x86: debugctlmsr arch_has_block_step

This implements user-mode step-until-branch on x86 using the BTF bit
in MSR_IA32_DEBUGCTLMSR.  It's just like single-step, only less so.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

3 files changed, 73 insertions, 3 deletions

diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 243bff650ca5..cf4b9dac4a05 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -107,7 +107,10 @@ static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
         return 0;
 }
 
-void user_enable_single_step(struct task_struct *child)
+/*
+ * Enable single-stepping.  Return nonzero if user mode is not using TF itself.
+ */
+static int enable_single_step(struct task_struct *child)
 {
         struct pt_regs *regs = task_pt_regs(child);
 
@@ -122,7 +125,7 @@ void user_enable_single_step(struct task_struct *child)
          * If TF was already set, don't do anything else
          */
         if (regs->eflags & X86_EFLAGS_TF)
-                return;
+                return 0;
 
         /* Set TF on the kernel stack.. */
         regs->eflags |= X86_EFLAGS_TF;
@@ -133,13 +136,68 @@ void user_enable_single_step(struct task_struct *child)
          * won't clear it by hand later.
          */
         if (is_setting_trap_flag(child, regs))
-                return;
+                return 0;
 
         set_tsk_thread_flag(child, TIF_FORCED_TF);
+
+        return 1;
+}
+
+/*
+ * Install this value in MSR_IA32_DEBUGCTLMSR whenever child is running.
+ */
+static void write_debugctlmsr(struct task_struct *child, unsigned long val)
+{
+        child->thread.debugctlmsr = val;
+
+        if (child != current)
+                return;
+
+#ifdef CONFIG_X86_64
+        wrmsrl(MSR_IA32_DEBUGCTLMSR, val);
+#else
+        wrmsr(MSR_IA32_DEBUGCTLMSR, val, 0);
+#endif
+}
+
+/*
+ * Enable single or block step.
+ */
+static void enable_step(struct task_struct *child, bool block)
+{
+        /*
+         * Make sure block stepping (BTF) is not enabled unless it should be.
+         * Note that we don't try to worry about any is_setting_trap_flag()
+         * instructions after the first when using block stepping.
+         * So noone should try to use debugger block stepping in a program
+         * that uses user-mode single stepping itself.
+         */
+        if (enable_single_step(child) && block) {
+                set_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
+                write_debugctlmsr(child, DEBUGCTLMSR_BTF);
+        } else if (test_and_clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR)) {
+                write_debugctlmsr(child, 0);
+        }
+}
+
+void user_enable_single_step(struct task_struct *child)
+{
+        enable_step(child, 0);
+}
+
+void user_enable_block_step(struct task_struct *child)
+{
+        enable_step(child, 1);
 }
 
 void user_disable_single_step(struct task_struct *child)
 {
+        /*
+         * Make sure block stepping (BTF) is disabled.
+         */
+        if (test_and_clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR))
+                write_debugctlmsr(child, 0);
+
         /* Always clear TIF_SINGLESTEP... */
         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 
diff --git a/arch/x86/kernel/traps_32.c b/arch/x86/kernel/traps_32.c
index 02d1e1e58e81..9b0bbd508cd5 100644
--- a/arch/x86/kernel/traps_32.c
+++ b/arch/x86/kernel/traps_32.c
@@ -837,6 +837,12 @@ fastcall void __kprobes do_debug(struct pt_regs * regs, long error_code)
 
         get_debugreg(condition, 6);
 
+        /*
+         * The processor cleared BTF, so don't mark that we need it set.
+         */
+        clear_tsk_thread_flag(tsk, TIF_DEBUGCTLMSR);
+        tsk->thread.debugctlmsr = 0;
+
         if (notify_die(DIE_DEBUG, "debug", regs, condition, error_code,
                                         SIGTRAP) == NOTIFY_STOP)
                 return;
diff --git a/arch/x86/kernel/traps_64.c b/arch/x86/kernel/traps_64.c
index 874aca397b02..610a64d6bdf0 100644
--- a/arch/x86/kernel/traps_64.c
+++ b/arch/x86/kernel/traps_64.c
@@ -850,6 +850,12 @@ asmlinkage void __kprobes do_debug(struct pt_regs * regs,
 
         get_debugreg(condition, 6);
 
+        /*
+         * The processor cleared BTF, so don't mark that we need it set.
+         */
+        clear_tsk_thread_flag(tsk, TIF_DEBUGCTLMSR);
+        tsk->thread.debugctlmsr = 0;
+
         if (notify_die(DIE_DEBUG, "debug", regs, condition, error_code,
                                                 SIGTRAP) == NOTIFY_STOP)
                 return;