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

From: [email-masked] (AKASHI Takahiro)
Date: Tue, 28 Oct 2014 09:18:22 +0900

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@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.

-Takahiro AKASHI

> Other than that it is OK.
>
>
> On Mon, Oct 27, 2014 at 4:04 PM, Will Deacon <[email-masked]> 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 <[email-masked]>
>>> ---
>>>   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 <[email-masked]>
>
> Regards
> Vijay
>