Subject: [RFC v2] arm64: kgdb: fix single stepping

From: [email-masked] (AKASHI Takahiro)
Date: Wed, 01 Oct 2014 20:17:14 +0900

Vijay,

Have you verified your code in mainline on real hardware?

On 09/29/2014 08:58 PM, Vijay Kilari wrote:
> Hi Akashi,
>
> On Fri, Sep 26, 2014 at 5:24 PM, AKASHI Takahiro
> <[email-masked]> 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).
>
> Have you please check the functionality by running KGDB test suit
> with multicores?

I only tested my patch on fast model with multicore configuration.

>> (2) 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.
>> (3) 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) is for v3.15, (2) and (3) with (1) for v3.16.
>>
>> With those changes, we will 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/entry.S |    1 -
>>   arch/arm64/kernel/kgdb.c  |   29 +++++++++++++++++++----------
>>   2 files changed, 19 insertions(+), 11 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
>> index fdd6eae..a935d5f 100644
>> --- a/arch/arm64/kernel/entry.S
>> +++ b/arch/arm64/kernel/entry.S
>> @@ -325,7 +325,6 @@ el1_dbg:
>>          mrs     x0, far_el1
>>          mov     x2, sp                          // struct pt_regs
>>          bl      do_debug_exception
>> -       enable_dbg
>>          kernel_exit 1
>>   el1_inv:
>>          // TODO: add support for undefined instructions in kernel mode
>> diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
>> index 75c9cf1..f1fc1d8 100644
>> --- a/arch/arm64/kernel/kgdb.c
>> +++ b/arch/arm64/kernel/kgdb.c
>> @@ -22,6 +22,7 @@
>>   #include <linux/irq.h>
>>   #include <linux/kdebug.h>
>>   #include <linux/kgdb.h>
>> +#include <asm/percpu.h>
>>   #include <asm/traps.h>
>>
>>   struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = {
>> @@ -95,6 +96,8 @@ struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = {
>>          { "fpcr", 4, -1 },
>>   };
>>
>> +static DEFINE_PER_CPU(unsigned int, kgdb_pstate);
>> +
>>   char *dbg_get_reg(int regno, void *mem, struct pt_regs *regs)
>>   {
>>          if (regno >= DBG_MAX_REG_NUM || regno < 0)
>> @@ -176,18 +179,14 @@ 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 |= (1 << 7);
>
> Hard coded values.

Yes, but this is a RFC.

>> +
>>                  /*
>>                   * Update step address value with address passed
>>                   * with step packet.
>> @@ -198,8 +197,6 @@ 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;
>
> why kgdb_single_step is not set?

I know what you mean, but
I never see differences at least on fast model.
In addition, I'm wondering why other major archs, including x86, don't care this variable.

>> -
>>                  /*
>>                   * Enable single step handling
>>                   */
>> @@ -229,6 +226,18 @@ 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 & (1 << 7))
>> +               regs->pstate |= (1 << 7);
>> +       else
>> +               regs->pstate &= ~(1 << 7);
>> +
> Same as above comment
>
>>          kgdb_handle_exception(1, SIGTRAP, 0, regs);
>>          return 0;
>>   }
>> --
>> 1.7.9.5
>>
>>
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