Re: [PATCH] riscv: entry: Fixup do_trap_break from kernel side

From: Peter Zijlstra <peterz@infradead.org>
To: Guo Ren <guoren@kernel.org>
Cc: Kees Cook <kees@kernel.org>,
	arnd@arndb.de, palmer@rivosinc.com, tglx@linutronix.de,
	luto@kernel.org, conor.dooley@microchip.com, heiko@sntech.de,
	jszhang@kernel.org, lazyparser@gmail.com, falcon@tinylab.org,
	chenhuacai@kernel.org, apatel@ventanamicro.com,
	atishp@atishpatra.org, mark.rutland@arm.com, bjorn@kernel.org,
	palmer@dabbelt.com, bjorn@rivosinc.com,
	daniel.thompson@linaro.org, linux-arch@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
	stable@vger.kernel.org, Guo Ren <guoren@linux.alibaba.com>
Subject: Re: [PATCH] riscv: entry: Fixup do_trap_break from kernel side
Date: Mon, 22 Jun 2026 10:28:41 +0200	[thread overview]
Message-ID: <20260622082841.GW49951@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <ajeKPpg2rwadVPY4@gmail.com>

On Sun, Jun 21, 2026 at 02:52:46AM -0400, Guo Ren wrote:
> On Fri, Jun 19, 2026 at 04:54:53PM -0700, Kees Cook wrote:
> > *thread encromancy*
> > 
> > On Sat, Jul 01, 2023 at 10:57:07PM -0400, guoren@kernel.org wrote:
> > > From: Guo Ren <guoren@linux.alibaba.com>
> > > 
> > > The irqentry_nmi_enter/exit would force the current context into in_interrupt.
> > > That would trigger the kernel to dead panic, but the kdb still needs "ebreak" to
> > > debug the kernel.
> > > 
> > > Move irqentry_nmi_enter/exit to exception_enter/exit could correct handle_break
> > > of the kernel side.
> > > 
> > > Before the fixup:
> > > $echo BUG > /sys/kernel/debug/provoke-crash/DIRECT
> > >   lkdtm: Performing direct entry BUG
> > >   ------------[ cut here ]------------
> > >   kernel BUG at drivers/misc/lkdtm/bugs.c:78!
> > > [...]
> > >   Kernel panic - not syncing: Aiee, killing interrupt handler!
> > 
> > This appears to still be unfixed. What's the blocker? The solutions in
> > this thread seem to work...
> > 
> > I'd like to be exercising an Oops path via KUnit (for KCFI), and riscv
> > just instantly falls over instead of thread-killing on the exception.
> Thanks for reviving this thread. At the time I didn’t fully understand
> Peter’s point. We should only use the NMI path when the trap occurs with
> interrupts disabled.
> Here’s the updated fix:
> 
>  do_trap_break(struct pt_regs *regs)
> ... 
>  		irqentry_exit_to_user_mode(regs);
>  	} else {
> -		irqentry_state_t state = irqentry_nmi_enter(regs);
> +		if (regs->status & SR_IE) {
> +			enum ctx_state prev_state = exception_enter();
>  
> -		handle_break(regs);
> +			handle_break(regs);
>  
> -		irqentry_nmi_exit(regs, state);
> +			exception_exit(prev_state);
> +		} else {
> +			irqentry_state_t state = irqentry_nmi_enter(regs);
> +
> +			handle_break(regs);
> +
> +			irqentry_nmi_exit(regs, state);
> +		}
>  	}
>  }
> 
> If you & Peter have no objection, I’ll post a v2.

I still don't understand it. This cannot fix anything. Consider:

 EBREAK
 raw_spin_lock_irq(&your_lock)
 EBREAK

So now the first 'works', but the second will crash. Additionally,
having the EBREAK context differ so dramatically between invocations
seems like a very bad deal to me.

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