Re: [PATCH 2/4] arm64: Extract kprobes_save_local_irqflag() and kprobes_restore_local_irqflag()

[Doug Anderson]

Hi,

On Sat, May 9, 2020 at 6:49 AM Wei Li <liwei391@xxxxxxxxxx> wrote:
>
> PSTATE.I and PSTATE.D are very important for single-step working.
>
> Without disabling interrupt on local CPU, there is a chance of
> interrupt occurrence in the period of exception return and start of
> out-of-line single-step, that result in wrongly single stepping
> into the interrupt handler. And if D bit is set then, it results into
> undefined exception and when it's handler enables dbg then single-step
> exception is generated, not as expected.
>
> As they are maintained well in kprobes_save_local_irqflag() and
> kprobes_restore_local_irqflag() for kprobe module, extract them as
> kernel_prepare_single_step() and kernel_cleanup_single_step() for
> general use.
>
> Signed-off-by: Wei Li <liwei391@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/debug-monitors.h |  4 ++++
>  arch/arm64/kernel/debug-monitors.c      | 26 +++++++++++++++++++++++
>  arch/arm64/kernel/probes/kprobes.c      | 28 ++-----------------------
>  3 files changed, 32 insertions(+), 26 deletions(-)
>
> diff --git a/arch/arm64/include/asm/debug-monitors.h b/arch/arm64/include/asm/debug-monitors.h
> index 7619f473155f..b62469f3475b 100644
> --- a/arch/arm64/include/asm/debug-monitors.h
> +++ b/arch/arm64/include/asm/debug-monitors.h
> @@ -113,6 +113,10 @@ void user_fastforward_single_step(struct task_struct *task);
>  void kernel_enable_single_step(struct pt_regs *regs);
>  void kernel_disable_single_step(void);
>  int kernel_active_single_step(void);
> +void kernel_prepare_single_step(unsigned long *flags,
> +                                               struct pt_regs *regs);
> +void kernel_cleanup_single_step(unsigned long flags,
> +                                               struct pt_regs *regs);
>
>  #ifdef CONFIG_HAVE_HW_BREAKPOINT
>  int reinstall_suspended_bps(struct pt_regs *regs);
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index 48222a4760c2..25ce6b5a52d2 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -429,6 +429,32 @@ int kernel_active_single_step(void)
>  }
>  NOKPROBE_SYMBOL(kernel_active_single_step);
>
> +/*
> + * Interrupts need to be disabled before single-step mode is set, and not
> + * reenabled until after single-step mode ends.
> + * Without disabling interrupt on local CPU, there is a chance of
> + * interrupt occurrence in the period of exception return and  start of
> + * out-of-line single-step, that result in wrongly single stepping
> + * into the interrupt handler.
> + */
> +void kernel_prepare_single_step(unsigned long *flags,
> +                                               struct pt_regs *regs)
> +{
> +       *flags = regs->pstate & DAIF_MASK;
> +       regs->pstate |= PSR_I_BIT;
> +       /* Unmask PSTATE.D for enabling software step exceptions. */
> +       regs->pstate &= ~PSR_D_BIT;
> +}
> +NOKPROBE_SYMBOL(kernel_prepare_single_step);

nit: why not just return unsigned long rather than passing by reference?


> +
> +void kernel_cleanup_single_step(unsigned long flags,
> +                                               struct pt_regs *regs)
> +{
> +       regs->pstate &= ~DAIF_MASK;
> +       regs->pstate |= flags;
> +}
> +NOKPROBE_SYMBOL(kernel_cleanup_single_step);
> +
>  /* ptrace API */
>  void user_enable_single_step(struct task_struct *task)
>  {
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index d1c95dcf1d78..c655b6b543e3 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -168,30 +168,6 @@ static void __kprobes set_current_kprobe(struct kprobe *p)
>         __this_cpu_write(current_kprobe, p);
>  }
>
> -/*
> - * Interrupts need to be disabled before single-step mode is set, and not
> - * reenabled until after single-step mode ends.
> - * Without disabling interrupt on local CPU, there is a chance of
> - * interrupt occurrence in the period of exception return and  start of
> - * out-of-line single-step, that result in wrongly single stepping
> - * into the interrupt handler.
> - */
> -static void __kprobes kprobes_save_local_irqflag(struct kprobe_ctlblk *kcb,
> -                                               struct pt_regs *regs)
> -{
> -       kcb->saved_irqflag = regs->pstate & DAIF_MASK;
> -       regs->pstate |= PSR_I_BIT;
> -       /* Unmask PSTATE.D for enabling software step exceptions. */
> -       regs->pstate &= ~PSR_D_BIT;
> -}
> -
> -static void __kprobes kprobes_restore_local_irqflag(struct kprobe_ctlblk *kcb,
> -                                               struct pt_regs *regs)
> -{
> -       regs->pstate &= ~DAIF_MASK;
> -       regs->pstate |= kcb->saved_irqflag;
> -}
> -
>  static void __kprobes
>  set_ss_context(struct kprobe_ctlblk *kcb, unsigned long addr)
>  {
> @@ -227,7 +203,7 @@ static void __kprobes setup_singlestep(struct kprobe *p,
>                 set_ss_context(kcb, slot);      /* mark pending ss */
>
>                 /* IRQs and single stepping do not mix well. */
> -               kprobes_save_local_irqflag(kcb, regs);
> +               kernel_prepare_single_step(&kcb->saved_irqflag, regs);

Is there some reason to have two functions?  It seems like every time
you call kernel_enable_single_step() you'd want to call
kernel_prepare_single_step().  ...and every time you call
kernel_disable_single_step() you'd want to call
kernel_cleanup_single_step().

Maybe you can just add the flags parameter to
kernel_enable_single_step() / kernel_disable_single_step() and put the
code in there?


-Doug