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From: Frederic Weisbecker <fweisbec@gmail.com>
To: keescook@chromium.org, peterz@infradead.org,
	vda.linux@googlemail.com, mingo@kernel.org, brgerst@gmail.com,
	luto@amacapital.net, torvalds@linux-foundation.org, bp@alien8.de,
	luto@kernel.org, oleg@redhat.com, hpa@zytor.com,
	linux-kernel@vger.kernel.org, dvlasenk@redhat.com,
	tglx@linutronix.de, riel@redhat.com
Cc: 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 01:07:27 +0200
Message-ID: <20150714230727.GD29441@lerouge> (raw)
In-Reply-To: <tip-c5c46f59e4e7c1ab244b8d38f2b61d317df90bba@git.kernel.org>

On Tue, Jul 07, 2015 at 03:51:48AM -0700, tip-bot for Andy Lutomirski wrote:
> Commit-ID:  c5c46f59e4e7c1ab244b8d38f2b61d317df90bba
> Gitweb:     http://git.kernel.org/tip/c5c46f59e4e7c1ab244b8d38f2b61d317df90bba
> Author:     Andy Lutomirski <luto@kernel.org>
> AuthorDate: Fri, 3 Jul 2015 12:44:26 -0700
> Committer:  Ingo Molnar <mingo@kernel.org>
> CommitDate: Tue, 7 Jul 2015 10:59:06 +0200
> 
> x86/entry: Add new, comprehensible entry and exit handlers written in C
> 
> The current x86 entry and exit code, written in a mixture of assembly and
> C code, is incomprehensible due to being open-coded in a lot of places
> without coherent documentation.
> 
> It appears to work primary by luck and duct tape: i.e. obvious runtime
> failures were fixed on-demand, without re-thinking the design.
> 
> Due to those reasons our confidence level in that code is low, and it is
> very difficult to incrementally improve.
> 
> Add new code written in C, in preparation for simply deleting the old
> entry code.
> 
> prepare_exit_to_usermode() is a new function that will handle all
> slow path exits to user mode.  It is called with IRQs disabled
> and it leaves us in a state in which it is safe to immediately
> return to user mode.  IRQs must not be re-enabled at any point
> after prepare_exit_to_usermode() returns and user mode is actually
> entered. (We can, of course, fail to enter user mode and treat
> that failure as a fresh entry to kernel mode.)
> 
> All callers of do_notify_resume() will be migrated to call
> prepare_exit_to_usermode() instead; prepare_exit_to_usermode() needs
> to do everything that do_notify_resume() does today, but it also
> takes care of scheduling and context tracking.  Unlike
> do_notify_resume(), it does not need to be called in a loop.
> 
> syscall_return_slowpath() is exactly what it sounds like: it will
> be called on any syscall exit slow path. It will replace
> syscall_trace_leave() and it calls prepare_exit_to_usermode() on the
> way out.
> 
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: Denys Vlasenko <vda.linux@googlemail.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: paulmck@linux.vnet.ibm.com
> Link: http://lkml.kernel.org/r/c57c8b87661a4152801d7d3786eac2d1a2f209dd.1435952415.git.luto@kernel.org
> [ Improved the changelog a bit. ]
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
>  arch/x86/entry/common.c | 112 +++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 111 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 9a327ee..febc530 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -207,6 +207,7 @@ long syscall_trace_enter(struct pt_regs *regs)
>  		return syscall_trace_enter_phase2(regs, arch, phase1_result);
>  }
>  
> +/* Deprecated. */
>  void syscall_trace_leave(struct pt_regs *regs)
>  {
>  	bool step;
> @@ -237,8 +238,117 @@ void syscall_trace_leave(struct pt_regs *regs)
>  	user_enter();
>  }
>  
> +static struct thread_info *pt_regs_to_thread_info(struct pt_regs *regs)
> +{
> +	unsigned long top_of_stack =
> +		(unsigned long)(regs + 1) + TOP_OF_KERNEL_STACK_PADDING;
> +	return (struct thread_info *)(top_of_stack - THREAD_SIZE);
> +}
> +
> +/* Called with IRQs disabled. */
> +__visible void prepare_exit_to_usermode(struct pt_regs *regs)
> +{
> +	if (WARN_ON(!irqs_disabled()))
> +		local_irq_disable();
> +
> +	/*
> +	 * In order to return to user mode, we need to have IRQs off with
> +	 * none of _TIF_SIGPENDING, _TIF_NOTIFY_RESUME, _TIF_USER_RETURN_NOTIFY,
> +	 * _TIF_UPROBE, or _TIF_NEED_RESCHED set.  Several of these flags
> +	 * can be set at any time on preemptable kernels if we have IRQs on,
> +	 * so we need to loop.  Disabling preemption wouldn't help: doing the
> +	 * work to clear some of the flags can sleep.
> +	 */
> +	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!

> +
> +	user_enter();

So now we are sure that we have only one call to user_enter() before
resuming userspace, once we've completed everything, rescheduling, signals,
etc... No more context tracking hacky round on signals and rescheduling?

That's great. I need to check if other archs still need schedule_user().

Thanks a lot!

  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 [this message]
2015-07-15 19:56       ` Linus Torvalds
2015-07-15 20:46         ` Andy Lutomirski
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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