[PATCH] arm64: kgdb: Ensure atomic single-step execution

[Mengchen Li <mengchenli64@gmail.com>]

The existing KGDB single-step handling on ARM64 is susceptible to
interference from external interrupts. If an interrupt arrives in the
narrow time window between the execution of the instruction under test
and the generation of the step exception, the CPU will vector to the
interrupt handler (e.g., el1_interrupt -> __el1_irq) instead of
triggering the debug exception immediately.

When the step exception is finally taken, the context is no longer that
of the original instruction. This causes the debugger to appear "stuck",
as it repeatedly tries to single-step through the interrupt handler's
code (e.g., irq_enter_rcu()) instead of the target code.

The fix is to make the single-step operation atomic by masking interrupts
for its duration:
1. Upon receiving a step ('s') request from GDB, save the current
   PSTATE and then mask IRQs by setting the PSTATE.I bit.
2. After the single-step exception is taken, in kgdb_single_step_handler(),
   disable the kernel's single-step mechanism and meticulously restore
   the original interrupt mask state from the saved PSTATE.

This guarantees the instruction is executed without interruption and the
debug exception is taken in the correct context.

As a result of this new approach, the following cleanups are also made:
- The global `kgdb_single_step` flag is removed, as state is now precisely
  managed by `kgdb_cpu_doing_single_step` and the interrupt mask.
- The logic to disable single-step and manage the flag in the 'c'ontinue
  case is removed, as it is rendered redundant.
- The call to `kernel_rewind_single_step()` is unnecessary and is removed.

Tested on OrangePi 3B (RK3566) via serial console (kgdboc);
allows reliable single-stepping with GDB where it previously failed.

Signed-off-by: Mengchen Li <mengchenli64@gmail.com>
---
 arch/arm64/kernel/kgdb.c | 49 ++++++++++++++++++++----------------------------
 1 file changed, 20 insertions(+), 29 deletions(-)

diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
index 968324a..ee8a7e3 100644
--- a/arch/arm64/kernel/kgdb.c
+++ b/arch/arm64/kernel/kgdb.c
@@ -101,6 +101,8 @@ 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)
@@ -128,25 +130,15 @@ int dbg_set_reg(int regno, void *mem, struct pt_regs *regs)
 void
 sleeping_thread_to_gdb_regs(unsigned long *gdb_regs, struct task_struct *task)
 {
-	struct cpu_context *cpu_context = &task->thread.cpu_context;
+	struct pt_regs *thread_regs;
 
 	/* Initialize to zero */
 	memset((char *)gdb_regs, 0, NUMREGBYTES);
 
-	gdb_regs[19] = cpu_context->x19;
-	gdb_regs[20] = cpu_context->x20;
-	gdb_regs[21] = cpu_context->x21;
-	gdb_regs[22] = cpu_context->x22;
-	gdb_regs[23] = cpu_context->x23;
-	gdb_regs[24] = cpu_context->x24;
-	gdb_regs[25] = cpu_context->x25;
-	gdb_regs[26] = cpu_context->x26;
-	gdb_regs[27] = cpu_context->x27;
-	gdb_regs[28] = cpu_context->x28;
-	gdb_regs[29] = cpu_context->fp;
-
-	gdb_regs[31] = cpu_context->sp;
-	gdb_regs[32] = cpu_context->pc;
+	thread_regs = task_pt_regs(task);
+	memcpy((void *)gdb_regs, (void *)thread_regs->regs, GP_REG_BYTES);
+	/* Special case for PSTATE */
+	dbg_get_reg(33, gdb_regs + GP_REG_BYTES, thread_regs);
 }
 
 void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long pc)
@@ -195,18 +187,12 @@ 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);
 		/*
 		 * Update step address value with address passed
 		 * with step packet.
@@ -217,15 +203,12 @@ 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
 		 */
 		if (!kernel_active_single_step())
 			kernel_enable_single_step(linux_regs);
-		else
-			kernel_rewind_single_step(linux_regs);
 		err = 0;
 		break;
 	default:
@@ -252,9 +235,17 @@ NOKPROBE_SYMBOL(kgdb_compiled_brk_handler);
 
 int kgdb_single_step_handler(struct pt_regs *regs, unsigned long esr)
 {
-	if (!kgdb_single_step)
-		return DBG_HOOK_ERROR;
+	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);
 	kgdb_handle_exception(0, SIGTRAP, 0, regs);
 	return DBG_HOOK_HANDLED;
 }
-- 
2.7.4