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

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Message-ID: <1970736614.19996.1589403401588.JavaMail.zimbra@efficios.com>
Date:   Wed, 13 May 2020 16:56:41 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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>, rostedt <rostedt@...dmis.org>,
        "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 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);
> +
> 		if (cached_flags & _TIF_USER_RETURN_NOTIFY)
>  			fire_user_return_notifiers();

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