kernel, maybe these pages are readable using "write()"?
>       In each case that is labelled, I'm not sure from reading the  
> code.
>       (Pages are always readable using ptrace(), that's ok, but write()
>       and other kernel reads shouldn't be able to read PROT_NONE  
> pages).

This is wrong for PPC32 and PPC64, see the email written earlier today:

On June 30, 2004, at 00:47, Paul Mackerras wrote:
> > 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.

Cheers,
Kyle Moffett



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