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

[Daniel Thompson]

On Tue, Jul 04, 2023 at 06:40:03PM +0200, Peter Zijlstra wrote:
> On Sat, Jul 01, 2023 at 10:57:07PM -0400, guoren@xxxxxxxxxx wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > 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.