Re: [PATCH] arm64: kgdb: fix single stepping

[Will Deacon]

On Tue, Oct 21, 2014 at 06:07:30AM +0100, AKASHI Takahiro wrote:
> I tried to verify kgdb in vanilla kernel on fast model, but it seems that
> the single stepping with kgdb doesn't work correctly since its first
> appearance at v3.15.
> 
> On v3.15, 'stepi' command after breaking the kernel at some breakpoint
> steps forward to the next instruction, but the succeeding 'stepi' never
> goes beyond that.
> On v3.16, 'stepi' moves forward and stops at the next instruction just
> after enable_dbg in el1_dbg, and never goes beyond that. This variance of
> behavior seems to come in with the following patch in v3.16:
> 
>     commit 2a2830703a23 ("arm64: debug: avoid accessing mdscr_el1 on fault
>     paths where possible")
> 
> This patch
> (1) moves kgdb_disable_single_step() from 'c' command handling to single
>     step handler.
>     This makes sure that single stepping gets effective at every 's' command.
>     Please note that, under the current implementation, single step bit in
>     spsr, which is cleared by the first single stepping, will not be set
>     again for the consecutive 's' commands because single step bit in mdscr
>     is still kept on (that is, kernel_active_single_step() in
>     kgdb_arch_handle_exception() is true).
> (2) re-implements kgdb_roundup_cpus() because the current implementation
>     enabled interrupts naively. See below.
> (3) removes 'enable_dbg' in el1_dbg.
>     Single step bit in mdscr is turned on in do_handle_exception()->
>     kgdb_handle_expection() before returning to debugged context, and if
>     debug exception is enabled in el1_dbg, we will see unexpected single-
>     stepping in el1_dbg.
>     Since v3.18, the following patch does the same:
>       commit 1059c6bf8534 ("arm64: debug: don't re-enable debug exceptions
>       on return from el1_dbg)
> (4) masks interrupts while single-stepping one instruction.
>     If an interrupt is caught during processing a single-stepping, debug
>     exception is unintentionally enabled by el1_irq's 'enable_dbg' before
>     returning to debugged context.
>     Thus, like in (2), we will see unexpected single-stepping in el1_irq.
> 
> Basically (1) and (2) are for v3.15, (3) and (4) for v3.1[67].

(3) was CC'd for stable, so I don't think you need to mention that here.

I'd like an ack from a KGDB person before I take this via the arm64 tree.

Will

> 
> * issue fixed by (2):
> Without (2), we would see another problem if a breakpoint is set at
> interrupt-sensible places, like gic_handle_irq():
> 
>     KGDB: re-enter error: breakpoint removed ffffffc000081258
>     ------------[ cut here ]------------
>     WARNING: CPU: 0 PID: 650 at kernel/debug/debug_core.c:435
> 					kgdb_handle_exception+0x1dc/0x1f4()
>     Modules linked in:
>     CPU: 0 PID: 650 Comm: sh Not tainted 3.17.0-rc2+ #177
>     Call trace:
>     [<ffffffc000087fac>] dump_backtrace+0x0/0x130
>     [<ffffffc0000880ec>] show_stack+0x10/0x1c
>     [<ffffffc0004d683c>] dump_stack+0x74/0xb8
>     [<ffffffc0000ab824>] warn_slowpath_common+0x8c/0xb4
>     [<ffffffc0000ab90c>] warn_slowpath_null+0x14/0x20
>     [<ffffffc000121bfc>] kgdb_handle_exception+0x1d8/0x1f4
>     [<ffffffc000092ffc>] kgdb_brk_fn+0x18/0x28
>     [<ffffffc0000821c8>] brk_handler+0x9c/0xe8
>     [<ffffffc0000811e8>] do_debug_exception+0x3c/0xac
>     Exception stack(0xffffffc07e027650 to 0xffffffc07e027770)
>     ...
>     [<ffffffc000083cac>] el1_dbg+0x14/0x68
>     [<ffffffc00012178c>] kgdb_cpu_enter+0x464/0x5c0
>     [<ffffffc000121bb4>] kgdb_handle_exception+0x190/0x1f4
>     [<ffffffc000092ffc>] kgdb_brk_fn+0x18/0x28
>     [<ffffffc0000821c8>] brk_handler+0x9c/0xe8
>     [<ffffffc0000811e8>] do_debug_exception+0x3c/0xac
>     Exception stack(0xffffffc07e027ac0 to 0xffffffc07e027be0)
>     ...
>     [<ffffffc000083cac>] el1_dbg+0x14/0x68
>     [<ffffffc00032e4b4>] __handle_sysrq+0x11c/0x190
>     [<ffffffc00032e93c>] write_sysrq_trigger+0x4c/0x60
>     [<ffffffc0001e7d58>] proc_reg_write+0x54/0x84
>     [<ffffffc000192fa4>] vfs_write+0x98/0x1c8
>     [<ffffffc0001939b0>] SyS_write+0x40/0xa0
> 
> Once some interrupt occurs, a breakpoint at gic_handle_irq() triggers kgdb.
> Kgdb then calls kgdb_roundup_cpus() to sync with other cpus.
> Current kgdb_roundup_cpus() unmasks interrupts temporarily to
> use smp_call_function().
> This eventually allows another interrupt to occur and likely results in
> hitting a breakpoint at gic_handle_irq() again since debug exception is
> always enabled in el1_irq.
> 
> We can avoid this issue by specifying "nokgdbroundup" in kernel parameter,
> but this will also leave other cpus be in unknown state in terms of kgdb,
> and may result in interfering with kgdb activity.
> 
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> ---
>  arch/arm64/kernel/kgdb.c |   60 +++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 46 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
> index a0d10c5..81b5910 100644
> --- a/arch/arm64/kernel/kgdb.c
> +++ b/arch/arm64/kernel/kgdb.c
> @@ -19,9 +19,13 @@
>   * along with this program.  If not, see <http://www.gnu.org/licenses/>.
>   */
>  
> +#include <linux/cpumask.h>
>  #include <linux/irq.h>
> +#include <linux/irq_work.h>
>  #include <linux/kdebug.h>
>  #include <linux/kgdb.h>
> +#include <linux/percpu.h>
> +#include <asm/ptrace.h>
>  #include <asm/traps.h>
>  
>  struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = {
> @@ -95,6 +99,9 @@ struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = {
>  	{ "fpcr", 4, -1 },
>  };
>  
> +static DEFINE_PER_CPU(unsigned int, kgdb_pstate);
> +static DEFINE_PER_CPU(struct irq_work, kgdb_irq_work);
> +
>  char *dbg_get_reg(int regno, void *mem, struct pt_regs *regs)
>  {
>  	if (regno >= DBG_MAX_REG_NUM || regno < 0)
> @@ -176,18 +183,14 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
>  		 * over and over again.
>  		 */
>  		kgdb_arch_update_addr(linux_regs, remcom_in_buffer);
> -		atomic_set(&kgdb_cpu_doing_single_step, -1);
> -		kgdb_single_step =  0;
> -
> -		/*
> -		 * Received continue command, disable single step
> -		 */
> -		if (kernel_active_single_step())
> -			kernel_disable_single_step();
>  
>  		err = 0;
>  		break;
>  	case 's':
> +		/* mask interrupts while single stepping */
> +		__this_cpu_write(kgdb_pstate, linux_regs->pstate);
> +		linux_regs->pstate |= PSR_I_BIT;
> +
>  		/*
>  		 * Update step address value with address passed
>  		 * with step packet.
> @@ -198,8 +201,6 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
>  		 */
>  		kgdb_arch_update_addr(linux_regs, remcom_in_buffer);
>  		atomic_set(&kgdb_cpu_doing_single_step, raw_smp_processor_id());
> -		kgdb_single_step =  1;
> -
>  		/*
>  		 * Enable single step handling
>  		 */
> @@ -229,6 +230,18 @@ static int kgdb_compiled_brk_fn(struct pt_regs *regs, unsigned int esr)
>  
>  static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned int esr)
>  {
> +	unsigned int pstate;
> +
> +	kernel_disable_single_step();
> +	atomic_set(&kgdb_cpu_doing_single_step, -1);
> +
> +	/* restore interrupt mask status */
> +	pstate = __this_cpu_read(kgdb_pstate);
> +	if (pstate & PSR_I_BIT)
> +		regs->pstate |= PSR_I_BIT;
> +	else
> +		regs->pstate &= ~PSR_I_BIT;
> +
>  	kgdb_handle_exception(1, SIGTRAP, 0, regs);
>  	return 0;
>  }
> @@ -249,16 +262,27 @@ static struct step_hook kgdb_step_hook = {
>  	.fn		= kgdb_step_brk_fn
>  };
>  
> -static void kgdb_call_nmi_hook(void *ignored)
> +static void kgdb_roundup_hook(struct irq_work *work)
>  {
>  	kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
>  }
>  
>  void kgdb_roundup_cpus(unsigned long flags)
>  {
> -	local_irq_enable();
> -	smp_call_function(kgdb_call_nmi_hook, NULL, 0);
> -	local_irq_disable();
> +	int cpu;
> +	struct cpumask mask;
> +	struct irq_work *work;
> +
> +	mask = *cpu_online_mask;
> +	cpumask_clear_cpu(smp_processor_id(), &mask);
> +	cpu = cpumask_first(&mask);
> +	if (cpu >= nr_cpu_ids)
> +		return;
> +
> +	for_each_cpu(cpu, &mask) {
> +		work = per_cpu_ptr(&kgdb_irq_work, cpu);
> +		irq_work_queue_on(work, cpu);
> +	}
>  }
>  
>  static int __kgdb_notify(struct die_args *args, unsigned long cmd)
> @@ -299,6 +323,8 @@ static struct notifier_block kgdb_notifier = {
>  int kgdb_arch_init(void)
>  {
>  	int ret = register_die_notifier(&kgdb_notifier);
> +	int cpu;
> +	struct irq_work *work;
>  
>  	if (ret != 0)
>  		return ret;
> @@ -306,6 +332,12 @@ int kgdb_arch_init(void)
>  	register_break_hook(&kgdb_brkpt_hook);
>  	register_break_hook(&kgdb_compiled_brkpt_hook);
>  	register_step_hook(&kgdb_step_hook);
> +
> +	for_each_possible_cpu(cpu) {
> +		work = per_cpu_ptr(&kgdb_irq_work, cpu);
> +		init_irq_work(work, kgdb_roundup_hook);
> +	}
> +
>  	return 0;
>  }
>  
> -- 
> 1.7.9.5
> 
> 
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