[PATCH] x86: fix TF bit corner case with ptrace and signals

If a debugger set the TF bit, make sure to clear it when creating a
signal context.  Otherwise, TF will be incorrectly restored by
sigreturn.
Signed-off-by: Daniel Jacobowitz <dan@debian.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/signal.c

@@ -277,6 +277,18 @@ setup_sigcontext(struct sigcontext __user *sc, struct _fpstate __user *fpstate,
 {
 	int tmp, err = 0;
 
+	/*
+	 * If TF is set due to a debugger (PT_DTRACE), 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;
+		}
+	}
+
 	tmp = 0;
 	__asm__("movl %%gs,%0" : "=r"(tmp): "0"(tmp));
 	err |= __put_user(tmp, (unsigned int __user *)&sc->gs);