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, 5 insertions, 9 deletions

diff --git a/arch/x86/kernel/signal_64.c b/arch/x86/kernel/signal_64.c
index 38d806467c0f..ab0178ebe00a 100644
--- a/arch/x86/kernel/signal_64.c
+++ b/arch/x86/kernel/signal_64.c
@@ -349,16 +349,12 @@ handle_signal(unsigned long sig, siginfo_t *info, struct k_sigaction *ka,
         }
 
         /*
-         * If TF is set due to a debugger (PT_DTRACE), clear the TF
-         * flag so that register information in the sigcontext is
-         * correct.
+         * If TF is set due to a debugger (TIF_FORCED_TF), clear the TF
+         * flag so that register information in the sigcontext is correct.
          */
-        if (unlikely(regs->eflags & TF_MASK)) {
-                if (likely(current->ptrace & PT_DTRACE)) {
-                        current->ptrace &= ~PT_DTRACE;
-                        regs->eflags &= ~TF_MASK;
-                }
-        }
+        if (unlikely(regs->eflags & X86_EFLAGS_TF) &&
+            likely(test_and_clear_thread_flag(TIF_FORCED_TF)))
+                regs->eflags &= ~X86_EFLAGS_TF;
 
 #ifdef CONFIG_IA32_EMULATION
         if (test_thread_flag(TIF_IA32)) {