kgdb, x86, arm, mips, powerpc: ignore user space single stepping

On the x86 arch, user space single step exceptions should be ignored
if they occur in the kernel space, such as ptrace stepping through a
system call.

First check if it is kgdb that is executing a single step, then ensure
it is not an accidental traversal into the user space, while in kgdb,
any other time the TIF_SINGLESTEP is set, kgdb should ignore the
exception.

On x86, arm, mips and powerpc, the kgdb_contthread usage was
inconsistent with the way single stepping is implemented in the kgdb
core. The arch specific stub should always set the
kgdb_cpu_doing_single_step correctly if it is single stepping. This
allows kgdb to correctly process an instruction steps if ptrace
happens to be requesting an instruction step over a system call.

Signed-off-by: Jason Wessel <[e-mail]>

arch/arm/kernel/kgdb.c

···
 	case 'D':
 	case 'k':
 	case 'c':
-		kgdb_contthread = NULL;
-
 		/*
 		 * Try to read optional parameter, pc unchanged if no parm.
 		 * If this was a compiled breakpoint, we need to move

arch/mips/kernel/kgdb.c

···
 
 		atomic_set(&kgdb_cpu_doing_single_step, -1);
 		if (remcom_in_buffer[0] == 's')
-			if (kgdb_contthread)
-				atomic_set(&kgdb_cpu_doing_single_step, cpu);
+			atomic_set(&kgdb_cpu_doing_single_step, cpu);
 
 		return 0;
 	}

arch/powerpc/kernel/kgdb.c

···
 			linux_regs->msr |= MSR_SE;
 #endif
 			kgdb_single_step = 1;
-			if (kgdb_contthread)
-				atomic_set(&kgdb_cpu_doing_single_step,
-					   raw_smp_processor_id());
+			atomic_set(&kgdb_cpu_doing_single_step,
+				   raw_smp_processor_id());
 		}
 		return 0;
 	}

arch/x86/kernel/kgdb.c

···
 		if (remcomInBuffer[0] == 's') {
 			linux_regs->flags |= X86_EFLAGS_TF;
 			kgdb_single_step = 1;
-			if (kgdb_contthread) {
-				atomic_set(&kgdb_cpu_doing_single_step,
-					   raw_smp_processor_id());
-			}
+			atomic_set(&kgdb_cpu_doing_single_step,
+				   raw_smp_processor_id());
 		}
 
 		get_debugreg(dr6, 6);
···
 
 	case DIE_DEBUG:
 		if (atomic_read(&kgdb_cpu_doing_single_step) ==
-			raw_smp_processor_id() &&
-			user_mode(regs))
-			return single_step_cont(regs, args);
+		    raw_smp_processor_id()) {
+			if (user_mode(regs))
+				return single_step_cont(regs, args);
+			break;
+		} else if (test_thread_flag(TIF_SINGLESTEP))
+			/* This means a user thread is single stepping
+			 * a system call which should be ignored
+			 */
+			return NOTIFY_DONE;
 		/* fall through */
 	default:
 		if (user_mode(regs))

kernel/kgdb.c

···
 	 * Get the passive CPU lock which will hold all the non-primary
 	 * CPU in a spin state while the debugger is active
 	 */
-	if (!kgdb_single_step || !kgdb_contthread) {
+	if (!kgdb_single_step) {
 		for (i = 0; i < NR_CPUS; i++)
 			atomic_set(&passive_cpu_wait[i], 1);
 	}
···
 
 #ifdef CONFIG_SMP
 	/* Signal the other CPUs to enter kgdb_wait() */
-	if ((!kgdb_single_step || !kgdb_contthread) && kgdb_do_roundup)
+	if ((!kgdb_single_step) && kgdb_do_roundup)
 		kgdb_roundup_cpus(flags);
 #endif
 
···
 	kgdb_post_primary_code(ks->linux_regs, ks->ex_vector, ks->err_code);
 	kgdb_deactivate_sw_breakpoints();
 	kgdb_single_step = 0;
-	kgdb_contthread = NULL;
+	kgdb_contthread = current;
 	exception_level = 0;
 
 	/* Talk to debugger with gdbserial protocol */
···
 	kgdb_info[ks->cpu].task = NULL;
 	atomic_set(&cpu_in_kgdb[ks->cpu], 0);
 
-	if (!kgdb_single_step || !kgdb_contthread) {
+	if (!kgdb_single_step) {
 		for (i = NR_CPUS-1; i >= 0; i--)
 			atomic_set(&passive_cpu_wait[i], 0);
 		/*