Getting rid of dynamic TASK_SIZE (on x86, at least)

[Andy Lutomirski]

Hi all-

I'm trying to get rid of x86's dynamic TASK_SIZE and just redefine it
to TASK_SIZE_MAX.  So far, these are the TASK_SIZE users that actually
seem to care about the task in question:

get_unmapped_area.  This is used by mmap, mremap, exec, uprobe XOL,
and maybe some other things.

 - mmap, mremap, etc: IMO this should check in_compat_syscall, not
TIF_ADDR32.  If a 64-bit task does an explicit 32-bit mmap (using int
$0x80, for example), it should get a 32-bit address back.

 - xol_add_vma: This one is weird: uprobes really is doing something
behind the task's back, and the addresses need to be consistent with
the address width.  I'm not quite sure what to do here.

 - exec.  This wants to set up mappings that are appropriate for the new task.

My inclination would be add a new 'limit' parameter to all the
get_unmapped_area variants and possible to vm_brk and friends and to
thus push the decision into the callers.  For the syscalls, we could
add:

static inline unsigned long this_syscall_addr_limit(void) { return TASK_SIZE; }

and override it on x86.

I'm not super excited to write that patch, though...

--Andy