From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: paulmck <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>, x86 <x86@kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
rostedt <rostedt@goodmis.org>,
Masami Hiramatsu <mhiramat@kernel.org>,
bristot <bristot@redhat.com>, jbaron <jbaron@akamai.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>, Nadav Amit <namit@vmware.com>,
"H. Peter Anvin" <hpa@zytor.com>,
Andy Lutomirski <luto@kernel.org>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Jessica Yu <jeyu@kernel.org>,
Alexei Starovoitov <alexei.starovoitov@gmail.com>
Subject: Re: [PATCH -v5 12/17] x86/kprobes: Fix ordering
Date: Thu, 14 Nov 2019 10:22:24 -0500 (EST) [thread overview]
Message-ID: <135240750.136.1573744944568.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20191114151323.GK2865@paulmck-ThinkPad-P72>
----- On Nov 14, 2019, at 10:13 AM, paulmck paulmck@kernel.org wrote:
> On Thu, Nov 14, 2019 at 10:06:17AM -0500, Mathieu Desnoyers wrote:
>> ----- On Nov 14, 2019, at 8:53 AM, Peter Zijlstra peterz@infradead.org wrote:
>>
>> > On Wed, Nov 13, 2019 at 10:42:32AM -0500, Mathieu Desnoyers wrote:
>> >> ----- On Nov 11, 2019, at 8:13 AM, Peter Zijlstra peterz@infradead.org wrote:
>> >>
>> >> > Kprobes does something like:
>> >> >
>> >> > register:
>> >> > arch_arm_kprobe()
>> >> > text_poke(INT3)
>> >> > /* guarantees nothing, INT3 will become visible at some point, maybe */
>> >> >
>> >> > kprobe_optimizer()
>> >> > /* guarantees the bytes after INT3 are unused */
>> >> > syncrhonize_rcu_tasks();
>> >>
>> >> syncrhonize -> synchronize
>> >
>> > Fixed.
>> >
>> >> > text_poke_bp(JMP32);
>> >> > /* implies IPI-sync, kprobe really is enabled */
>> >> >
>> >> >
>> >> > unregister:
>> >> > __disarm_kprobe()
>> >> > unoptimize_kprobe()
>> >> > text_poke_bp(INT3 + tail);
>> >> > /* implies IPI-sync, so tail is guaranteed visible */
>> >> > arch_disarm_kprobe()
>> >> > text_poke(old);
>> >> > /* guarantees nothing, old will maybe become visible */
>> >> >
>> >> > synchronize_rcu()
>> >> >
>> >> > free-stuff
>> >> >
>> >> > Now the problem is that on register, the synchronize_rcu_tasks() does
>> >> > not imply sufficient to guarantee all CPUs have already observed INT3
>> >> > (although in practise this is exceedingly unlikely not to have
>> >>
>> >> practise -> practice
>> >
>> > And fixed.
>> >
>> >> > happened) (similar to how MEMBARRIER_CMD_PRIVATE_EXPEDITED does not
>> >> > imply MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE).
>> >> >
>> >> > Worse, even if it did, we'd have to do 2 synchronize calls to provide
>> >> > the guarantee we're looking for, the first to ensure INT3 is visible,
>> >> > the second to guarantee nobody is then still using the instruction
>> >> > bytes after INT3.
>> >>
>> >> I'm not entirely convinced about this last statement though. AFAIU:
>> >>
>> >> - Swapping between some instruction and INT3 is OK,
>> >> - Swapping back between that INT3 and _original_ instruction is OK,
>> >> - Anything else needs to have explicit core serialization.
>> >
>> > So far, agreed.
>> >
>> >> So I understand the part about requiring the synchronize call to guarantee
>> >> that nobody is then still using the instruction bytes following INT3, but not
>> >> the rationale for the first synchronization. What operation would theoretically
>> >> follow this first synchronize call ?
>> >
>> > I'm not completely sure what you're asking, so I'm going to explain too
>> > much and hope I answered your question along the way. If not, please be
>> > a little more specific.
>> >
>> > First:
>> >
>> > So what can happen with optimized kprobes is that the original
>> > instruction is <5 bytes and we want to write a JMP.d32 (5 bytes).
>> > Something like:
>> >
>> > 83e: 48 89 e5 mov %rsp,%rbp
>> > 841: 48 85 c0 test %rax,%rax
>> >
>> > And we put a kprobe on the MOV. Then we poke INT3 at 0x83e and IPI-sync.
>> > At that point the kprobe is active:
>> >
>> > 83e: cc 89 e5 int3 ; 2 byte garbage
>> > 841: 48 85 c0 test %rax,%rax
>> >
>> > Now we want to optimize that thing. But imagine a task being preempted
>> > at 0x841. If we poke in the JMP.d32 the above turns into gibberish
>> >
>> > 83e: e9 12 34 56 78 jmp +0x12345678
>> >
>> > Then our task resumes, loads the instruction at 0x841, which then reads:
>> >
>> > 841: 56 78 c0
>> >
>> > And goes *bang*.
>>
>> Thanks for the reminder. I somehow forgot that optimized kprobes covered
>> instructions smaller than 5 bytes.
>>
>> >
>> > So what we do, after enabling the regular kprobe, is call
>> > synchronize_rcu_tasks() to wait for each task to have passed through
>> > schedule(). That guarantees no task is preempted inside the kprobe
>> > shadow (when it triggers it ensures it resumes execution at an
>> > instruction boundary further than 5 bytes away).
>>
>> Indeed, given that synchronize_rcu_tasks() awaits for voluntary context
>> switches (or user-space execution), it guarantees that no task was preempted
>> within the kprobe shadow.
>>
>> Considering that synchronize_rcu_tasks() is meant only for code rewriting,
>> I wonder if it would make sense to include the core serializing guarantees
>> within this RCU API ?
>
> As in have synchronize_rcu_tasks() do the IPI-sync love before doing
> the current wait-for-voluntary-context-switch work?
This is what I have in mind, yes, based on the assumption that the only
intended use-case for synchronize_rcu_tasks() is code patching.
> I don't know of
> any objection to that approach. Certainly there is no possible
> argument based on latency. ;-)
Indeed! :)
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2019-11-14 15:22 UTC|newest]
Thread overview: 106+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-11 13:12 [PATCH -v5 00/17] Rewrite x86/ftrace to use text_poke (and more) Peter Zijlstra
2019-11-11 13:12 ` [PATCH -v5 01/17] x86/alternatives: Teach text_poke_bp() to emulate instructions Peter Zijlstra
2019-11-15 9:43 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-11-11 13:12 ` [PATCH -v5 02/17] x86/alternatives: Update int3_emulate_push() comment Peter Zijlstra
2019-11-15 9:43 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:12 ` [PATCH -v5 03/17] x86/alternatives,jump_label: Provide better text_poke() batching interface Peter Zijlstra
2019-11-15 9:43 ` [tip: core/kprobes] x86/alternatives, jump_label: " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:12 ` [PATCH -v5 04/17] x86/alternatives: Add and use text_gen_insn() helper Peter Zijlstra
2019-11-12 17:10 ` Steven Rostedt
2019-11-12 22:25 ` Peter Zijlstra
2019-11-15 9:43 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:12 ` [PATCH -v5 05/17] x86/ftrace: Use text_poke() Peter Zijlstra
2019-11-12 18:25 ` Steven Rostedt
2019-11-12 22:24 ` Peter Zijlstra
2019-11-12 22:48 ` Steven Rostedt
2019-11-13 9:01 ` Peter Zijlstra
2019-11-13 14:27 ` Steven Rostedt
2019-11-14 13:18 ` Peter Zijlstra
2019-11-14 13:56 ` Steven Rostedt
2019-11-14 14:05 ` Peter Zijlstra
2019-11-13 8:53 ` Peter Zijlstra
2019-11-15 9:43 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-11-16 20:46 ` Borislav Petkov
2019-11-18 17:35 ` [PATCH] x86/ftrace: Mark ftrace_modify_code_direct() __ref Borislav Petkov
2019-11-18 17:52 ` Steven Rostedt
2019-11-19 9:55 ` [tip: core/kprobes] " tip-bot2 for Borislav Petkov
2019-12-04 8:33 ` tip-bot2 for Borislav Petkov
2019-12-04 8:33 ` [tip: core/kprobes] x86/ftrace: Use text_poke() tip-bot2 for Peter Zijlstra
2019-11-11 13:12 ` [PATCH -v5 06/17] x86/mm: Remove set_kernel_text_r[ow]() Peter Zijlstra
2019-11-15 9:43 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:12 ` [PATCH -v5 07/17] x86/alternative: Add text_opcode_size() Peter Zijlstra
2019-11-15 9:43 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:13 ` [PATCH -v5 08/17] x86/ftrace: Use text_gen_insn() Peter Zijlstra
2019-11-15 9:43 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:13 ` [PATCH -v5 09/17] x86/alternative: Remove text_poke_loc::len Peter Zijlstra
2019-11-15 9:43 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:13 ` [PATCH -v5 10/17] x86/alternative: Shrink text_poke_loc Peter Zijlstra
2019-11-15 9:43 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:13 ` [PATCH -v5 11/17] x86/kprobes: Convert to text-patching.h Peter Zijlstra
2019-11-19 16:56 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:13 ` [PATCH -v5 12/17] x86/kprobes: Fix ordering Peter Zijlstra
2019-11-13 14:31 ` Paul E. McKenney
2019-11-13 15:42 ` Mathieu Desnoyers
2019-11-14 13:53 ` Peter Zijlstra
2019-11-14 15:06 ` Mathieu Desnoyers
2019-11-14 15:13 ` Paul E. McKenney
2019-11-14 15:22 ` Mathieu Desnoyers [this message]
2019-11-14 15:28 ` Peter Zijlstra
2019-11-14 15:30 ` Mathieu Desnoyers
2019-11-14 15:42 ` Peter Zijlstra
2019-11-14 15:58 ` Peter Zijlstra
2019-11-19 16:56 ` [tip: core/kprobes] x86/kprobes: Fix ordering while text-patching tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:13 ` [PATCH -v5 13/17] arm/ftrace: Use __patch_text_real() Peter Zijlstra
2019-11-11 16:47 ` Will Deacon
2019-11-11 17:19 ` Peter Zijlstra
2019-11-11 17:25 ` Peter Zijlstra
2019-11-12 11:29 ` Will Deacon
2019-11-13 9:26 ` [PATCH -v5mkII 13/17] arm/ftrace: Use __patch_text() Peter Zijlstra
2019-11-19 16:56 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2020-01-22 21:26 ` Dmitry Osipenko
2020-02-07 10:17 ` Peter Zijlstra
2020-02-07 10:26 ` Peter Zijlstra
2020-02-07 11:27 ` Peter Zijlstra
2020-02-07 16:47 ` Dmitry Osipenko
2020-01-08 12:22 ` [PATCH -v5mkII 13/17] " Arnd Bergmann
2020-01-08 14:16 ` Steven Rostedt
2020-01-08 14:22 ` Arnd Bergmann
2019-11-11 13:13 ` [PATCH -v5 14/17] module: Remove set_all_modules_text_*() Peter Zijlstra
2019-11-19 16:56 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:13 ` [PATCH -v5 15/17] ftrace: Rework event_create_dir() Peter Zijlstra
2019-11-14 14:07 ` Steven Rostedt
2019-11-19 16:56 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:13 ` [PATCH -v5 16/17] x86/kprobe: Add comments to arch_{,un}optimize_kprobes() Peter Zijlstra
2019-11-19 16:56 ` [tip: core/kprobes] " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 13:13 ` [PATCH -v5 17/17] x86/alternative: Use INT3_INSN_SIZE Peter Zijlstra
2019-11-19 16:56 ` [tip: core/kprobes] x86/alternatives: " tip-bot2 for Peter Zijlstra
2019-12-04 8:33 ` tip-bot2 for Peter Zijlstra
2019-11-11 19:47 ` [PATCH -v5 00/17] Rewrite x86/ftrace to use text_poke (and more) Alexei Starovoitov
2019-11-11 20:39 ` Peter Zijlstra
2019-11-11 20:42 ` Peter Zijlstra
2019-11-11 20:56 ` Alexei Starovoitov
2019-11-12 18:26 ` Steven Rostedt
2019-11-25 3:55 ` Masami Hiramatsu
2019-11-25 6:47 ` Masami Hiramatsu
2019-11-25 17:32 ` Steven Rostedt
2019-11-26 0:11 ` Masami Hiramatsu
2019-11-26 8:58 ` Masami Hiramatsu
2019-11-26 9:58 ` Masami Hiramatsu
2019-11-26 23:48 ` Masami Hiramatsu
[not found] ` <CAADnVQK4twuXzFhD-qLHmCVK0n1h-GDENQLu+4PVV3Hp++R6kQ@mail.gmail.com>
2019-11-27 4:32 ` Alexei Starovoitov
2019-11-27 5:01 ` Alexei Starovoitov
2019-11-27 6:41 ` Masami Hiramatsu
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