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From: Andy Lutomirski <luto@amacapital.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
	Kees Cook <keescook@chromium.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Denys Vlasenko <vda.linux@googlemail.com>,
	Ingo Molnar <mingo@kernel.org>, Brian Gerst <brgerst@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	Andrew Lutomirski <luto@kernel.org>,
	Oleg Nesterov <oleg@redhat.com>, Peter Anvin <hpa@zytor.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Rik van Riel <riel@redhat.com>,
	"linux-tip-commits@vger.kernel.org" 
	<linux-tip-commits@vger.kernel.org>
Subject: Re: [tip:x86/asm] x86/entry: Add new, comprehensible entry and exit handlers written in C
Date: Wed, 15 Jul 2015 13:46:58 -0700
Message-ID: <CALCETrXizJLmwpAa0z1K+Hie=vD_PTcssxpSwNO5XqLq=2_U7Q@mail.gmail.com> (raw)
In-Reply-To: <CA+55aFy7e=h0bUDL=TV3QRcxicLfgg5vJX9jf+wwSmPg+jYN4w@mail.gmail.com>

On Wed, Jul 15, 2015 at 12:56 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Tue, Jul 14, 2015 at 4:07 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
>> On Tue, Jul 07, 2015 at 03:51:48AM -0700, tip-bot for Andy Lutomirski wrote:
>>> +     while (true) {
>>> +             u32 cached_flags =
>>> +                     READ_ONCE(pt_regs_to_thread_info(regs)->flags);
>>> +
>>> +             if (!(cached_flags & (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME |
>>> +                                   _TIF_UPROBE | _TIF_NEED_RESCHED)))
>>> +                     break;
>>> +
>>> +             /* We have work to do. */
>>> +             local_irq_enable();
>>> +
>>> +             if (cached_flags & _TIF_NEED_RESCHED)
>>> +                     schedule();
>>> +
>>> +             if (cached_flags & _TIF_UPROBE)
>>> +                     uprobe_notify_resume(regs);
>>> +
>>> +             /* 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);
>>> +             }
>>> +
>>> +             if (cached_flags & _TIF_USER_RETURN_NOTIFY)
>>> +                     fire_user_return_notifiers();
>>> +
>>> +             /* Disable IRQs and retry */
>>> +             local_irq_disable();
>>> +     }
>>
>> I dreamed so many times about this loop in C!
>
> So this made me look at it again, and now I'm worried.
>
> There's that "early break", but it doesn't check
> _TIF_USER_RETURN_NOTIFY. So if *only* USER_RETURN_NOTIFY is set, we're
> screwed.

Crap, that's a bug.  I'll send a patch.

>
> It migth be that that doesn't happen for some reason, but I'm not
> seeing what that reason would be.
>
> The other thing that worries me is that this depends on all the
> handler routines to clear the flags (except for
> tracehook_notify_resume()). Which they hopefully do. But that means
> that just looking at this locally, it's not at all obvious that it
> works right.

The old do_notify_resume work loop worked more or less the same way,
so we should be okay here.  See below.

>
> So wouldn't it be much nicer to do:
>
>         u32 cached_flags = READ_ONCE(pt_regs_to_thread_info(regs)->flags);
>
>         cached_flags &= _TIF_SIGPENDING | _TIF_NOTIFY_RESUME |
> _TIF_USER_RETURN_NOTIFY | _TIF_UPROBE | _TIF_NEED_RESCHED;
>
>         if (!cached_flags)
>                 break;
>
>         atomic_clear_mask(cached_flags, &pt_regs_to_thread_info(regs)->flags);
>
> and then have those bit tests after that?
>

Yes, but it would be a slowdown unless we converted all the various
handlers stopped clearing the bits separately (two atomics instead of
one).  And to do that, we'd probably want to change all the arches,

Signal handling has all the recalc_sigpending stuff.  schedule() had
better clear TIF_NEED_RESCHED.  fire_user_return_notifiers is totally
absurd but it does clear the bit.  uprobes clears the bit directly.

I'd be all for changing this, but coordinating with the generic code
could be annoying.

--Andy

  reply index

Thread overview: 70+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-07-03 19:44 [PATCH v5 00/17] x86: Rewrite exit-to-userspace code Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 01/17] selftests/x86: Add a test for 32-bit fast syscall arg faults Andy Lutomirski
2015-07-07 10:49   ` [tip:x86/asm] x86/entry, " tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 02/17] x86/entry/64/compat: Fix bad fast syscall arg failure path Andy Lutomirski
2015-07-07 10:49   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 03/17] uml: Fix do_signal() prototype Andy Lutomirski
2015-07-07 10:49   ` [tip:x86/asm] um: " tip-bot for Ingo Molnar
2015-07-03 19:44 ` [PATCH v5 04/17] context_tracking: Add ct_state and CT_WARN_ON Andy Lutomirski
2015-07-07 10:50   ` [tip:x86/asm] context_tracking: Add ct_state() and CT_WARN_ON() tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 05/17] notifiers: Assert that RCU is watching in notify_die Andy Lutomirski
2015-07-07 10:50   ` [tip:x86/asm] notifiers, RCU: Assert that RCU is watching in notify_die() tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 06/17] x86: Move C entry and exit code to arch/x86/entry/common.c Andy Lutomirski
2015-07-07 10:50   ` [tip:x86/asm] x86/entry: Move C entry and exit code to arch/x86/ entry/common.c tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 07/17] x86/traps: Assert that we're in CONTEXT_KERNEL in exception entries Andy Lutomirski
2015-07-07 10:51   ` [tip:x86/asm] x86/traps, context_tracking: Assert that we' re " tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 08/17] x86/entry: Add enter_from_user_mode and use it in syscalls Andy Lutomirski
2015-07-07 10:51   ` [tip:x86/asm] x86/entry: Add enter_from_user_mode() " tip-bot for Andy Lutomirski
2015-07-14 23:00     ` Frederic Weisbecker
2015-07-14 23:04       ` Andy Lutomirski
2015-07-14 23:28         ` Frederic Weisbecker
2015-12-21 20:50   ` [PATCH v5 08/17] x86/entry: Add enter_from_user_mode " Sasha Levin
2015-12-21 22:44     ` Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 09/17] x86/entry: Add new, comprehensible entry and exit hooks Andy Lutomirski
2015-07-07 10:51   ` [tip:x86/asm] x86/entry: Add new, comprehensible entry and exit handlers written in C tip-bot for Andy Lutomirski
2015-07-14 23:07     ` Frederic Weisbecker
2015-07-15 19:56       ` Linus Torvalds
2015-07-15 20:46         ` Andy Lutomirski [this message]
2015-07-15 21:25           ` [PATCH] x86/entry: Fix _TIF_USER_RETURN_NOTIFY check in prepare_exit_to_usermode Andy Lutomirski
2015-07-18  3:25             ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 10/17] x86/entry/64: Really create an error-entry-from-usermode code path Andy Lutomirski
2015-07-07 10:52   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 11/17] x86/entry/64: Migrate 64-bit and compat syscalls to new exit hooks Andy Lutomirski
2015-07-07 10:52   ` [tip:x86/asm] x86/entry/64: Migrate 64-bit and compat syscalls to the new exit handlers and remove old assembly code tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 12/17] x86/asm/entry/64: Save all regs on interrupt entry Andy Lutomirski
2015-07-07 10:52   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 13/17] x86/asm/entry/64: Simplify irq stack pt_regs handling Andy Lutomirski
2015-07-07 10:53   ` [tip:x86/asm] x86/asm/entry/64: Simplify IRQ " tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 14/17] x86/asm/entry/64: Migrate error and interrupt exit work to C Andy Lutomirski
2015-07-07 10:53   ` [tip:x86/asm] x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code tip-bot for Andy Lutomirski
2015-08-11 22:18     ` Frederic Weisbecker
2015-08-11 22:25       ` Andy Lutomirski
2015-08-11 22:49         ` Frederic Weisbecker
2015-08-11 22:59           ` Andy Lutomirski
2015-08-12  1:02             ` Paul E. McKenney
2015-08-12 13:13             ` Frederic Weisbecker
2015-08-11 22:38     ` Frederic Weisbecker
2015-08-11 22:51       ` Andy Lutomirski
2015-08-11 23:22         ` Frederic Weisbecker
2015-08-11 23:33           ` Andy Lutomirski
2015-08-12 13:32             ` Frederic Weisbecker
2015-08-12 14:59               ` Andy Lutomirski
2015-08-18 22:34                 ` Frederic Weisbecker
2015-08-18 22:40                   ` Andy Lutomirski
2015-08-19 17:18                     ` Frederic Weisbecker
2015-08-19 18:02                       ` Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 15/17] x86/entry: Remove exception_enter from most trap handlers Andy Lutomirski
2015-07-07 10:53   ` [tip:x86/asm] x86/entry: Remove exception_enter() " tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 16/17] x86/entry: Remove SCHEDULE_USER and asm/context-tracking.h Andy Lutomirski
2015-07-07 10:54   ` [tip:x86/asm] x86/entry: Remove SCHEDULE_USER and asm/ context-tracking.h tip-bot for Andy Lutomirski
2015-07-03 19:44 ` [PATCH v5 17/17] x86/irq: Document how IRQ context tracking works and add an assertion Andy Lutomirski
2015-07-07 10:54   ` [tip:x86/asm] x86/irq, context_tracking: Document how IRQ context tracking works and add an RCU assertion tip-bot for Andy Lutomirski
2015-07-14 23:26     ` Frederic Weisbecker
2015-07-14 23:33       ` Andy Lutomirski
2015-07-18 13:23         ` Frederic Weisbecker
2015-07-18 14:10           ` Paul E. McKenney
2015-07-07 11:12 ` [PATCH v5 00/17] x86: Rewrite exit-to-userspace code Ingo Molnar
2015-07-07 16:03   ` Andy Lutomirski
2015-07-07 17:55     ` [PATCH] x86/entry/64: Fix warning on compat syscalls with CONFIG_AUDITSYSCALL=n Andy Lutomirski
2015-07-08  9:57       ` [tip:x86/asm] x86/entry/64: Fix IRQ state confusion and related warning on compat syscalls with CONFIG_AUDITSYSCALL =n tip-bot for Andy Lutomirski
2015-07-08 19:12       ` tip-bot for Andy Lutomirski

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