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

[Vijay Kilari]

Hi Akashil,



On Tue, Oct 28, 2014 at 5:48 AM, AKASHI Takahiro
<takahiro.akashi@xxxxxxxxxx> wrote:
> Vijay,
>
> On 10/27/2014 09:45 PM, Vijay Kilari wrote:
>>
>> 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.
>
>
> V1F1000 doesn't reveal this issue even on my environment (FVP_VE_AEM8A),
> but I can easily reproduce it with vanilla v3.15:
>    (gdb) b sys_sync
>    (gdb) c
> At the target,
>    # sync
> At gdb,
>    (gdb) info reg pc
>    (gdb) si
>    (gdb) info reg pc  <= (a)
>    (gdb) si
>    (gdb) info reg pc  <= (b)
>
> Here you can see that you are still at the same address as (a), that is
>    <sys_sync+4>
> and you can never go forward with stepi.
>
> Please try these steps and let me know the result.

 Are you testing on hardware or simulator?
On my simulator  arm64 I see warnings in boot test

>
> -Takahiro AKASHI
>
>
>
>> 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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