[PATCH 4/6] ptrace/x86: ptrace_write_dr7() should create bp if !disabled

[Oleg Nesterov <oleg@redhat.com>]

24f1e32c "hw-breakpoints: Rewrite the hw-breakpoints layer on top
of perf events" introduced the minor regression. Before this commit

	PTRACE_POKEUSER DR7, enableDR0
	PTRACE_POKEUSER DR0, address

was perfectly valid, now PTRACE_POKEUSER(DR7) fails if DR0 was not
previously initialized by PTRACE_POKEUSER(DR0).

Change ptrace_write_dr7() to do ptrace_register_breakpoint(addr => 0)
if !bp && !disabled. This fixes watchpoint-zeroaddr from ptrace-tests,
see https://bugzilla.redhat.com/show_bug.cgi?id=660204.

Reported-by: Jan Kratochvil <jan.kratochvil@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 arch/x86/kernel/ptrace.c |   17 ++++++++++-------
 1 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 0526368..5c387b3 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -670,13 +670,16 @@ restore:
 		if (!bp) {
 			if (disabled)
 				continue;
-			/*
-			 * We should have at least an inactive breakpoint at
-			 * this slot. It means the user is writing dr7 without
-			 * having written the address register first.
-			 */
-			rc = -EINVAL;
-			break;
+
+			bp = ptrace_register_breakpoint(tsk,
+					len, type, 0, disabled);
+			if (IS_ERR(bp)) {
+				rc = PTR_ERR(bp);
+				break;
+			}
+
+			thread->ptrace_bps[i] = bp;
+			continue;
 		}
 
 		rc = ptrace_modify_breakpoint(bp, len, type, disabled);
-- 
1.5.5.1