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

    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 <>
    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();


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