Re: [PATCHv2] x86/mm: set x32 syscall bit in SET_PERSONALITY()

[Dmitry Safonov]

On 03/21/2017 10:31 PM, Andy Lutomirski wrote:
> On Tue, Mar 21, 2017 at 11:09 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> wrote:
> > On 03/21/2017 08:45 PM, Andy Lutomirski wrote:
> > > On Tue, Mar 21, 2017 at 10:17 AM, Cyrill Gorcunov <gorcunov@xxxxxxxxx>
> > > wrote:
> > > > On Tue, Mar 21, 2017 at 07:37:12PM +0300, Dmitry Safonov wrote:
> > > > ...
> > > > > diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> > > > > index d6b784a5520d..d3d4d9abcaf8 100644
> > > > > --- a/arch/x86/kernel/process_64.c
> > > > > +++ b/arch/x86/kernel/process_64.c
> > > > > @@ -519,8 +519,14 @@ void set_personality_ia32(bool x32)
> > > > >               if (current->mm)
> > > > >                       current->mm->context.ia32_compat = TIF_X32;
> > > > >               current->personality &= ~READ_IMPLIES_EXEC;
> > > > > -             /* in_compat_syscall() uses the presence of the x32
> > > > > -                syscall bit flag to determine compat status */
> > > > > +             /*
> > > > > +              * in_compat_syscall() uses the presence of the x32
> > > > > +              * syscall bit flag to determine compat status.
> > > > > +              * On the bitness of syscall relies x86 mmap() code,
> > > > > +              * so set x32 syscall bit right here to make
> > > > > +              * in_compat_syscall() work during exec().
> > > > > +              */
> > > > > +             task_pt_regs(current)->orig_ax |= __X32_SYSCALL_BIT;
> > > > >               current->thread.status &= ~TS_COMPAT;
> > > >
> > > >
> > > > Hi! I must admit I didn't follow close the overall series (so can't
> > > > comment much here :) but I have a slightly unrelated question -- is
> > > > there a way to figure out if task is running in x32 mode say with
> > > > some ptrace or procfs sign?
> > >
> > >
> > > You should be able to figure out of a *syscall* is x32 by simply
> > > looking at bit 30 in the syscall number.  (This is unlike i386, which
> > > is currently not reflected in ptrace.)
> >
> >
> > The process could be stopped with PTRACE_SEIZE and I think, it'll not
> > have x32 syscall bit at that moment.
> >
> > I guess the question comes from that we're releasing CRIU 3.0 with
> > 32-bit C/R and some other cool stuff, but we don't support x32 yet.
> > As we don't want release a thing that we aren't properly testing.
> > So for a while we should error on dumping x32 applications.
>
> I'm curious: shouldn't x32 CRIU just work?  What goes wrong?

I also think, it should be quite easy to add, as we have arch_prctl() 
for vdso and etc.
But there are things, which will not work if we just dump application
as 64-bit.

For example, what comes to mind: sys_get_robust_list(), it has different 
pointers for 64-bit or for x32/ia32 applications: robust_list
and compat_robust_list. So during C/R we should sometimes call
compatible syscalls for x32 applications to dump/restore, as for futex
list e.g., native will return NULL or empty list.

> --Andy


--
             Dmitry