From: Petr Mladek <pmladek@suse.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Jessica Yu <jeyu@redhat.com>, Jiri Kosina <jikos@kernel.org>,
Miroslav Benes <mbenes@suse.cz>, Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Heiko Carstens <heiko.carstens@de.ibm.com>,
live-patching@vger.kernel.org, linux-kernel@vger.kernel.org,
x86@kernel.org, linuxppc-dev@lists.ozlabs.org,
linux-s390@vger.kernel.org, Vojtech Pavlik <vojtech@suse.com>,
Jiri Slaby <jslaby@suse.cz>,
Chris J Arges <chris.j.arges@canonical.com>,
Andy Lutomirski <luto@kernel.org>
Subject: Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model
Date: Thu, 5 May 2016 13:21:32 +0200 [thread overview]
Message-ID: <20160505112132.GX2749@pathway.suse.cz> (raw)
In-Reply-To: <20160504172517.tdatoj2nlkqwyd4g@treble>
On Wed 2016-05-04 12:25:17, Josh Poimboeuf wrote:
> On Wed, May 04, 2016 at 04:12:05PM +0200, Petr Mladek wrote:
> > On Wed 2016-05-04 14:39:40, Petr Mladek wrote:
> > > *
> > > * Note that the task must never be migrated to the target
> > > * state when being inside this ftrace handler.
> > > */
> > >
> > > We might want to move the second paragraph on top of the function.
> > > It is a basic and important fact. It actually explains why the first
> > > read barrier is not needed when the patch is being disabled.
> >
> > I wrote the statement partly intuitively. I think that it is really
> > somehow important. And I am slightly in doubts if we are on the safe side.
> >
> > First, why is it important that the task->patch_state is not switched
> > when being inside the ftrace handler?
> >
> > If we are inside the handler, we are kind-of inside the called
> > function. And the basic idea of this consistency model is that
> > we must not switch a task when it is inside a patched function.
> > This is normally decided by the stack.
> >
> > The handler is a bit special because it is called right before the
> > function. If it was the only patched function on the stack, it would
> > not matter if we choose the new or old code. Both decisions would
> > be safe for the moment.
> >
> > The fun starts when the function calls another patched function.
> > The other patched function must be called consistently with
> > the first one. If the first function was from the patch,
> > the other must be from the patch as well and vice versa.
> >
> > This is why we must not switch task->patch_state dangerously
> > when being inside the ftrace handler.
> >
> > Now I am not sure if this condition is fulfilled. The ftrace handler
> > is called as the very first instruction of the function. Does not
> > it break the stack validity? Could we sleep inside the ftrace
> > handler? Will the patched function be detected on the stack?
> >
> > Or is my brain already too far in the fantasy world?
>
> I think this isn't a possibility.
>
> In today's code base, this can't happen because task patch states are
> only switched when sleeping or when exiting the kernel. The ftrace
> handler doesn't sleep directly.
>
> If it were preempted, it couldn't be switched there either because we
> consider preempted stacks to be unreliable.
This was the missing piece.
> In theory, a DWARF stack trace of a preempted task *could* be reliable.
> But then the DWARF unwinder should be smart enough to see that the
> original function called the ftrace handler. Right? So the stack would
> be reliable, but then livepatch would see the original function on the
> stack and wouldn't switch the task.
>
> Does that make sense?
Yup. I think that we are on the safe side. Thanks for explanation.
Best Regards,
Petr
next prev parent reply other threads:[~2016-05-05 11:21 UTC|newest]
Thread overview: 118+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-28 20:44 [RFC PATCH v2 00/18] livepatch: hybrid consistency model Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 01/18] x86/asm/head: clean up initial stack variable Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 02/18] x86/asm/head: use a common function for starting CPUs Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks Josh Poimboeuf
2016-04-29 18:46 ` Brian Gerst
2016-04-29 20:28 ` Josh Poimboeuf
2016-04-29 19:39 ` Andy Lutomirski
2016-04-29 20:50 ` Josh Poimboeuf
2016-04-29 21:38 ` Andy Lutomirski
2016-04-29 23:27 ` Josh Poimboeuf
2016-04-30 0:10 ` Andy Lutomirski
2016-04-28 20:44 ` [RFC PATCH v2 04/18] x86: move _stext marker before head code Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Josh Poimboeuf
2016-04-29 18:06 ` Andy Lutomirski
2016-04-29 20:11 ` Josh Poimboeuf
2016-04-29 20:19 ` Andy Lutomirski
2016-04-29 20:27 ` Josh Poimboeuf
2016-04-29 20:32 ` Andy Lutomirski
2016-04-29 21:25 ` Josh Poimboeuf
2016-04-29 21:37 ` Andy Lutomirski
2016-04-29 22:11 ` Jiri Kosina
2016-04-29 22:57 ` Josh Poimboeuf
2016-04-30 0:09 ` Andy Lutomirski
2016-04-29 22:41 ` Josh Poimboeuf
2016-04-30 0:08 ` Andy Lutomirski
2016-05-02 13:52 ` Josh Poimboeuf
2016-05-02 15:52 ` Andy Lutomirski
2016-05-02 17:31 ` Josh Poimboeuf
2016-05-02 18:12 ` Andy Lutomirski
2016-05-02 18:34 ` Ingo Molnar
2016-05-02 19:44 ` Josh Poimboeuf
2016-05-02 19:54 ` Jiri Kosina
2016-05-02 20:00 ` Jiri Kosina
2016-05-03 0:39 ` Andy Lutomirski
2016-05-04 15:16 ` David Laight
2016-05-19 23:15 ` Josh Poimboeuf
2016-05-19 23:39 ` Andy Lutomirski
2016-05-20 14:05 ` Josh Poimboeuf
2016-05-20 15:41 ` Andy Lutomirski
2016-05-20 16:41 ` Josh Poimboeuf
2016-05-20 16:59 ` Andy Lutomirski
2016-05-20 17:49 ` Josh Poimboeuf
2016-05-23 23:02 ` Jiri Kosina
2016-05-24 1:42 ` Andy Lutomirski
2016-05-23 21:34 ` Andy Lutomirski
2016-05-24 2:28 ` Josh Poimboeuf
2016-05-24 3:52 ` Andy Lutomirski
2016-06-22 16:30 ` Josh Poimboeuf
2016-06-22 17:59 ` Andy Lutomirski
2016-06-22 18:22 ` Josh Poimboeuf
2016-06-22 18:26 ` Andy Lutomirski
2016-06-22 18:40 ` Josh Poimboeuf
2016-06-22 19:17 ` Andy Lutomirski
2016-06-23 16:19 ` Josh Poimboeuf
2016-06-23 16:35 ` Andy Lutomirski
2016-06-23 18:31 ` Josh Poimboeuf
2016-06-23 20:40 ` Josh Poimboeuf
2016-06-23 22:00 ` Andy Lutomirski
2016-06-23 0:09 ` Andy Lutomirski
2016-06-23 15:55 ` Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 06/18] x86: dump_trace() error handling Josh Poimboeuf
2016-04-29 13:45 ` Minfei Huang
2016-04-29 14:00 ` Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 07/18] stacktrace/x86: function for detecting reliable stack traces Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 08/18] livepatch: temporary stubs for klp_patch_pending() and klp_patch_task() Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 09/18] livepatch/x86: add TIF_PATCH_PENDING thread flag Josh Poimboeuf
2016-04-29 18:08 ` Andy Lutomirski
2016-04-29 20:18 ` Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 10/18] livepatch/powerpc: " Josh Poimboeuf
2016-05-03 9:07 ` Petr Mladek
2016-05-03 12:06 ` Miroslav Benes
2016-04-28 20:44 ` [RFC PATCH v2 11/18] livepatch/s390: reorganize TIF thread flag bits Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 12/18] livepatch/s390: add TIF_PATCH_PENDING thread flag Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 13/18] livepatch: separate enabled and patched states Josh Poimboeuf
2016-05-03 9:30 ` Petr Mladek
2016-05-03 13:48 ` Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 14/18] livepatch: remove unnecessary object loaded check Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 15/18] livepatch: move patching functions into patch.c Josh Poimboeuf
2016-05-03 9:39 ` Petr Mladek
2016-04-28 20:44 ` [RFC PATCH v2 16/18] livepatch: store function sizes Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Josh Poimboeuf
2016-05-04 8:42 ` Petr Mladek
2016-05-04 15:51 ` Josh Poimboeuf
2016-05-05 9:41 ` Miroslav Benes
2016-05-05 13:06 ` Petr Mladek
2016-05-04 12:39 ` barriers: was: " Petr Mladek
2016-05-04 13:53 ` Peter Zijlstra
2016-05-04 16:51 ` Josh Poimboeuf
2016-05-04 14:12 ` Petr Mladek
2016-05-04 17:25 ` Josh Poimboeuf
2016-05-05 11:21 ` Petr Mladek [this message]
2016-05-09 15:42 ` Miroslav Benes
2016-05-04 17:02 ` Josh Poimboeuf
2016-05-05 10:21 ` Petr Mladek
2016-05-04 14:48 ` klp_task_patch: " Petr Mladek
2016-05-04 14:56 ` Jiri Kosina
2016-05-04 17:57 ` Josh Poimboeuf
2016-05-05 11:57 ` Petr Mladek
2016-05-06 12:38 ` Josh Poimboeuf
2016-05-09 12:23 ` Petr Mladek
2016-05-16 18:12 ` Josh Poimboeuf
2016-05-18 13:12 ` Petr Mladek
2016-05-06 11:33 ` Petr Mladek
2016-05-06 12:44 ` Josh Poimboeuf
2016-05-09 9:41 ` Miroslav Benes
2016-05-16 17:27 ` Josh Poimboeuf
2016-05-10 11:39 ` Miroslav Benes
2016-05-17 22:53 ` Jessica Yu
2016-05-18 8:16 ` Jiri Kosina
2016-05-18 16:51 ` Josh Poimboeuf
2016-05-18 20:22 ` Jiri Kosina
2016-05-23 9:42 ` David Laight
2016-05-23 18:44 ` Jiri Kosina
2016-05-24 15:06 ` David Laight
2016-05-24 22:45 ` Jiri Kosina
2016-06-06 13:54 ` [RFC PATCH v2 17/18] " Petr Mladek
2016-06-06 14:29 ` Josh Poimboeuf
2016-04-28 20:44 ` [RFC PATCH v2 18/18] livepatch: add /proc/<pid>/patch_state Josh Poimboeuf
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