x86 single_step: TIF_FORCED_TF

This changes the single-step support to use a new thread_info flag
TIF_FORCED_TF instead of the PT_DTRACE flag in task_struct.ptrace.
This keeps arch implementation uses out of this non-arch field.

This changes the ptrace access to eflags to mask TF and maintain
the TIF_FORCED_TF flag directly if userland sets TF, instead of
relying on ptrace_signal_deliver.  The 64-bit and 32-bit kernels
are harmonized on this same behavior.  The ptrace_signal_deliver
approach works now, but this change makes the low-level register
access code reliable when called from different contexts than a
ptrace stop, which will be possible in the future.

The 64-bit do_debug exception handler is also changed not to clear TF
from user-mode registers.  This matches the 32-bit kernel's behavior.

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

1 files changed, 3 insertions, 6 deletions

diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 6732272e3479..243bff650ca5 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -135,7 +135,7 @@ void user_enable_single_step(struct task_struct *child)
 	if (is_setting_trap_flag(child, regs))
 		return;
 
-	child->ptrace |= PT_DTRACE;
+	set_tsk_thread_flag(child, TIF_FORCED_TF);
 }
 
 void user_disable_single_step(struct task_struct *child)
@@ -144,9 +144,6 @@ void user_disable_single_step(struct task_struct *child)
 	clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 
 	/* But touch TF only if it was set by us.. */
-	if (child->ptrace & PT_DTRACE) {
-		struct pt_regs *regs = task_pt_regs(child);
-		regs->eflags &= ~X86_EFLAGS_TF;
-		child->ptrace &= ~PT_DTRACE;
-	}
+	if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
+		task_pt_regs(child)->eflags &= ~X86_EFLAGS_TF;
 }