linux-kernel - [Q] Figuring out task mode

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Message-ID: <20170321180525.GC21564@uranus.lan>
Date:   Tue, 21 Mar 2017 21:05:25 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Dmitry Safonov <dsafonov@...tuozzo.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Adam Borowski <kilobyte@...band.pl>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrei Vagin <avagin@...il.com>, Borislav Petkov <bp@...e.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [Q] Figuring out task mode

/I renamed the mail's subject/

On Tue, Mar 21, 2017 at 10:45:57AM -0700, Andy Lutomirski wrote:
> >> +             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.)

Yes, syscall number will help but from criu perpspective (until
Dima's patches are merged into mainlie) we need to figure out
if we can dump x32 tasks without running parasite code inside,
ie via plain ptrace call or some procfs output. But looks like
it's impossible for now.

> Do we actually have an x32 per-task mode at all?  If so, maybe we can
> just remove it on top of Dmitry's series.

Don't think so, x32 should be set upon exec and without Dima's series
it is immutable I think.