* Re: [RFC v3 2/2] arm64: kprobes: Allow reentering kprobes while single-stepping
[not found] ` <20260710063242.228714-3-hupu@transsion.com>
@ 2026-07-10 9:38 ` Masami Hiramatsu
0 siblings, 0 replies; 2+ messages in thread
From: Masami Hiramatsu @ 2026-07-10 9:38 UTC (permalink / raw)
To: Pu Hu
Cc: Hongyan Xia, Jiazi Li, catalin.marinas@arm.com,
linux-kernel@vger.kernel.org, naveen@kernel.org,
yang@os.amperecomputing.com, will@kernel.org, davem@davemloft.net,
linux-arm-kernel@lists.infradead.org,
linux-trace-kernel@vger.kernel.org
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!
> Signed-off-by: Pu Hu <hupu@transsion.com>
> Signed-off-by: Hongyan Xia <hongyan.xia@transsion.com>
> ---
> arch/arm64/include/asm/kprobes.h | 6 ++++++
> arch/arm64/kernel/probes/kprobes.c | 23 ++++++++++++++++++++++-
> 2 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/kprobes.h b/arch/arm64/include/asm/kprobes.h
> index f2782560647b..35ce2c94040e 100644
> --- a/arch/arm64/include/asm/kprobes.h
> +++ b/arch/arm64/include/asm/kprobes.h
> @@ -26,6 +26,12 @@
> struct prev_kprobe {
> struct kprobe *kp;
> unsigned int status;
> +
> + /*
> + * The original DAIF state of the outer kprobe, saved here before
> + * a nested kprobe overwrites kcb->saved_irqflag during reentry.
> + */
> + unsigned long saved_irqflag;
> };
>
> /* per-cpu kprobe control block */
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index 798e4b091d1a..4e0efad5caf2 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -174,12 +174,27 @@ static void __kprobes save_previous_kprobe(struct kprobe_ctlblk *kcb)
> {
> kcb->prev_kprobe.kp = kprobe_running();
> kcb->prev_kprobe.status = kcb->kprobe_status;
> +
> + /*
> + * Save the outer kprobe's original DAIF flags before the nested
> + * kprobe calls kprobes_save_local_irqflag() and overwrites
> + * kcb->saved_irqflag. Without this, the outer kprobe will restore
> + * the wrong DAIF state and leave interrupts permanently masked.
> + */
> + kcb->prev_kprobe.saved_irqflag = kcb->saved_irqflag;
> }
>
> static void __kprobes restore_previous_kprobe(struct kprobe_ctlblk *kcb)
> {
> __this_cpu_write(current_kprobe, kcb->prev_kprobe.kp);
> kcb->kprobe_status = kcb->prev_kprobe.status;
> +
> + /*
> + * Restore the outer kprobe's saved_irqflag so that when its
> + * single-step completes, kprobes_restore_local_irqflag() uses
> + * the correct original DAIF value.
> + */
> + kcb->saved_irqflag = kcb->prev_kprobe.saved_irqflag;
> }
>
> static void __kprobes set_current_kprobe(struct kprobe *p)
> @@ -240,10 +255,16 @@ static int __kprobes reenter_kprobe(struct kprobe *p,
> switch (kcb->kprobe_status) {
> case KPROBE_HIT_SSDONE:
> case KPROBE_HIT_ACTIVE:
> + case KPROBE_HIT_SS:
> + /*
> + * A probe can be hit while another kprobe is preparing or
> + * executing its XOL single-step instruction. This is still a
> + * recoverable one-level reentry, so handle it in the same way as
> + * reentry from KPROBE_HIT_ACTIVE or KPROBE_HIT_SSDONE.
> + */
> kprobes_inc_nmissed_count(p);
> setup_singlestep(p, regs, kcb, 1);
> break;
> - case KPROBE_HIT_SS:
> case KPROBE_REENTER:
> pr_warn("Failed to recover from reentered kprobes.\n");
> dump_kprobe(p);
> --
> 2.43.0
>
--
Masami Hiramatsu (Google) <mhiramat@kernel.org>
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