5 files changed, 204 insertions, 78 deletions

diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index db6720edfdd0..8bfb6146f753 100644
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c
@@ -43,6 +43,8 @@
 #include <linux/smp.h>
 #include <linux/nmi.h>
 #include <linux/hw_breakpoint.h>
+#include <linux/uaccess.h>
+#include <linux/memory.h>
 
 #include <asm/debugreg.h>
 #include <asm/apicdef.h>
@@ -741,6 +743,64 @@ void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long ip)
         regs->ip = ip;
 }
 
+int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
+{
+        int err;
+        char opc[BREAK_INSTR_SIZE];
+
+        bpt->type = BP_BREAKPOINT;
+        err = probe_kernel_read(bpt->saved_instr, (char *)bpt->bpt_addr,
+                                BREAK_INSTR_SIZE);
+        if (err)
+                return err;
+        err = probe_kernel_write((char *)bpt->bpt_addr,
+                                 arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE);
+#ifdef CONFIG_DEBUG_RODATA
+        if (!err)
+                return err;
+        /*
+         * It is safe to call text_poke() because normal kernel execution
+         * is stopped on all cores, so long as the text_mutex is not locked.
+         */
+        if (mutex_is_locked(&text_mutex))
+                return -EBUSY;
+        text_poke((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
+                  BREAK_INSTR_SIZE);
+        err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
+        if (err)
+                return err;
+        if (memcmp(opc, arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE))
+                return -EINVAL;
+        bpt->type = BP_POKE_BREAKPOINT;
+#endif /* CONFIG_DEBUG_RODATA */
+        return err;
+}
+
+int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
+{
+#ifdef CONFIG_DEBUG_RODATA
+        int err;
+        char opc[BREAK_INSTR_SIZE];
+
+        if (bpt->type != BP_POKE_BREAKPOINT)
+                goto knl_write;
+        /*
+         * It is safe to call text_poke() because normal kernel execution
+         * is stopped on all cores, so long as the text_mutex is not locked.
+         */
+        if (mutex_is_locked(&text_mutex))
+                goto knl_write;
+        text_poke((void *)bpt->bpt_addr, bpt->saved_instr, BREAK_INSTR_SIZE);
+        err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
+        if (err || memcmp(opc, bpt->saved_instr, BREAK_INSTR_SIZE))
+                goto knl_write;
+        return err;
+knl_write:
+#endif /* CONFIG_DEBUG_RODATA */
+        return probe_kernel_write((char *)bpt->bpt_addr,
+                                  (char *)bpt->saved_instr, BREAK_INSTR_SIZE);
+}
+
 struct kgdb_arch arch_kgdb_ops = {
         /* Breakpoint instruction: */
         .gdb_bpt_instr          = { 0xcc },
diff --git a/drivers/misc/kgdbts.c b/drivers/misc/kgdbts.c
index 3f7ad83ed740..3aa9a969b373 100644
--- a/drivers/misc/kgdbts.c
+++ b/drivers/misc/kgdbts.c
@@ -134,12 +134,17 @@ static int force_hwbrks;
 static int hwbreaks_ok;
 static int hw_break_val;
 static int hw_break_val2;
+static int cont_instead_of_sstep;
+static unsigned long cont_thread_id;
+static unsigned long sstep_thread_id;
 #if defined(CONFIG_ARM) || defined(CONFIG_MIPS) || defined(CONFIG_SPARC)
 static int arch_needs_sstep_emulation = 1;
 #else
 static int arch_needs_sstep_emulation;
 #endif
+static unsigned long cont_addr;
 static unsigned long sstep_addr;
+static int restart_from_top_after_write;
 static int sstep_state;
 
 /* Storage for the registers, in GDB format. */
@@ -187,7 +192,8 @@ static int kgdbts_unreg_thread(void *ptr)
          */
         while (!final_ack)
                 msleep_interruptible(1500);
-
+        /* Pause for any other threads to exit after final ack. */
+        msleep_interruptible(1000);
         if (configured)
                 kgdb_unregister_io_module(&kgdbts_io_ops);
         configured = 0;
@@ -211,7 +217,7 @@ static unsigned long lookup_addr(char *arg)
         if (!strcmp(arg, "kgdbts_break_test"))
                 addr = (unsigned long)kgdbts_break_test;
         else if (!strcmp(arg, "sys_open"))
-                addr = (unsigned long)sys_open;
+                addr = (unsigned long)do_sys_open;
         else if (!strcmp(arg, "do_fork"))
                 addr = (unsigned long)do_fork;
         else if (!strcmp(arg, "hw_break_val"))
@@ -283,6 +289,16 @@ static void hw_break_val_write(void)
         hw_break_val++;
 }
 
+static int get_thread_id_continue(char *put_str, char *arg)
+{
+        char *ptr = &put_str[11];
+
+        if (put_str[1] != 'T' || put_str[2] != '0')
+                return 1;
+        kgdb_hex2long(&ptr, &cont_thread_id);
+        return 0;
+}
+
 static int check_and_rewind_pc(char *put_str, char *arg)
 {
         unsigned long addr = lookup_addr(arg);
@@ -299,13 +315,21 @@ static int check_and_rewind_pc(char *put_str, char *arg)
         if (addr + BREAK_INSTR_SIZE == ip)
                 offset = -BREAK_INSTR_SIZE;
 #endif
-        if (strcmp(arg, "silent") && ip + offset != addr) {
+
+        if (arch_needs_sstep_emulation && sstep_addr &&
+            ip + offset == sstep_addr &&
+            ((!strcmp(arg, "sys_open") || !strcmp(arg, "do_fork")))) {
+                /* This is special case for emulated single step */
+                v2printk("Emul: rewind hit single step bp\n");
+                restart_from_top_after_write = 1;
+        } else if (strcmp(arg, "silent") && ip + offset != addr) {
                 eprintk("kgdbts: BP mismatch %lx expected %lx\n",
                            ip + offset, addr);
                 return 1;
         }
         /* Readjust the instruction pointer if needed */
         ip += offset;
+        cont_addr = ip;
 #ifdef GDB_ADJUSTS_BREAK_OFFSET
         instruction_pointer_set(&kgdbts_regs, ip);
 #endif
@@ -315,6 +339,8 @@ static int check_and_rewind_pc(char *put_str, char *arg)
 static int check_single_step(char *put_str, char *arg)
 {
         unsigned long addr = lookup_addr(arg);
+        static int matched_id;
+
         /*
          * From an arch indepent point of view the instruction pointer
          * should be on a different instruction
@@ -324,6 +350,29 @@ static int check_single_step(char *put_str, char *arg)
         gdb_regs_to_pt_regs(kgdbts_gdb_regs, &kgdbts_regs);
         v2printk("Singlestep stopped at IP: %lx\n",
                    instruction_pointer(&kgdbts_regs));
+
+        if (sstep_thread_id != cont_thread_id) {
+                /*
+                 * Ensure we stopped in the same thread id as before, else the
+                 * debugger should continue until the original thread that was
+                 * single stepped is scheduled again, emulating gdb's behavior.
+                 */
+                v2printk("ThrID does not match: %lx\n", cont_thread_id);
+                if (arch_needs_sstep_emulation) {
+                        if (matched_id &&
+                            instruction_pointer(&kgdbts_regs) != addr)
+                                goto continue_test;
+                        matched_id++;
+                        ts.idx -= 2;
+                        sstep_state = 0;
+                        return 0;
+                }
+                cont_instead_of_sstep = 1;
+                ts.idx -= 4;
+                return 0;
+        }
+continue_test:
+        matched_id = 0;
         if (instruction_pointer(&kgdbts_regs) == addr) {
                 eprintk("kgdbts: SingleStep failed at %lx\n",
                            instruction_pointer(&kgdbts_regs));
@@ -365,10 +414,40 @@ static int got_break(char *put_str, char *arg)
         return 1;
 }
 
+static void get_cont_catch(char *arg)
+{
+        /* Always send detach because the test is completed at this point */
+        fill_get_buf("D");
+}
+
+static int put_cont_catch(char *put_str, char *arg)
+{
+        /* This is at the end of the test and we catch any and all input */
+        v2printk("kgdbts: cleanup task: %lx\n", sstep_thread_id);
+        ts.idx--;
+        return 0;
+}
+
+static int emul_reset(char *put_str, char *arg)
+{
+        if (strncmp(put_str, "$OK", 3))
+                return 1;
+        if (restart_from_top_after_write) {
+                restart_from_top_after_write = 0;
+                ts.idx = -1;
+        }
+        return 0;
+}
+
 static void emul_sstep_get(char *arg)
 {
         if (!arch_needs_sstep_emulation) {
-                fill_get_buf(arg);
+                if (cont_instead_of_sstep) {
+                        cont_instead_of_sstep = 0;
+                        fill_get_buf("c");
+                } else {
+                        fill_get_buf(arg);
+                }
                 return;
         }
         switch (sstep_state) {
@@ -398,9 +477,11 @@ static void emul_sstep_get(char *arg)
 static int emul_sstep_put(char *put_str, char *arg)
 {
         if (!arch_needs_sstep_emulation) {
-                if (!strncmp(put_str+1, arg, 2))
-                        return 0;
-                return 1;
+                char *ptr = &put_str[11];
+                if (put_str[1] != 'T' || put_str[2] != '0')
+                        return 1;
+                kgdb_hex2long(&ptr, &sstep_thread_id);
+                return 0;
         }
         switch (sstep_state) {
         case 1:
@@ -411,8 +492,7 @@ static int emul_sstep_put(char *put_str, char *arg)
                 v2printk("Stopped at IP: %lx\n",
                          instruction_pointer(&kgdbts_regs));
                 /* Want to stop at IP + break instruction size by default */
-                sstep_addr = instruction_pointer(&kgdbts_regs) +
-                        BREAK_INSTR_SIZE;
+                sstep_addr = cont_addr + BREAK_INSTR_SIZE;
                 break;
         case 2:
                 if (strncmp(put_str, "$OK", 3)) {
@@ -424,6 +504,9 @@ static int emul_sstep_put(char *put_str, char *arg)
                 if (strncmp(put_str, "$T0", 3)) {
                         eprintk("kgdbts: failed continue sstep\n");
                         return 1;
+                } else {
+                        char *ptr = &put_str[11];
+                        kgdb_hex2long(&ptr, &sstep_thread_id);
                 }
                 break;
         case 4:
@@ -502,10 +585,10 @@ static struct test_struct bad_read_test[] = {
 static struct test_struct singlestep_break_test[] = {
         { "?", "S0*" }, /* Clear break points */
         { "kgdbts_break_test", "OK", sw_break, }, /* set sw breakpoint */
-        { "c", "T0*", }, /* Continue */
+        { "c", "T0*", NULL, get_thread_id_continue }, /* Continue */
+        { "kgdbts_break_test", "OK", sw_rem_break }, /*remove breakpoint */
         { "g", "kgdbts_break_test", NULL, check_and_rewind_pc },
         { "write", "OK", write_regs }, /* Write registers */
-        { "kgdbts_break_test", "OK", sw_rem_break }, /*remove breakpoint */
         { "s", "T0*", emul_sstep_get, emul_sstep_put }, /* Single step */
         { "g", "kgdbts_break_test", NULL, check_single_step },
         { "kgdbts_break_test", "OK", sw_break, }, /* set sw breakpoint */
@@ -523,16 +606,16 @@ static struct test_struct singlestep_break_test[] = {
 static struct test_struct do_fork_test[] = {
         { "?", "S0*" }, /* Clear break points */
         { "do_fork", "OK", sw_break, }, /* set sw breakpoint */
-        { "c", "T0*", }, /* Continue */
-        { "g", "do_fork", NULL, check_and_rewind_pc }, /* check location */
-        { "write", "OK", write_regs }, /* Write registers */
+        { "c", "T0*", NULL, get_thread_id_continue }, /* Continue */
         { "do_fork", "OK", sw_rem_break }, /*remove breakpoint */
+        { "g", "do_fork", NULL, check_and_rewind_pc }, /* check location */
+        { "write", "OK", write_regs, emul_reset }, /* Write registers */
         { "s", "T0*", emul_sstep_get, emul_sstep_put }, /* Single step */
         { "g", "do_fork", NULL, check_single_step },
         { "do_fork", "OK", sw_break, }, /* set sw breakpoint */
         { "7", "T0*", skip_back_repeat_test }, /* Loop based on repeat_test */
         { "D", "OK", NULL, final_ack_set }, /* detach and unregister I/O */
-        { "", "" },
+        { "", "", get_cont_catch, put_cont_catch },
 };
 
 /* Test for hitting a breakpoint at sys_open for what ever the number
@@ -541,16 +624,16 @@ static struct test_struct do_fork_test[] = {
 static struct test_struct sys_open_test[] = {
         { "?", "S0*" }, /* Clear break points */
         { "sys_open", "OK", sw_break, }, /* set sw breakpoint */
-        { "c", "T0*", }, /* Continue */
-        { "g", "sys_open", NULL, check_and_rewind_pc }, /* check location */
-        { "write", "OK", write_regs }, /* Write registers */
+        { "c", "T0*", NULL, get_thread_id_continue }, /* Continue */
         { "sys_open", "OK", sw_rem_break }, /*remove breakpoint */
+        { "g", "sys_open", NULL, check_and_rewind_pc }, /* check location */
+        { "write", "OK", write_regs, emul_reset }, /* Write registers */
         { "s", "T0*", emul_sstep_get, emul_sstep_put }, /* Single step */
         { "g", "sys_open", NULL, check_single_step },
         { "sys_open", "OK", sw_break, }, /* set sw breakpoint */
         { "7", "T0*", skip_back_repeat_test }, /* Loop based on repeat_test */
         { "D", "OK", NULL, final_ack_set }, /* detach and unregister I/O */
-        { "", "" },
+        { "", "", get_cont_catch, put_cont_catch },
 };
 
 /*
@@ -693,8 +776,8 @@ static int run_simple_test(int is_get_char, int chr)
         /* This callback is a put char which is when kgdb sends data to
          * this I/O module.
          */
-        if (ts.tst[ts.idx].get[0] == '\0' &&
-                ts.tst[ts.idx].put[0] == '\0') {
+        if (ts.tst[ts.idx].get[0] == '\0' && ts.tst[ts.idx].put[0] == '\0' &&
+            !ts.tst[ts.idx].get_handler) {
                 eprintk("kgdbts: ERROR: beyond end of test on"
                            " '%s' line %i\n", ts.name, ts.idx);
                 return 0;
@@ -907,6 +990,17 @@ static void kgdbts_run_tests(void)
         if (ptr)
                 sstep_test = simple_strtol(ptr+1, NULL, 10);
 
+        /* All HW break point tests */
+        if (arch_kgdb_ops.flags & KGDB_HW_BREAKPOINT) {
+                hwbreaks_ok = 1;
+                v1printk("kgdbts:RUN hw breakpoint test\n");
+                run_breakpoint_test(1);
+                v1printk("kgdbts:RUN hw write breakpoint test\n");
+                run_hw_break_test(1);
+                v1printk("kgdbts:RUN access write breakpoint test\n");
+                run_hw_break_test(0);
+        }
+
         /* required internal KGDB tests */
         v1printk("kgdbts:RUN plant and detach test\n");
         run_plant_and_detach_test(0);
@@ -924,35 +1018,11 @@ static void kgdbts_run_tests(void)
 
         /* ===Optional tests=== */
 
-        /* All HW break point tests */
-        if (arch_kgdb_ops.flags & KGDB_HW_BREAKPOINT) {
-                hwbreaks_ok = 1;
-                v1printk("kgdbts:RUN hw breakpoint test\n");
-                run_breakpoint_test(1);
-                v1printk("kgdbts:RUN hw write breakpoint test\n");
-                run_hw_break_test(1);
-                v1printk("kgdbts:RUN access write breakpoint test\n");
-                run_hw_break_test(0);
-        }
-
         if (nmi_sleep) {
                 v1printk("kgdbts:RUN NMI sleep %i seconds test\n", nmi_sleep);
                 run_nmi_sleep_test(nmi_sleep);
         }
 
-#ifdef CONFIG_DEBUG_RODATA
-        /* Until there is an api to write to read-only text segments, use
-         * HW breakpoints for the remainder of any tests, else print a
-         * failure message if hw breakpoints do not work.
-         */
-        if (!(arch_kgdb_ops.flags & KGDB_HW_BREAKPOINT && hwbreaks_ok)) {
-                eprintk("kgdbts: HW breakpoints do not work,"
-                        "skipping remaining tests\n");
-                return;
-        }
-        force_hwbrks = 1;
-#endif /* CONFIG_DEBUG_RODATA */
-
         /* If the do_fork test is run it will be the last test that is
          * executed because a kernel thread will be spawned at the very
          * end to unregister the debug hooks.
diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
index fa391835508d..c4d2fc194ede 100644
--- a/include/linux/kgdb.h
+++ b/include/linux/kgdb.h
@@ -63,7 +63,8 @@ enum kgdb_bptype {
         BP_HARDWARE_BREAKPOINT,
         BP_WRITE_WATCHPOINT,
         BP_READ_WATCHPOINT,
-        BP_ACCESS_WATCHPOINT
+        BP_ACCESS_WATCHPOINT,
+        BP_POKE_BREAKPOINT,
 };
 
 enum kgdb_bpstate {
@@ -207,8 +208,8 @@ extern void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long pc);
 
 /* Optional functions. */
 extern int kgdb_validate_break_address(unsigned long addr);
-extern int kgdb_arch_set_breakpoint(unsigned long addr, char *saved_instr);
-extern int kgdb_arch_remove_breakpoint(unsigned long addr, char *bundle);
+extern int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt);
+extern int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt);
 
 /**
  *      kgdb_arch_late - Perform any architecture specific initalization.
diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index 1dc53bae56e1..0557f24c6bca 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -160,37 +160,39 @@ early_param("nokgdbroundup", opt_nokgdbroundup);
  * Weak aliases for breakpoint management,
  * can be overriden by architectures when needed:
  */
-int __weak kgdb_arch_set_breakpoint(unsigned long addr, char *saved_instr)
+int __weak kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
 {
         int err;
 
-        err = probe_kernel_read(saved_instr, (char *)addr, BREAK_INSTR_SIZE);
+        err = probe_kernel_read(bpt->saved_instr, (char *)bpt->bpt_addr,
+                                BREAK_INSTR_SIZE);
         if (err)
                 return err;
-
-        return probe_kernel_write((char *)addr, arch_kgdb_ops.gdb_bpt_instr,
-                                  BREAK_INSTR_SIZE);
+        err = probe_kernel_write((char *)bpt->bpt_addr,
+                                 arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE);
+        return err;
 }
 
-int __weak kgdb_arch_remove_breakpoint(unsigned long addr, char *bundle)
+int __weak kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
 {
-        return probe_kernel_write((char *)addr,
-                                  (char *)bundle, BREAK_INSTR_SIZE);
+        return probe_kernel_write((char *)bpt->bpt_addr,
+                                  (char *)bpt->saved_instr, BREAK_INSTR_SIZE);
 }
 
 int __weak kgdb_validate_break_address(unsigned long addr)
 {
-        char tmp_variable[BREAK_INSTR_SIZE];
+        struct kgdb_bkpt tmp;
         int err;
-        /* Validate setting the breakpoint and then removing it.  In the
+        /* Validate setting the breakpoint and then removing it.  If the
          * remove fails, the kernel needs to emit a bad message because we
          * are deep trouble not being able to put things back the way we
          * found them.
          */
-        err = kgdb_arch_set_breakpoint(addr, tmp_variable);
+        tmp.bpt_addr = addr;
+        err = kgdb_arch_set_breakpoint(&tmp);
         if (err)
                 return err;
-        err = kgdb_arch_remove_breakpoint(addr, tmp_variable);
+        err = kgdb_arch_remove_breakpoint(&tmp);
         if (err)
                 printk(KERN_ERR "KGDB: Critical breakpoint error, kernel "
                    "memory destroyed at: %lx", addr);
@@ -234,7 +236,6 @@ static void kgdb_flush_swbreak_addr(unsigned long addr)
  */
 int dbg_activate_sw_breakpoints(void)
 {
-        unsigned long addr;
         int error;
         int ret = 0;
         int i;
@@ -243,16 +244,15 @@ int dbg_activate_sw_breakpoints(void)
                 if (kgdb_break[i].state != BP_SET)
                         continue;
 
-                addr = kgdb_break[i].bpt_addr;
-                error = kgdb_arch_set_breakpoint(addr,
-                                kgdb_break[i].saved_instr);
+                error = kgdb_arch_set_breakpoint(&kgdb_break[i]);
                 if (error) {
                         ret = error;
-                        printk(KERN_INFO "KGDB: BP install failed: %lx", addr);
+                        printk(KERN_INFO "KGDB: BP install failed: %lx",
+                               kgdb_break[i].bpt_addr);
                         continue;
                 }
 
-                kgdb_flush_swbreak_addr(addr);
+                kgdb_flush_swbreak_addr(kgdb_break[i].bpt_addr);
                 kgdb_break[i].state = BP_ACTIVE;
         }
         return ret;
@@ -301,7 +301,6 @@ int dbg_set_sw_break(unsigned long addr)
 
 int dbg_deactivate_sw_breakpoints(void)
 {
-        unsigned long addr;
         int error;
         int ret = 0;
         int i;
@@ -309,15 +308,14 @@ int dbg_deactivate_sw_breakpoints(void)
         for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                 if (kgdb_break[i].state != BP_ACTIVE)
                         continue;
-                addr = kgdb_break[i].bpt_addr;
-                error = kgdb_arch_remove_breakpoint(addr,
-                                        kgdb_break[i].saved_instr);
+                error = kgdb_arch_remove_breakpoint(&kgdb_break[i]);
                 if (error) {
-                        printk(KERN_INFO "KGDB: BP remove failed: %lx\n", addr);
+                        printk(KERN_INFO "KGDB: BP remove failed: %lx\n",
+                               kgdb_break[i].bpt_addr);
                         ret = error;
                 }
 
-                kgdb_flush_swbreak_addr(addr);
+                kgdb_flush_swbreak_addr(kgdb_break[i].bpt_addr);
                 kgdb_break[i].state = BP_SET;
         }
         return ret;
@@ -351,7 +349,6 @@ int kgdb_isremovedbreak(unsigned long addr)
 
 int dbg_remove_all_break(void)
 {
-        unsigned long addr;
         int error;
         int i;
 
@@ -359,12 +356,10 @@ int dbg_remove_all_break(void)
         for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                 if (kgdb_break[i].state != BP_ACTIVE)
                         goto setundefined;
-                addr = kgdb_break[i].bpt_addr;
-                error = kgdb_arch_remove_breakpoint(addr,
-                                kgdb_break[i].saved_instr);
+                error = kgdb_arch_remove_breakpoint(&kgdb_break[i]);
                 if (error)
                         printk(KERN_ERR "KGDB: breakpoint remove failed: %lx\n",
-                           addr);
+                               kgdb_break[i].bpt_addr);
 setundefined:
                 kgdb_break[i].state = BP_UNDEFINED;
         }
diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 9b5f17da1c56..bb9520f0f6ff 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -743,7 +743,7 @@ kdb_printit:
                 kdb_input_flush();
                 c = console_drivers;
 
-                if (!dbg_io_ops->is_console) {
+                if (dbg_io_ops && !dbg_io_ops->is_console) {
                         len = strlen(moreprompt);
                         cp = moreprompt;
                         while (len--) {