[PATCH v4 0/2] arm64: Fix pending single-step debugging issues

[PATCH v4 0/2] arm64: Fix pending single-step debugging issues

[Sumit Garg]

This patch-set reworks pending fixes from Wei's series [1] to make
single-step debugging via kgdb/kdb on arm64 work as expected. There was
a prior discussion on ML [2] regarding if we should keep the interrupts
enabled during single-stepping. So patch #1 follows suggestion from Will
[3] to not disable interrupts during single stepping but rather skip
single stepping within interrupt handler.

[1] https://lore.kernel.org/all/20200509214159.19680-1-liwei391@huawei.com/
[2] https://lore.kernel.org/all/CAD=FV=Voyfq3Qz0T3RY+aYWYJ0utdH=P_AweB=13rcV8GDBeyQ@mail.gmail.com/
[3] https://lore.kernel.org/all/20200626095551.GA9312@willie-the-truck/

Changes in v4:
- Rebased to the tip of mainline.
- Picked up Doug's Tested-by tag.

Changes in v3:
- Reword commit descriptions as per Daniel's suggestions.

Changes in v2:
- Replace patch #1 to rather follow Will's suggestion.

Sumit Garg (2):
  arm64: entry: Skip single stepping into interrupt handlers
  arm64: kgdb: Set PSTATE.SS to 1 to re-enable single-step

 arch/arm64/include/asm/debug-monitors.h |  1 +
 arch/arm64/kernel/debug-monitors.c      |  5 +++++
 arch/arm64/kernel/entry-common.c        | 18 +++++++++++++++++-
 arch/arm64/kernel/kgdb.c                |  2 ++
 4 files changed, 25 insertions(+), 1 deletion(-)

-- 
2.34.1

[PATCH v4 1/2] arm64: entry: Skip single stepping into interrupt handlers

[Sumit Garg]

Currently on systems where the timer interrupt (or any other
fast-at-human-scale periodic interrupt) is active then it is impossible
to step any code with interrupts unlocked because we will always end up
stepping into the timer interrupt instead of stepping the user code.

The common user's goal while single stepping is that when they step then
the system will stop at PC+4 or PC+I for a branch that gets taken
relative to the instruction they are stepping. So, fix broken single step
implementation via skipping single stepping into interrupt handlers.

The methodology is when we receive an interrupt from EL1, check if we
are single stepping (pstate.SS). If yes then we save MDSCR_EL1.SS and
clear the register bit if it was set. Then unmask only D and leave I set.
On return from the interrupt, set D and restore MDSCR_EL1.SS. Along with
this skip reschedule if we were stepping.

Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
---
 arch/arm64/kernel/entry-common.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index cce1167199e3..53bcb1902f2f 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -471,19 +471,35 @@ static __always_inline void __el1_irq(struct pt_regs *regs,
 	do_interrupt_handler(regs, handler);
 	irq_exit_rcu();
 
-	arm64_preempt_schedule_irq();
+	/* Don't reschedule in case we are single stepping */
+	if (!(regs->pstate & DBG_SPSR_SS))
+		arm64_preempt_schedule_irq();
 
 	exit_to_kernel_mode(regs);
 }
+
 static void noinstr el1_interrupt(struct pt_regs *regs,
 				  void (*handler)(struct pt_regs *))
 {
+	unsigned long reg;
+
+	/* Disable single stepping within interrupt handler */
+	if (regs->pstate & DBG_SPSR_SS) {
+		reg = read_sysreg(mdscr_el1);
+		write_sysreg(reg & ~DBG_MDSCR_SS, mdscr_el1);
+	}
+
 	write_sysreg(DAIF_PROCCTX_NOIRQ, daif);
 
 	if (IS_ENABLED(CONFIG_ARM64_PSEUDO_NMI) && !interrupts_enabled(regs))
 		__el1_pnmi(regs, handler);
 	else
 		__el1_irq(regs, handler);
+
+	if (regs->pstate & DBG_SPSR_SS) {
+		write_sysreg(DAIF_PROCCTX_NOIRQ | PSR_D_BIT, daif);
+		write_sysreg(reg, mdscr_el1);
+	}
 }
 
 asmlinkage void noinstr el1h_64_irq_handler(struct pt_regs *regs)
-- 
2.34.1

Re: [PATCH v4 1/2] arm64: entry: Skip single stepping into interrupt handlers

[Mark Rutland]

On Thu, Dec 15, 2022 at 07:59:02PM +0530, Sumit Garg wrote:
> Currently on systems where the timer interrupt (or any other
> fast-at-human-scale periodic interrupt) is active then it is impossible
> to step any code with interrupts unlocked because we will always end up
> stepping into the timer interrupt instead of stepping the user code.
> 
> The common user's goal while single stepping is that when they step then
> the system will stop at PC+4 or PC+I for a branch that gets taken
> relative to the instruction they are stepping. So, fix broken single step
> implementation via skipping single stepping into interrupt handlers.
> 
> The methodology is when we receive an interrupt from EL1, check if we
> are single stepping (pstate.SS). If yes then we save MDSCR_EL1.SS and
> clear the register bit if it was set. Then unmask only D and leave I set.
> On return from the interrupt, set D and restore MDSCR_EL1.SS. Along with
> this skip reschedule if we were stepping.
> 
> Suggested-by: Will Deacon <will@kernel.org>
> Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
> Tested-by: Douglas Anderson <dianders@chromium.org>

FWIW, this looks reasonable to me; I have a couple of minor style/structure
comments below.

> ---
>  arch/arm64/kernel/entry-common.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index cce1167199e3..53bcb1902f2f 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -471,19 +471,35 @@ static __always_inline void __el1_irq(struct pt_regs *regs,
>  	do_interrupt_handler(regs, handler);
>  	irq_exit_rcu();
>  
> -	arm64_preempt_schedule_irq();
> +	/* Don't reschedule in case we are single stepping */
> +	if (!(regs->pstate & DBG_SPSR_SS))
> +		arm64_preempt_schedule_irq();

Please change arm64_preempt_schedule_irq() to take the regs as an argument, and
put this test inside arm64_preempt_schedule_irq(). That way all the
decision-making about whether to preempt is in one place.

That can go immediately after the need_irq_preemption() test.

>  
>  	exit_to_kernel_mode(regs);
>  }
> +
>  static void noinstr el1_interrupt(struct pt_regs *regs,
>  				  void (*handler)(struct pt_regs *))
>  {
> +	unsigned long reg;

Please s/reg/mdscr/. That way it's harder to confuse with 'regs', it's clearer
that it's the MDSCR_ELx value, and if we have to save/restore any other
registers in future it'll be obvious how to name things.

Thanks,
Mark.

> +
> +	/* Disable single stepping within interrupt handler */
> +	if (regs->pstate & DBG_SPSR_SS) {
> +		reg = read_sysreg(mdscr_el1);
> +		write_sysreg(reg & ~DBG_MDSCR_SS, mdscr_el1);
> +	}
> +
>  	write_sysreg(DAIF_PROCCTX_NOIRQ, daif);
>  
>  	if (IS_ENABLED(CONFIG_ARM64_PSEUDO_NMI) && !interrupts_enabled(regs))
>  		__el1_pnmi(regs, handler);
>  	else
>  		__el1_irq(regs, handler);
> +
> +	if (regs->pstate & DBG_SPSR_SS) {
> +		write_sysreg(DAIF_PROCCTX_NOIRQ | PSR_D_BIT, daif);
> +		write_sysreg(reg, mdscr_el1);
> +	}
>  }
>  
>  asmlinkage void noinstr el1h_64_irq_handler(struct pt_regs *regs)
> -- 
> 2.34.1
> 

Re: [PATCH v4 1/2] arm64: entry: Skip single stepping into interrupt handlers

[Sumit Garg]

 On Fri, 16 Dec 2022 at 22:14, Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Thu, Dec 15, 2022 at 07:59:02PM +0530, Sumit Garg wrote:
> > Currently on systems where the timer interrupt (or any other
> > fast-at-human-scale periodic interrupt) is active then it is impossible
> > to step any code with interrupts unlocked because we will always end up
> > stepping into the timer interrupt instead of stepping the user code.
> >
> > The common user's goal while single stepping is that when they step then
> > the system will stop at PC+4 or PC+I for a branch that gets taken
> > relative to the instruction they are stepping. So, fix broken single step
> > implementation via skipping single stepping into interrupt handlers.
> >
> > The methodology is when we receive an interrupt from EL1, check if we
> > are single stepping (pstate.SS). If yes then we save MDSCR_EL1.SS and
> > clear the register bit if it was set. Then unmask only D and leave I set.
> > On return from the interrupt, set D and restore MDSCR_EL1.SS. Along with
> > this skip reschedule if we were stepping.
> >
> > Suggested-by: Will Deacon <will@kernel.org>
> > Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
> > Tested-by: Douglas Anderson <dianders@chromium.org>
>
> FWIW, this looks reasonable to me; I have a couple of minor style/structure
> comments below.

Thanks Mark for your review.

>
> > ---
> >  arch/arm64/kernel/entry-common.c | 18 +++++++++++++++++-
> >  1 file changed, 17 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> > index cce1167199e3..53bcb1902f2f 100644
> > --- a/arch/arm64/kernel/entry-common.c
> > +++ b/arch/arm64/kernel/entry-common.c
> > @@ -471,19 +471,35 @@ static __always_inline void __el1_irq(struct pt_regs *regs,
> >       do_interrupt_handler(regs, handler);
> >       irq_exit_rcu();
> >
> > -     arm64_preempt_schedule_irq();
> > +     /* Don't reschedule in case we are single stepping */
> > +     if (!(regs->pstate & DBG_SPSR_SS))
> > +             arm64_preempt_schedule_irq();
>
> Please change arm64_preempt_schedule_irq() to take the regs as an argument, and
> put this test inside arm64_preempt_schedule_irq(). That way all the
> decision-making about whether to preempt is in one place.
>
> That can go immediately after the need_irq_preemption() test.

Okay, I will change arm64_preempt_schedule_irq() accordingly.

>
> >
> >       exit_to_kernel_mode(regs);
> >  }
> > +
> >  static void noinstr el1_interrupt(struct pt_regs *regs,
> >                                 void (*handler)(struct pt_regs *))
> >  {
> > +     unsigned long reg;
>
> Please s/reg/mdscr/. That way it's harder to confuse with 'regs', it's clearer
> that it's the MDSCR_ELx value, and if we have to save/restore any other
> registers in future it'll be obvious how to name things.
>

Ack.

-Sumit

> Thanks,
> Mark.
>
> > +
> > +     /* Disable single stepping within interrupt handler */
> > +     if (regs->pstate & DBG_SPSR_SS) {
> > +             reg = read_sysreg(mdscr_el1);
> > +             write_sysreg(reg & ~DBG_MDSCR_SS, mdscr_el1);
> > +     }
> > +
> >       write_sysreg(DAIF_PROCCTX_NOIRQ, daif);
> >
> >       if (IS_ENABLED(CONFIG_ARM64_PSEUDO_NMI) && !interrupts_enabled(regs))
> >               __el1_pnmi(regs, handler);
> >       else
> >               __el1_irq(regs, handler);
> > +
> > +     if (regs->pstate & DBG_SPSR_SS) {
> > +             write_sysreg(DAIF_PROCCTX_NOIRQ | PSR_D_BIT, daif);
> > +             write_sysreg(reg, mdscr_el1);
> > +     }
> >  }
> >
> >  asmlinkage void noinstr el1h_64_irq_handler(struct pt_regs *regs)
> > --
> > 2.34.1
> >

[PATCH v4 2/2] arm64: kgdb: Set PSTATE.SS to 1 to re-enable single-step

[Sumit Garg]

Currently only the first attempt to single-step has any effect. After
that all further stepping remains "stuck" at the same program counter
value.

Refer to the ARM Architecture Reference Manual (ARM DDI 0487E.a) D2.12,
PSTATE.SS=1 should be set at each step before 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@huawei.com>
Signed-off-by: Wei Li <liwei391@huawei.com>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Douglas Anderson <dianders@chromium.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 7b7e05c02691..ce3875ad5cd3 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 3da09778267e..9af898b22ed4 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 cda9c1e9864f..51f204bbcf87 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:
-- 
2.34.1