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

On Mon, Jan 05, 2015 at 11:52:42PM +0000, AKASHI Takahiro wrote:

Hi Will,

Hi Akashi,

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.

As I asked here:

http://lists.infradead.org/pipermail/linux-arm-kernel/2014-October/297237.ht...

I'd like to see an ack/feedback from a core kgdb person on this patch, because we're basically disabling interrupts in the low-level handler to deal with potential re-entrancy problems that aren't handled or prevented by the higher levels of the stack.

I've added Jason for feedback... (main question is whether kgdb expects the architecture to disable interrupts during a single step, otherwise we can get a recursive debug exception due to something like a breakpoint firing on the interrupt handler)

Will

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 takahiro.akashi@linaro.org

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; }