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>

1 files changed, 12 insertions, 2 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,