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

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Date:   Mon, 17 Jul 2023 12:45:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     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, daniel.thompson@...aro.org,
        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 Mon, Jul 17, 2023 at 07:33:25AM +0800, Guo Ren wrote:
> On Mon, Jul 10, 2023 at 4:02 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Sun, Jul 09, 2023 at 10:30:22AM +0800, Guo Ren wrote:
> > > On Wed, Jul 5, 2023 at 12:40 AM Peter Zijlstra <peterz@...radead.org> 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?
> > > EBREAK is just an instruction of riscv which would rise breakpoint exception.
> > >
> > >
> > > >
> > > > 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.
> > > When the ebreak happend out of local_irq_disable region, but
> > > __nmi_enter forces handle_break() into in_interupt() state. So how
> >
> > And why is that a problem? I think I'm missing something fundamental
> > here...
> The irqentry_nmi_enter() would force the current context to get
> in_interrupt=true, although ebreak happens in the context which is
> in_interrupt=false.
> A lot of checking codes, such as:
>         if (in_interrupt())
>                 panic("Fatal exception in interrupt");

Why would you do that?!?

Are you're trying to differentiate between an exception and an
interrupt?

You *could* have ebreak in an interrupt, right? So why panic the machine
if that happens?

> It would make the kernel panic, but we don't panic; we want back to the shell.
> eg:
> echo BUG > /sys/kernel/debug/provoke-crash/DIRECT