Re: [RFC v3 2/2] arm64: kprobes: Allow reentering kprobes while single-stepping

[Pu Hu <hupu@transsion.com>]

On 7/10/2026 5:38 PM, Masami Hiramatsu wrote:
> On Fri, 10 Jul 2026 06:32:55 +0000
> Pu Hu <hupu@transsion.com> wrote:
> 
>> From: Pu Hu <hupu@transsion.com>
>>
>> A kprobe can be hit while another kprobe is in KPROBE_HIT_SS state. This
>> can happen when tracing or perf code runs from the debug exception path
>> while the first kprobe is preparing or executing its out-of-line
>> single-step instruction.
>>
>> Currently arm64 treats a kprobe hit in KPROBE_HIT_SS as unrecoverable,
>> the same as a hit in KPROBE_REENTER. This is too strict. A hit in
>> KPROBE_HIT_SS is still a one-level reentry and can be handled by saving
>> the current kprobe state and setting up single-step for the new probe,
>> just like reentry from KPROBE_HIT_ACTIVE or KPROBE_HIT_SSDONE.
>>
>> The truly unrecoverable case is hitting another kprobe while already in
>> KPROBE_REENTER, because the reentry save area has already been consumed.
>>
>> Move KPROBE_HIT_SS to the recoverable reentry cases and leave
>> KPROBE_REENTER as the unrecoverable nested reentry case.
>>
>> This change also requires saving saved_irqflag in struct prev_kprobe.
>> When a nested kprobe calls kprobes_save_local_irqflag(), it overwrites
>> kcb->saved_irqflag with the currently masked DAIF value, losing the
>> outer kprobe's original DAIF state. Without this fix, when the outer
>> kprobe's single-step finishes, kprobes_restore_local_irqflag() applies
>> the wrong DAIF mask and leaves interrupts permanently disabled.
>>
>> Extend struct prev_kprobe with a saved_irqflag field and save/restore it
>> alongside kp and status. This ensures the outer kprobe's original
>> interrupt state is preserved across reentry.
>>
>> This mirrors the x86 fix in commit 6a5022a56ac3
>> ("kprobes/x86: Allow to handle reentered kprobe on single-stepping").
>>
> 
> OK, this looks good to me.
> 
> Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> for this series.
> 
> Will, Catalin, can you pick this series?
> 
> Thanks!
> 

Hi Will, Catalin,

Just a gentle ping on this series.

Masami has kindly reviewed it and provided his Reviewed-by tag.
I was wondering if you had a chance to take a look, or if there is
anything else I should address before it can be picked up.

No rush, and apologies for the reminder if this is already on your radar.

Thanks a lot for your time!

Thanks,
Pu Hu