linux-kernel - Re: x32 processes, with CONFIG_X86

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Message-ID: <CALCETrXzhmT+Ly19bhEv8S=OtGafYkUMcLatwscCYiGHp_hezA@mail.gmail.com>
Date:	Thu, 7 Apr 2016 17:20:50 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Dave Jones <davej@...emonkey.org.uk>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: x32 processes, with CONFIG_X86_X32 not set

On Fri, Mar 25, 2016 at 8:58 AM, Dave Jones <davej@...emonkey.org.uk> wrote:
> On Fri, Mar 25, 2016 at 08:51:39AM -0700, Andy Lutomirski wrote:
>  > On Fri, Mar 25, 2016 at 8:48 AM, Dave Jones <davej@...emonkey.org.uk> wrote:
>  > > I had a trinity process get stuck last overnight.
>  > > The reason for it getting stuck is my bug (I think), but
>  > > there's an odd unrelated thing I noticed while debugging this..
>  > >
>  > > $ strace -p 20966
>  > > strace: Process 20966 attached
>  > > strace: [ Process PID=20966 runs in x32 mode. ]
>  > >
>  > > So I don't use that new-fangled x32 stuff.
>  > > I don't even have CONFIG_X86_X32 compiled in.
>  > >
>  > > Is this strace getting confused, or did we somehow screw
>  > > up the syscall entry code ?
>  > >
>  > >         Dave
>  > >
>  >
>  > I think you're just seeing an oddity of how x32 works.  Unlike
>  > "compat", x32-ness of the current syscall isn't a special magic state
>  > variable; it's just but 31 in the syscall nr.  So trying to do an x32
>  > syscall on a non-x32 syscall should still show bit 31 set to ptracers,
>  > and the strace probably decodes this as being in x32 mode.
>
> But this is an x86-64 binary, and it's the main process, not one of the fuzzing
> child processes. It shouldn't be even trying to do anything weird.
> It creates a bunch of fd's, then enters a loop forking/reaping children.
> (In this case it actually hung while creating the fd's)
>
> Trinity doesn't actually have any knowledge of x32 at all, mostly because
> it's been irrelevant to me (and most other people).
>

Hmm.  Do you have the next couple lines of strace output by any
chance?  I'm wondering if this is a classic bug/misfeature/confusion
in the way that orig_ax works.

FWIW, the way that strace detects 32-bit mode is bogus, and I don't
actually know how strace detects x32 mode.

>         Dave
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC