On Thu, 29 Jun 2006, Pavel Machek wrote:
> Hi!
>
> > > > PROT_READ to be used or implicitly adding it. Don't confuse people
> > > > with wrong statement like yours.
> > >
> > > Can you quote part of POSIX where it says that PROT_WRITE must imply
> > > PROT_READ?
> >
> > I don't believe POSIX cares either way
> >
> > "An implementation may permit accesses other than those specified by
> > prot; however, if the Memory Protection option is supported, the
> > implementation shall not permit a write to succeed where PROT_WRITE has
> > not been set or shall not permit any access where PROT_NONE alone has
> > been set."
> >
> > However the current behaviour of "write to map read might work some days
> > depending on the execution order of instructions" (and in some cases the
> > order that the specific CPU does its tests for access rights) is not
> > sane, not conducive to application stability and not good practice.
>
> Well, some architectures may have working PROT_WRITE without
> PROT_READ. If you are careful and code your userland application in
> assembly, it should work okay.
>
> On processors where that combination depends randomly depending on
> phase of moon (i386, apparently), I guess change is okay. But the
> patch disabled PROT_WRITE w/o PROT_READ on _all_ processors.
>
> Pavel
>
ok, the following patch should make x86 self-consistent, making PROT_WRITE
imply PROT_READ.
i can produce patches for other architectures, if people agree with this
approach.
thanks,
-Jason
--- linux-2.6/arch/i386/mm/fault.c.bak 2006-06-29 16:48:25.000000000 -0400
+++ linux-2.6/arch/i386/mm/fault.c 2006-06-29 16:49:51.000000000 -0400
@@ -449,7 +449,7 @@
case 1: /* read, present */
goto bad_area;
case 0: /* read, not present */
- if (!(vma->vm_flags & (VM_READ | VM_EXEC)))
+ if (!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE)))
goto bad_area;
}
-
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