linux-kernel - Re: [PATCH] riscv: entry: Fixup do_trap

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Message-ID: <20230704173451.GD385243@aspen.lan>
Date:   Tue, 4 Jul 2023 18:34:51 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     guoren@...nel.org, arnd@...db.de, palmer@...osinc.com,
        tglx@...utronix.de, luto@...nel.org, conor.dooley@...rochip.com,
        heiko@...ech.de, jszhang@...nel.org, lazyparser@...il.com,
        falcon@...ylab.org, chenhuacai@...nel.org, apatel@...tanamicro.com,
        atishp@...shpatra.org, mark.rutland@....com, bjorn@...nel.org,
        palmer@...belt.com, bjorn@...osinc.com, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        stable@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH] riscv: entry: Fixup do_trap_break from kernel side

On Tue, Jul 04, 2023 at 06:40:03PM +0200, Peter Zijlstra wrote:
> On Sat, Jul 01, 2023 at 10:57:07PM -0400, guoren@...nel.org wrote:
> > From: Guo Ren <guoren@...ux.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.
>
> This doesn't explain much if anything :/
>
> I'm confused (probably because I don't know RISC-V very well), what's
> EBREAK and how does it happen?

Among other things ebreak is part of the BUG() macro (although it is
also used to programmatically enter kgdb).


> Specifically, if EBREAK can happen inside an local_irq_disable() region,
> then the below change is actively wrong. Any exception/interrupt that
> can happen while local_irq_disable() must be treated like an NMI.
>
> If that makes kdb unhappy, fix kdb.

The only relationship this problem has to kgdb/kdb is that is was found
using the kgdb test suite. However the panic is absolutely nothing to
do with kgdb.

I would never normally be so sure regarding the absence of bugs in kgdb
but in this case it can be reproduced when kgdb is not enabled in the
KConfig which I think puts it in the clear!

Reproduction is simply:

  /bin/echo BUG > /sys/kernel/debug/provoke-crash/DIRECT

Above will panic the kernel but, absent options specifically requesting
a panic, this should kill the echo process rather than killing the kernel.


Daniel.