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Date:   Thu, 20 Jul 2023 07:22:08 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@kernel.org>
Cc:     Petr Mladek <pmladek@suse.com>, Brian Gerst <brgerst@gmail.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        alyssa.milburn@linux.intel.com, keescook@chromium.org,
        joao@overdrivepizza.com, tim.c.chen@linux.intel.com,
        live-patching@vger.kernel.org
Subject: Re: [PATCH v2 2/2] x86: Rewrite ret_from_fork() in C
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On Wed, Jul 19, 2023 at 04:31:11PM -0700, Josh Poimboeuf wrote:
> On Wed, Jul 19, 2023 at 10:50:50PM +0200, Peter Zijlstra wrote:
> > > The below cures things; Josh, did I miss anything?
> > > 
> > > ---
> > >  arch/x86/entry/entry_64.S | 14 +++++++++++++-
> > >  1 file changed, 13 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> > > index 91f6818884fa..cfe7882ea9ae 100644
> > > --- a/arch/x86/entry/entry_64.S
> > > +++ b/arch/x86/entry/entry_64.S
> > > @@ -285,7 +285,14 @@ SYM_FUNC_END(__switch_to_asm)
> > >   */
> > >  .pushsection .text, "ax"
> > >  SYM_CODE_START(ret_from_fork_asm)
> > > -	UNWIND_HINT_REGS
> > > +	/*
> > > +	 * This is the start of the kernel stack; even through there's a regs
> > > +	 * set at the top, there is no real exception frame and one cannot
> > > +	 * unwind further. This is the end.
> > > +	 *
> > > +	 * This ensures stack unwinds of kernel threads hit a known good state.
> > > +	 */
> > > +	UNWIND_HINT_END_OF_STACK
> 
> The comments may be a bit superfluous (to me at least) but the patch
> looks fine.

Right, well, it took me a minute to figure out how it was all supposed
to work, I figured I'd stick a comment on it.

The bit I missed is that if you reach the return-to-user part, you will
actually have user_mode() true on the regset.

> > So unwind_orc.c:unwind_next_frame() will terminate on this hint *or* on
> > user_mode(state->regs).
> > 
> > AFAICT way things are set up in copy_thread(), user_mode() will not be
> > true -- after all there is no usermode, the kthread would first have to
> > exec() something to create a usermode.
> > 
> > Yet I'm wondering if perhaps we should spoof the regs to make
> > user_mode() true and auto-terminate without this explicit hint.
> 
> I'm not sure that would be worth the trouble / cleverness.  The hint is
> straightforward IMO.

I tried, it doesn't work, clearly I missed something.