linux-kernel - [PATCH RFC] riscv: Do not handle break traps from kernel as nmi

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Message-Id: <20250903-dev-alex-break_nmi_v1-v1-1-4a3d81c29598@rivosinc.com>
Date: Wed, 03 Sep 2025 19:54:29 +0000
From: Alexandre Ghiti <alexghiti@...osinc.com>
To: Peter Zijlstra <peterz@...radead.org>, 
 Steven Rostedt <rostedt@...dmis.org>, 
 Paul Walmsley <paul.walmsley@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
 Alexandre Ghiti <alex@...ti.fr>, Guo Ren <guoren@...nel.org>, 
 Björn Töpel <bjorn@...osinc.com>
Cc: Palmer Dabbelt <palmer@...osinc.com>, linux-riscv@...ts.infradead.org, 
 linux-kernel@...r.kernel.org, stable@...r.kernel.org, 
 Alexandre Ghiti <alexghiti@...osinc.com>
Subject: [PATCH RFC] riscv: Do not handle break traps from kernel as nmi

kprobe has been broken on riscv for quite some time. There is an attempt
[1] to fix that which actually works. This patch works because it enables
ARCH_HAVE_NMI_SAFE_CMPXCHG and that makes the ring buffer allocation
succeed when handling a kprobe because we handle *all* kprobes in nmi
context. We do so because Peter advised us to treat all kernel traps as
nmi [2].

But that does not seem right for kprobe handling, so instead, treat
break traps from kernel as non-nmi.

Link: https://lore.kernel.org/linux-riscv/20250711090443.1688404-1-pulehui@huaweicloud.com/ [1]
Link: https://lore.kernel.org/linux-riscv/20250422094419.GC14170@noisy.programming.kicks-ass.net/ [2]
Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
Cc: stable@...r.kernel.org
Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
---
This is clearly an RFC and this is likely not the right way to go, it is
just a way to trigger a discussion about if handling kprobes in an nmi 
context is the right way or not.
---
 arch/riscv/kernel/traps.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 80230de167def3c33db5bc190347ec5f87dbb6e3..90f36bb9b12d4ba0db0f084f87899156e3c7dc6f 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -315,11 +315,11 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
 		local_irq_disable();
 		irqentry_exit_to_user_mode(regs);
 	} else {
-		irqentry_state_t state = irqentry_nmi_enter(regs);
+		irqentry_state_t state = irqentry_enter(regs);
 
 		handle_break(regs);
 
-		irqentry_nmi_exit(regs, state);
+		irqentry_exit(regs, state);
 	}
 }
 

---
base-commit: ae9a687664d965b13eeab276111b2f97dd02e090
change-id: 20250903-dev-alex-break_nmi_v1-57c5321f3e80

Best regards,
-- 
Alexandre Ghiti <alexghiti@...osinc.com>