linux-kernel - Re: [patch V4 part 1 05/36] x86/entry: Flip _TIF_SIGPENDING and _TIF_NOTIFY

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Message-ID: <1473818156.20126.1589410086731.JavaMail.zimbra@efficios.com>
Date:   Wed, 13 May 2020 18:48:06 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, paulmck <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch V4 part 1 05/36] x86/entry: Flip _TIF_SIGPENDING and
 _TIF_NOTIFY_RESUME handling

----- On May 13, 2020, at 5:10 PM, rostedt rostedt@...dmis.org wrote:

> On Wed, 13 May 2020 16:56:41 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
>> ----- On May 5, 2020, at 9:16 AM, Thomas Gleixner tglx@...utronix.de wrote:
>> 
>> > Make sure task_work runs before any kind of userspace -- very much
>> > including signals -- is invoked.
>> 
>> What is missing from this patch description is: _why_ is this deemed
>> useful ?
>> 
>> Also, color me confused: is "do_signal()" actually running any user-space,
>> or just setting up the user-space stack for eventual return to signal handler ?
>> 
>> Also, it might be OK, but we're changing the order of two things which
>> have effects on each other: restartable sequences abort fixup for preemption
>> and do_signal(), which also have effects on rseq abort.
>> 
>> Because those two will cause the abort to trigger, I suspect changing
>> the order might be OK, but we really need to think this through.
>> 
>> Thanks,
>> 
>> Mathieu
>> 
>> > 
>> > Suggested-by: Andy Lutomirski <luto@...nel.org>
>> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>> > ---
>> > arch/x86/entry/common.c |    8 ++++----
>> > 1 file changed, 4 insertions(+), 4 deletions(-)
>> > 
>> > --- a/arch/x86/entry/common.c
>> > +++ b/arch/x86/entry/common.c
>> > @@ -156,16 +156,16 @@ static void exit_to_usermode_loop(struct
>> > 		if (cached_flags & _TIF_PATCH_PENDING)
>> > 			klp_update_patch_state(current);
>> > 
>> > -		/* deal with pending signal delivery */
>> > -		if (cached_flags & _TIF_SIGPENDING)
>> > -			do_signal(regs);
>> > -
>> > 		if (cached_flags & _TIF_NOTIFY_RESUME) {
>> > 			clear_thread_flag(TIF_NOTIFY_RESUME);
>> > 			tracehook_notify_resume(regs);
>> > 			rseq_handle_notify_resume(NULL, regs);
>> > 		}
>> > 
>> > +		/* deal with pending signal delivery */
>> > +		if (cached_flags & _TIF_SIGPENDING)
>> > +			do_signal(regs);
> 
> Looking deeper into this, it appears that do_signal() can freeze or kill the
> task.
> 
> That is, it wont go back to user space here, but simply schedule out (being
> traced) or even exit (killed).
> 
> Before the resume hooks would never be called in such cases, and now they
> are.

Regarding swapping order of tracehook vs do_signal: Is the task really resumed
if it gets frozen or killed ? What is this change trying to accomplish, why
is it needed in the first place ?

Regarding swapping order of rseq wrt do_signal: why is it needed, why, and has
this been tested ?

Thanks,

Mathieu

> 
> -- Steve
> 
> 
>> > +
>> > 		if (cached_flags & _TIF_USER_RETURN_NOTIFY)
>> >  			fire_user_return_notifiers();

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com