x86, ptrace: Fix block-step

Implement ptrace-block-step using TIF_BLOCKSTEP which will set
DEBUGCTLMSR_BTF when set for a task while preserving any other
DEBUGCTLMSR bits.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100325135414.017536066@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

4 files changed, 48 insertions, 6 deletions

diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
index 7a880ad3a208..f2f56c0967b6 100644
--- a/arch/x86/kernel/kprobes.c
+++ b/arch/x86/kernel/kprobes.c
@@ -422,12 +422,22 @@ static void __kprobes set_current_kprobe(struct kprobe *p, struct pt_regs *regs,
 
 static void __kprobes clear_btf(void)
 {
-        /* XXX */
+        if (test_thread_flag(TIF_BLOCKSTEP)) {
+                unsigned long debugctl = get_debugctlmsr();
+
+                debugctl &= ~DEBUGCTLMSR_BTF;
+                update_debugctlmsr(debugctl);
+        }
 }
 
 static void __kprobes restore_btf(void)
 {
-        /* XXX */
+        if (test_thread_flag(TIF_BLOCKSTEP)) {
+                unsigned long debugctl = get_debugctlmsr();
+
+                debugctl |= DEBUGCTLMSR_BTF;
+                update_debugctlmsr(debugctl);
+        }
 }
 
 void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 1a60beb32ede..8328009416d7 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -195,6 +195,17 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
         prev = &prev_p->thread;
         next = &next_p->thread;
 
+        if (test_tsk_thread_flag(prev_p, TIF_BLOCKSTEP) ^
+            test_tsk_thread_flag(next_p, TIF_BLOCKSTEP)) {
+                unsigned long debugctl = get_debugctlmsr();
+
+                debugctl &= ~DEBUGCTLMSR_BTF;
+                if (test_tsk_thread_flag(next_p, TIF_BLOCKSTEP))
+                        debugctl |= DEBUGCTLMSR_BTF;
+
+                update_debugctlmsr(debugctl);
+        }
+
         if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
             test_tsk_thread_flag(next_p, TIF_NOTSC)) {
                 /* prev and next are different */
diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 7beba0769a8c..58de45ee08b6 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -169,9 +169,19 @@ static void enable_step(struct task_struct *child, bool block)
          * So noone should try to use debugger block stepping in a program
          * that uses user-mode single stepping itself.
          */
-        if (!enable_single_step(child))
-                return;
-        /* XXX */
+        if (enable_single_step(child) && block) {
+                unsigned long debugctl = get_debugctlmsr();
+
+                debugctl |= DEBUGCTLMSR_BTF;
+                update_debugctlmsr(debugctl);
+                set_tsk_thread_flag(child, TIF_BLOCKSTEP);
+        } else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP)) {
+                unsigned long debugctl = get_debugctlmsr();
+
+                debugctl &= ~DEBUGCTLMSR_BTF;
+                update_debugctlmsr(debugctl);
+                clear_tsk_thread_flag(child, TIF_BLOCKSTEP);
+        }
 }
 
 void user_enable_single_step(struct task_struct *child)
@@ -189,7 +199,13 @@ void user_disable_single_step(struct task_struct *child)
         /*
          * Make sure block stepping (BTF) is disabled.
          */
-        /* XXX */
+        if (test_tsk_thread_flag(child, TIF_BLOCKSTEP)) {
+                unsigned long debugctl = get_debugctlmsr();
+
+                debugctl &= ~DEBUGCTLMSR_BTF;
+                update_debugctlmsr(debugctl);
+                clear_tsk_thread_flag(child, TIF_BLOCKSTEP);
+        }
 
         /* Always clear TIF_SINGLESTEP... */
         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index e3da5d726a37..36f1bd9f8e76 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -544,6 +544,11 @@ dotraplinkage void __kprobes do_debug(struct pt_regs *regs, long error_code)
         /* DR6 may or may not be cleared by the CPU */
         set_debugreg(0, 6);
 
+        /*
+         * The processor cleared BTF, so don't mark that we need it set.
+         */
+        clear_tsk_thread_flag(tsk, TIF_BLOCKSTEP);
+
         /* Store the virtualized DR6 value */
         tsk->thread.debugreg6 = dr6;