[PATCH 3/3] x86/debug: Fix PTRACE_{BLOCK,SINGLE}STEP vs ptrace_get_debugreg(6)

[Peter Zijlstra <peterz@infradead.org>]

Commit d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to
thread.virtual_dr6") changed the semantics of the variable from random
collection of bits, to exactly only those bits that ptrace() needs.

Unfortunately we lost DR_STEP for PTRACE_{BLOCK,SINGLE}STEP.

Fixes: d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to thread.virtual_dr6")
Reported-by: Kyle Huey <me@kylehuey.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/include/asm/ptrace.h |    2 ++
 arch/x86/kernel/step.c        |    9 +++++++++
 arch/x86/kernel/traps.c       |    2 +-
 3 files changed, 12 insertions(+), 1 deletion(-)

--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -355,6 +355,8 @@ static inline unsigned long regs_get_ker
 #define arch_has_block_step()	(boot_cpu_data.x86 >= 6)
 #endif
 
+extern unsigned long user_dr_step(unsigned long dr6);
+
 #define ARCH_HAS_USER_SINGLE_STEP_REPORT
 
 struct user_desc;
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -235,3 +235,12 @@ void user_disable_single_step(struct tas
 	if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
 		task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
 }
+
+unsigned long user_dr_step(unsigned long dr6)
+{
+	if (test_thread_flag(TIF_BLOCKSTEP) ||
+	    test_thread_flag(TIF_SINGLESTEP))
+		return dr6 & DR_STEP;
+
+	return 0;
+}
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -940,7 +940,7 @@ static __always_inline void exc_debug_us
 	 * Clear the virtual DR6 value, ptrace() routines will set bits here
 	 * for things it wants signals for.
 	 */
-	current->thread.virtual_dr6 = 0;
+	current->thread.virtual_dr6 = user_dr_step(dr6);
 
 	/*
 	 * The SDM says "The processor clears the BTF flag when it