kernel: tweak USER_SIZE.

Change-Id: Id0a4ad785bbd195e425dd32ca891dc4525bb3e18
Reviewed-on: https://review.haiku-os.org/c/1151
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

vm: place commpage and team data near the top of user address space

Placing commpage and team user data somewhere at the top of the user accessible
virtual address space prevents these areas from conflicting with elf images
that require to be mapped at exact address (in most cases: runtime_loader).

Improved safety for user memory accesses.

* Changed IS_USER_ADDRESS to check an address using USER_BASE and
USER_SIZE, rather than just !IS_KERNEL_ADDRESS. The old check would
allow user buffers to point into the physical memory map area.
* Added an unmapped hole at the end of the bottom half of the address
space which catches buffers that cross into the uncanonical address
region. This also removes the need to check for uncanonical return
addresses in the syscall handler, it is no longer possible for the
return address to be uncanonical under normal circumstances. All
cases in which the return address might be changed by the kernel
are still handled via the IRET path.

Added and modified kernel stubs and headers for arch mipsel. Correctness not included.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@32621 a95241bf-73f2-0310-859d-f6bbb57e9c96

vm: place commpage and team data near the top of user address space

Placing commpage and team user data somewhere at the top of the user accessible
virtual address space prevents these areas from conflicting with elf images
that require to be mapped at exact address (in most cases: runtime_loader).

Improved safety for user memory accesses.

* Changed IS_USER_ADDRESS to check an address using USER_BASE and
USER_SIZE, rather than just !IS_KERNEL_ADDRESS. The old check would
allow user buffers to point into the physical memory map area.
* Added an unmapped hole at the end of the bottom half of the address
space which catches buffers that cross into the uncanonical address
region. This also removes the need to check for uncanonical return
addresses in the syscall handler, it is no longer possible for the
return address to be uncanonical under normal circumstances. All
cases in which the return address might be changed by the kernel
are still handled via the IRET path.

Added and modified kernel stubs and headers for arch mipsel. Correctness not included.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@32621 a95241bf-73f2-0310-859d-f6bbb57e9c96