linux-kernel - Re: [PATCH 2/4] arm64: Extract kprobes_save_local_irqflag() and kprobes_restore_local

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Date:   Sat, 16 May 2020 16:47:07 +0800
From:   "liwei (GF)" <liwei391@...wei.com>
To:     Doug Anderson <dianders@...omium.org>
CC:     Daniel Thompson <daniel.thompson@...aro.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        David Miller <davem@...emloft.net>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>, <liwei1412@....com>
Subject: Re: [PATCH 2/4] arm64: Extract kprobes_save_local_irqflag() and
 kprobes_restore_local_irqflag()

Hi Douglas,

On 2020/5/14 8:21, Doug Anderson wrote:
(SNIP)
>> +/*
>> + * 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?
Because i just extract this function from kprobes_save_local_irqflag(), i think
return unsigned long is fine.

> 
>> +
>> +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?
> 

As kernel_enable_single_step() / kernel_disable_single_step() are also called in
breakpoint_handler() and watchpoint_handler(), i am not sure it's a right thing
to put the daif flag prepare/cleanup into them, especially we don't have a context
to save the flags.

Thanks,
Wei