From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>,
Peter Zijlstra <peterz@infradead.org>,
Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
Anton Arapov <anton@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Roland McGrath <roland@hack.frob.com>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/7] ptrace: Partly fix set_task_blockstep()->update_debugctlmsr() logic
Date: Fri, 7 Sep 2012 20:44:26 +0530 [thread overview]
Message-ID: <20120907151426.GR30238@linux.vnet.ibm.com> (raw)
In-Reply-To: <20120903152609.GA9071@redhat.com>
* Oleg Nesterov <oleg@redhat.com> [2012-09-03 17:26:09]:
> Afaics the usage of update_debugctlmsr() and TIF_BLOCKSTEP in
> step.c was always very wrong.
>
> 1. update_debugctlmsr() was simply unneeded. The child sleeps
> TASK_TRACED, __switch_to_xtra(next_p => child) should notice
> TIF_BLOCKSTEP and set/clear DEBUGCTLMSR_BTF after resume if
> needed.
>
> 2. It is wrong. The state of DEBUGCTLMSR_BTF bit in CPU register
> should always match the state of current's TIF_BLOCKSTEP bit.
>
> 3. Even get_debugctlmsr() + update_debugctlmsr() itself does not
> look right. Irq can change other bits in MSR_IA32_DEBUGCTLMSR
> register or the caller can be preempted in between.
>
> 4. It is not safe to play with TIF_BLOCKSTEP if task != current.
> DEBUGCTLMSR_BTF and TIF_BLOCKSTEP should always match each
> other if the task is running. The tracee is stopped but it
> can be SIGKILL'ed right before set/clear_tsk_thread_flag().
>
> However, now that uprobes uses user_enable_single_step(current)
> we can't simply remove update_debugctlmsr(). So this patch adds
> the additional "task == current" check and disables irqs to avoid
> the race with interrupts/preemption.
>
> Unfortunately this patch doesn't solve the last problem, we need
> another fix. Probably we should teach ptrace_stop() to set/clear
> single/block stepping after resume.
>
> And afaics there is yet another problem: perf can play with
> MSR_IA32_DEBUGCTLMSR from nmi, this obviously means that even
> __switch_to_xtra() has problems.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
> arch/x86/kernel/step.c | 14 +++++++++++++-
> 1 files changed, 13 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
> index 7a51498..f89cdc6 100644
> --- a/arch/x86/kernel/step.c
> +++ b/arch/x86/kernel/step.c
> @@ -161,6 +161,16 @@ static void set_task_blockstep(struct task_struct *task, bool on)
> {
> unsigned long debugctl;
>
> + /*
> + * Ensure irq/preemption can't change debugctl in between.
> + * Note also that both TIF_BLOCKSTEP and debugctl should
> + * be changed atomically wrt preemption.
> + * FIXME: this means that set/clear TIF_BLOCKSTEP is simply
> + * wrong if task != current, SIGKILL can wakeup the stopped
> + * tracee and set/clear can play with the running task, this
> + * can confuse the next __switch_to_xtra().
> + */
> + local_irq_disable();
> debugctl = get_debugctlmsr();
> if (on) {
> debugctl |= DEBUGCTLMSR_BTF;
> @@ -169,7 +179,9 @@ static void set_task_blockstep(struct task_struct *task, bool on)
> debugctl &= ~DEBUGCTLMSR_BTF;
> clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
> }
> - update_debugctlmsr(debugctl);
> + if (task == current)
> + update_debugctlmsr(debugctl);
> + local_irq_enable();
> }
>
> /*
>
The changes look simple and neat. But I would prefer somebody with
better x86 knowledgde comment on this.
--
Thanks and Regards
Srikar
next prev parent reply other threads:[~2012-09-07 15:17 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-09-03 15:25 [PATCH 0/7] uprobes: single-step fixes Oleg Nesterov
2012-09-03 15:25 ` [PATCH 1/7] uprobes: Introduce arch_uprobe_enable/disable_step() Oleg Nesterov
2012-09-07 14:57 ` Srikar Dronamraju
2012-09-03 15:26 ` [PATCH 2/7] uprobes: x86: Implement x86 specific arch_uprobe_*_step Oleg Nesterov
2012-09-07 14:59 ` Srikar Dronamraju
2012-09-03 15:26 ` [PATCH 3/7] ptrace: Introduce set_task_blockstep() helper Oleg Nesterov
2012-09-07 15:00 ` Srikar Dronamraju
2012-09-03 15:26 ` [PATCH 4/7] ptrace: Partly fix set_task_blockstep()->update_debugctlmsr() logic Oleg Nesterov
2012-09-07 15:14 ` Srikar Dronamraju [this message]
2012-09-10 16:57 ` Sebastian Andrzej Siewior
2012-09-10 17:45 ` Peter Zijlstra
2012-09-10 17:27 ` Oleg Nesterov
2012-09-03 15:26 ` [PATCH 5/7] uprobes: Do not (ab)use TIF_SINGLESTEP/user_*_single_step() for single-stepping Oleg Nesterov
2012-09-07 15:11 ` Srikar Dronamraju
2012-09-07 15:50 ` Oleg Nesterov
2012-09-08 7:49 ` Srikar Dronamraju
2012-09-03 15:26 ` [PATCH 6/7] uprobes: Xol should send SIGTRAP if X86_EFLAGS_TF was set Oleg Nesterov
2012-09-12 12:08 ` Srikar Dronamraju
2012-09-12 14:45 ` Oleg Nesterov
2012-09-03 15:26 ` [PATCH 7/7] uprobes: Make arch_uprobe_task->saved_trap_nr "unsigned int" Oleg Nesterov
2012-09-12 12:27 ` Srikar Dronamraju
2012-09-08 17:06 ` [PATCH 0/7] uprobes: single-step fixes Oleg Nesterov
2012-09-12 12:33 ` Srikar Dronamraju
2012-09-08 17:06 ` [PATCH 8/7] uprobes: Fix arch_uprobe_disable_step() && UTASK_SSTEP_TRAPPED interaction Oleg Nesterov
2012-09-12 12:36 ` Srikar Dronamraju
2012-09-10 16:57 ` [PATCH 0/7] uprobes: single-step fixes Sebastian Andrzej Siewior
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