Address Collision using i386 4G/4G Memory Split

Brooks Davis brooks at
Tue Dec 18 08:34:28 UTC 2018

On Mon, Dec 17, 2018 at 03:58:05PM -0500, Kurt Lidl wrote:
> Alexander Lochmann writes:
> > According to git commit e3089a (
> > FreeBSD 12.0 i386 uses separate address spaces for kernel and user
> > space. So basically two memory areas, one in each space, can have the
> > same address.
> > Is this possible with FreeBSD 12.0? Is this likely to happen?
> If the userspace program and the kernel address happen to overlap, the 
> system will deal with it.  There's not anything to worry about.  As to
> whether or not it's likely to happen -- I'm not sure about that.  I
> expect the default stack and heap space locations for a fresh process
> have changed due to this change, but it should not matter.

4/4 does potentially alter the failure modes of buggy code that tries to
read directly from userspace addresses.  For example, correct calls to
the sysctls fixed in r342125 may panic prior to the fix because the
addresses in question aren't mapped in kernel space.  They might also
fail or behave bizarrely if the page is mapped and the value from the
kernel page is used.

-- Brooks
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