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LoongArch: Avoid preempt count underflow without probe

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LoongArch uses break 11 for the breakpoint placed after an instruction
that Kprobes executes out of line. Since userspace can issue the same
break instruction, do_bp() can reach kprobe_singlestep_handler() when
there is no current probe.

The handler actually returns false in this case, but it first calls
preempt_enable_no_resched(). The corresponding preempt_disable() is done
by kprobe_breakpoint_handler() on a real Kprobe hit, so it has not run
here. As a result, an ordinary userspace breakpoint (code 11) underflows
the current task's preempt count.

This also makes in_interrupt() return true until the task schedules. One
visible consequence is the socket cgroup attribution: cgroup_sk_alloc()
treats the allocation as interrupt context and assigns the socket to the
root cgroup. A socket opened from the SIGTRAP handler can then avoid a
BPF_CGROUP_INET_SOCK_CREATE policy attached to the task's own cgroup.

Return as soon as kprobe_running() reports no active probe.

The same check has appeared in [PATCH v10 2/4] of the original LoongArch
Kprobes series, but was dropped before the feature reached mainline.

Cc: stable@vger.kernel.org
Fixes: 6d4cc40fb5 ("LoongArch: Add kprobes support")
Link: https://lore.kernel.org/loongarch/1670575981-14389-3-git-send-email-yangtiezhu@loongson.cn/
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
This commit is contained in:
Jérémy Jean 2026-09-04 21:44:43 +08:00 committed by Huacai Chen
parent c3f2feace5
commit 72ce4b2467
1 changed files with 3 additions and 0 deletions
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3
arch/loongarch/kernel/kprobes.c
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@ -275,6 +275,9 @@ bool kprobe_singlestep_handler(struct pt_regs *regs)
struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
unsigned long addr = instruction_pointer(regs);
if (!cur)
return false;
if (cur && (kcb->kprobe_status & (KPROBE_HIT_SS | KPROBE_REENTER)) &&
((unsigned long)&cur->ainsn.insn[1] == addr)) {
restore_local_irqflag(kcb, regs);