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

From: [email-masked] (Yong Zhao)
Date: Tue, 6 Jan 2015 11:22:01 -0500

Hi Will,

I have used Takahiro's patch to fix the single stepping problem of KGDB 
on arm64. We had real hardware called AMD Seattle. The problem I had is 
exactly the same as the description of Takahiro. We are working on 
kernel 3.14, but we did port back many patches related to arm64 from 
higher kernel versions. The patches that we ported are:

1d70460 arm64: kgdb: fix single stepping
5f31df7 arm64: debug: don't re-enable debug exceptions on return from 
el1_dbg
All the arm64 KGDB patches from 3.16

The first one is Takahiro's patch.

Best,
Yong
On 15-01-05 06:52 PM, AKASHI Takahiro wrote:
> Hi Will,
>
> What's the current status of this patch?
> I'm pretty sure that the bug is reproducable and can be fixed
> with my patch since I've got a report from another guy
> who had the same problem and tried my patch.
>
> Thanks,
> -Takahiro AKASHI
>
> On 10/21/2014 02:07 PM, 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].
>>
>> * 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;
>>   }
>>
>>