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

[Cyrill Gorcunov]

On Tue, May 10, 2016 at 09:07:49AM -0700, Andy Lutomirski wrote:
> 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, could you please highlight what's wrong with TASK_SIZE helper
in first place? The idea behind is to clean up the code or there
some real problem?