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

[Andy Lutomirski]

On Tue, May 10, 2016 at 9:30 AM, Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> 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?

It's annoying and ugly.  It also makes the idea of doing 32-bit CRIU
restore by starting in 64-bit mode and switching to 32-bit more
complicated because it requires switching TASK_SIZE.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC