Re: Testing PROT_NONE and other protections, and a surprise

[Russell King]

On Thu, Jul 01, 2004 at 04:26:06AM +0100, Jamie Lokier wrote:
> When running it on i386, I got a *huge* surprise (to me).  A
> PROT_WRITE-only page can sometimes fault on read or exec.  This is the
> output:
> 
> Requested PROT | ---    R--    -W-    RW-    --X    R-X    -WX    RWX
> ========================================================================
> MAP_SHARED     | ---    r-x    !w!    rwx    r-x    r-x    rwx    rwx
> MAP_PRIVATE    | ---    r-x    !w!    rwx    r-x    r-x    rwx    rwx
> 
> The "!" means that a read or exec *sometimes* raises a signal.

Here are the ARM results:

Requested PROT | ---    R--    -W-    RW-    --X    R-X    -WX    RWX
========================================================================
MAP_SHARED     | ---    r-x    !w!    rwx    r-x    r-x    rwx    rwx
MAP_PRIVATE    | ---    r-x    !w!    rwx    r-x    r-x    rwx    rwx

As expected, the same as x86 since we have the same situation there -
we can not represent the write-only page permission in hardware.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 PCMCIA      - http://pcmcia.arm.linux.org.uk/
                 2.6 Serial core
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