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

[Vijay Kilari]

Hi Akashi,

   I could not reproduce this with my simulator.
It would be good  if you could post result of KGDB test suite.

>From sysfs you can launch using below command:

echo V1F1000 > /sys/module/kgdbts/parameters/kgdbts
or enable boot test.

Other than that it is OK.


On Mon, Oct 27, 2014 at 4:04 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> 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
>>

Acked-by: Vijaya Kumar K <Vijaya.Kumar@xxxxxxxxxxxxxxxxxx>

Regards
Vijay
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