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

From: [email-masked] (AKASHI Takahiro)
Date: Wed, 07 Jan 2015 09:12:19 +0900

Vijay,

On 01/07/2015 01:37 AM, Kumar, Vijaya wrote:
> Hi Will,
>
>    I could not reproduce this issue on my simulator.
>   For now if this is validated on any platform, it should be ok.

Let me make sure that you tried the steps that I suggested in:
http://lists.infradead.org/pipermail/linux-arm-kernel/2014-October/297567.html

but still couldn't reproduce this problem?

-Takahiro AKASHI

> Regards
> Vijay
>
> ________________________________________
> From: Yong Zhao <[email-masked]>
> Sent: Tuesday, January 6, 2015 9:52 PM
> To: AKASHI Takahiro; catalin.marinas at arm.com; will.deacon at arm.com
> Cc: dsaxena at linaro.org; Kumar, Vijaya; linux-arm-kernel at lists.infradead.org; linaro-kernel at lists.linaro.org; linux-kernel at vger.kernel.org
> Subject: Re: [PATCH] arm64: kgdb: fix single stepping
>
> 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;
>>>    }
>>>
>>>
>