linux-kernel - Re: x32 processes, with CONFIG_X86

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Message-ID: <20160408013228.GB567@codemonkey.org.uk>
Date:	Thu, 7 Apr 2016 21:32:28 -0400
From:	Dave Jones <davej@...emonkey.org.uk>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	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 Thu, Apr 07, 2016 at 05:20:50PM -0700, Andy Lutomirski 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.

I don't. That box got rebooted a couple dozen times since then.
FWIW, I've not seen this happen since. Very strange.

	Dave