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, 17 insertions, 0 deletions

diff --git a/arch/x86/kernel/ptrace_32.c b/arch/x86/kernel/ptrace_32.c
index b739608..bc7fd80 100644
--- a/arch/x86/kernel/ptrace_32.c
+++ b/arch/x86/kernel/ptrace_32.c
@@ -104,6 +104,15 @@ static int putreg(struct task_struct *child,
 			break;
 		case EFL:
 			value &= FLAG_MASK;
+			/*
+			 * If the user value contains TF, mark that
+			 * it was not "us" (the debugger) that set it.
+			 * If not, make sure it stays set if we had.
+			 */
+			if (value & X86_EFLAGS_TF)
+				clear_tsk_thread_flag(child, TIF_FORCED_TF);
+			else if (test_tsk_thread_flag(child, TIF_FORCED_TF))
+				value |= X86_EFLAGS_TF;
 			value |= get_stack_long(child, EFL_OFFSET) & ~FLAG_MASK;
 			break;
 	}
@@ -119,6 +128,14 @@ static unsigned long getreg(struct task_struct *child,
 	unsigned long retval = ~0UL;
 
 	switch (regno >> 2) {
+		case EFL:
+			/*
+			 * If the debugger set TF, hide it from the readout.
+			 */
+			retval = get_stack_long(child, EFL_OFFSET);
+			if (test_tsk_thread_flag(child, TIF_FORCED_TF))
+				retval &= ~X86_EFLAGS_TF;
+			break;
 		case GS:
 			retval = child->thread.gs;
 			break;