Re: [RFC v2 2/3] arm64: kprobes: Allow reentering kprobes while single-stepping

[Pu Hu <hupu@transsion.com>]

On 7/10/2026 1:09 PM, Masami Hiramatsu wrote:
> On Thu, 9 Jul 2026 14:22:25 +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 mirrors the x86 fix in commit 6a5022a56ac3
>> ("kprobes/x86: Allow to handle reentered kprobe on single-stepping").
>>
>
> Hi, as Sashiko commented, we have to save the saved_irqflag to
> prev_kprobbe.
>
> https://sashiko.dev/#/patchset/20260709142215.226872-1-hupu%40transsion.com?part=2
>
> Thank you,
>

Hi Masami,

We already added the saved_irqflag support in Patch 3 of this series:

   arm64: kprobes: Save and restore saved_irqflag in prev_kprobe

However, I see the issue. Patch 2 and Patch 3 are separate commits,
so Patch 2 alone is not bisectable. I'll prepare a v3 that folds them
together, so the reentry logic and the saved_irqflag preservation land
in a single commit.

Thanks for pointing this out.

Thanks,
Pu Hu