[PATCH v2 2/2] arm64: kgdb: Set PSTATE.SS to 1 to re-enable single-step

[Sumit Garg]

On Fri, 6 May 2022 at 21:43, Daniel Thompson <daniel.thompson at linaro.org> wrote:
>
> On Wed, Apr 13, 2022 at 12:24:58PM +0530, Sumit Garg wrote:
> > After fixing wrongly single-stepping into the irq handler, when we execute
> > single-step in kdb/kgdb, we can see only the first step can work.
>
> I might be nitpicking since, again, I've no problems with the code
> but... I'd rather this patch description focused on what this patch
> does rather than what the patch before it does!
>
> Something more like:
>
>   Currently only the first attempt to single-step has any effect.
>   After that all further stepping remains "stuck" at the same program
>   counter value.
>

Ack, I will use this instead.

-Sumit

>
> Daniel.
>
>
>
>
> > Refer to the ARM Architecture Reference Manual (ARM DDI 0487E.a) D2.12,
> > i think PSTATE.SS=1 should be set each step for transferring the PE to the
> > 'Active-not-pending' state. The problem here is PSTATE.SS=1 is not set
> > since the second single-step.
> >
> > After the first single-step, the PE transferes to the 'Inactive' state,
> > with PSTATE.SS=0 and MDSCR.SS=1, thus PSTATE.SS won't be set to 1 due to
> > kernel_active_single_step()=true. Then the PE transferes to the
> > 'Active-pending' state when ERET and returns to the debugger by step
> > exception.
> >
> > Before this patch:
> > ==================
> > Entering kdb (current=0xffff3376039f0000, pid 1) on processor 0 due to Keyboard Entry
> > [0]kdb>
> >
> > [0]kdb>
> > [0]kdb> bp write_sysrq_trigger
> > Instruction(i) BP #0 at 0xffffa45c13d09290 (write_sysrq_trigger)
> >     is enabled   addr at ffffa45c13d09290, hardtype=0 installed=0
> >
> > [0]kdb> go
> > $ echo h > /proc/sysrq-trigger
> >
> > Entering kdb (current=0xffff4f7e453f8000, pid 175) on processor 1 due to Breakpoint @ 0xffffad651a309290
> > [1]kdb> ss
> >
> > Entering kdb (current=0xffff4f7e453f8000, pid 175) on processor 1 due to SS trap @ 0xffffad651a309294
> > [1]kdb> ss
> >
> > Entering kdb (current=0xffff4f7e453f8000, pid 175) on processor 1 due to SS trap @ 0xffffad651a309294
> > [1]kdb>
> >
> > After this patch:
> > =================
> > Entering kdb (current=0xffff6851c39f0000, pid 1) on processor 0 due to Keyboard Entry
> > [0]kdb> bp write_sysrq_trigger
> > Instruction(i) BP #0 at 0xffffc02d2dd09290 (write_sysrq_trigger)
> >     is enabled   addr at ffffc02d2dd09290, hardtype=0 installed=0
> >
> > [0]kdb> go
> > $ echo h > /proc/sysrq-trigger
> >
> > Entering kdb (current=0xffff6851c53c1840, pid 174) on processor 1 due to Breakpoint @ 0xffffc02d2dd09290
> > [1]kdb> ss
> >
> > Entering kdb (current=0xffff6851c53c1840, pid 174) on processor 1 due to SS trap @ 0xffffc02d2dd09294
> > [1]kdb> ss
> >
> > Entering kdb (current=0xffff6851c53c1840, pid 174) on processor 1 due to SS trap @ 0xffffc02d2dd09298
> > [1]kdb> ss
> >
> > Entering kdb (current=0xffff6851c53c1840, pid 174) on processor 1 due to SS trap @ 0xffffc02d2dd0929c
> > [1]kdb>
> >
> > Fixes: 44679a4f142b ("arm64: KGDB: Add step debugging support")
> > Co-developed-by: Wei Li <liwei391 at huawei.com>
> > Signed-off-by: Wei Li <liwei391 at huawei.com>
> > Signed-off-by: Sumit Garg <sumit.garg at linaro.org>
> > ---
> >  arch/arm64/include/asm/debug-monitors.h | 1 +
> >  arch/arm64/kernel/debug-monitors.c      | 5 +++++
> >  arch/arm64/kernel/kgdb.c                | 2 ++
> >  3 files changed, 8 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/debug-monitors.h b/arch/arm64/include/asm/debug-monitors.h
> > index 00c291067e57..9e1e864d6440 100644
> > --- a/arch/arm64/include/asm/debug-monitors.h
> > +++ b/arch/arm64/include/asm/debug-monitors.h
> > @@ -104,6 +104,7 @@ void user_regs_reset_single_step(struct user_pt_regs *regs,
> >  void kernel_enable_single_step(struct pt_regs *regs);
> >  void kernel_disable_single_step(void);
> >  int kernel_active_single_step(void);
> > +void kernel_regs_reset_single_step(struct pt_regs *regs);
> >
> >  #ifdef CONFIG_HAVE_HW_BREAKPOINT
> >  int reinstall_suspended_bps(struct pt_regs *regs);
> > diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> > index 4f3661eeb7ec..ea3f410aa385 100644
> > --- a/arch/arm64/kernel/debug-monitors.c
> > +++ b/arch/arm64/kernel/debug-monitors.c
> > @@ -438,6 +438,11 @@ int kernel_active_single_step(void)
> >  }
> >  NOKPROBE_SYMBOL(kernel_active_single_step);
> >
> > +void kernel_regs_reset_single_step(struct pt_regs *regs)
> > +{
> > +     set_regs_spsr_ss(regs);
> > +}
> > +
> >  /* ptrace API */
> >  void user_enable_single_step(struct task_struct *task)
> >  {
> > diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
> > index 2aede780fb80..acf2196b1e9b 100644
> > --- a/arch/arm64/kernel/kgdb.c
> > +++ b/arch/arm64/kernel/kgdb.c
> > @@ -224,6 +224,8 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
> >                */
> >               if (!kernel_active_single_step())
> >                       kernel_enable_single_step(linux_regs);
> > +             else
> > +                     kernel_regs_reset_single_step(linux_regs);
> >               err = 0;
> >               break;
> >       default:
> > --
> > 2.25.1