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

[Guo Ren]

On Wed, Jul 5, 2023 at 12:40 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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?
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
about:

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d..69f7043a98b9 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -18,6 +18,7 @@
 #include <linux/irq.h>
 #include <linux/kexec.h>
 #include <linux/entry-common.h>
+#include <linux/context_tracking.h>

 #include <asm/asm-prototypes.h>
 #include <asm/bug.h>
@@ -285,12 +286,18 @@ asmlinkage __visible __trap_section void
do_trap_break(struct pt_regs *regs)
                handle_break(regs);

                irqentry_exit_to_user_mode(regs);
-       } else {
+       } else if (in_interrupt()){
                irqentry_state_t state = irqentry_nmi_enter(regs);

                handle_break(regs);

                irqentry_nmi_exit(regs, state);
+       } else {
+               enum ctx_state prev_state = exception_enter();
+
+               handle_break(regs);
+
+               exception_exit(prev_state);
        }
 }


>
> If that makes kdb unhappy, fix kdb.
>
> > Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
> > Reported-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> >  arch/riscv/kernel/traps.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> > index efc6b649985a..ed0eb9452f9e 100644
> > --- a/arch/riscv/kernel/traps.c
> > +++ b/arch/riscv/kernel/traps.c
> > @@ -18,6 +18,7 @@
> >  #include <linux/irq.h>
> >  #include <linux/kexec.h>
> >  #include <linux/entry-common.h>
> > +#include <linux/context_tracking.h>
> >
> >  #include <asm/asm-prototypes.h>
> >  #include <asm/bug.h>
> > @@ -257,11 +258,11 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
> >
> >               irqentry_exit_to_user_mode(regs);
> >       } else {
> > -             irqentry_state_t state = irqentry_nmi_enter(regs);
> > +             enum ctx_state prev_state = exception_enter();
> >
> >               handle_break(regs);
> >
> > -             irqentry_nmi_exit(regs, state);
> > +             exception_exit(prev_state);
> >       }
> >  }
> >
> > --
> > 2.36.1
> >



--
Best Regards
 Guo Ren