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    SubjectRe: [PATCH 4/7] ptrace: Partly fix set_task_blockstep()->update_debugctlmsr() logic
    * 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



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