[PATCH] Fix x86-64 singlestep through sigreturn system call

This patch just directly clones Davide Libenzi's i386 code for x86-64 in
both 64-bit and 32-bit cases.  With this, the behavior of single-stepping
all system calls is consistent.  

The syscall exit tracing caused by TIF_SINGLESTEP is superfluous in the
case of sysret returns, but harmlessly so (since continuing afterward with
PTRACE_CONT will have cleared TF as well as TIF_SINGLESTEP).  I figured
that little bit of extra processing in the single-step case was better than
adding code to ignore the flag in the sysret case.
Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: <ak@muc.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/entry.S

@@ -297,7 +297,7 @@ int_very_careful:
 	sti
 	SAVE_REST
 	/* Check for syscall exit trace */	
-	testl $(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT),%edx
+	testl $(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SINGLESTEP),%edx
 	jz int_signal
 	pushq %rdi
 	leaq 8(%rsp),%rdi	# &ptregs -> arg1	
@@ -305,6 +305,7 @@ int_very_careful:
 	popq %rdi
 	btr  $TIF_SYSCALL_TRACE,%edi
 	btr  $TIF_SYSCALL_AUDIT,%edi
+	btr  $TIF_SINGLESTEP,%edi
 	jmp int_restore_rest
 	
 int_signal:

arch/x86_64/kernel/ptrace.c

@@ -88,6 +88,7 @@ void ptrace_disable(struct task_struct *child)
 { 
 	long tmp;
 
+	clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 	tmp = get_stack_long(child, EFL_OFFSET) & ~TRAP_FLAG;
 	put_stack_long(child, EFL_OFFSET, tmp);
 }
@@ -344,6 +345,7 @@ asmlinkage long sys_ptrace(long request, long pid, unsigned long addr, long data
 			set_tsk_thread_flag(child,TIF_SYSCALL_TRACE);
 		else
 			clear_tsk_thread_flag(child,TIF_SYSCALL_TRACE);
+		clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 		child->exit_code = data;
 	/* make sure the single step bit is not set. */
 		tmp = get_stack_long(child, EFL_OFFSET);
@@ -395,6 +397,7 @@ asmlinkage long sys_ptrace(long request, long pid, unsigned long addr, long data
 		ret = 0;
 		if (child->state == TASK_ZOMBIE)	/* already dead */
 			break;
+		clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 		child->exit_code = SIGKILL;
 		/* make sure the single step bit is not set. */
 		tmp = get_stack_long(child, EFL_OFFSET) & ~TRAP_FLAG;
@@ -416,6 +419,7 @@ asmlinkage long sys_ptrace(long request, long pid, unsigned long addr, long data
 		}
 		tmp = get_stack_long(child, EFL_OFFSET) | TRAP_FLAG;
 		put_stack_long(child, EFL_OFFSET, tmp);
+		set_tsk_thread_flag(child, TIF_SINGLESTEP);
 		child->exit_code = data;
 		/* give it a chance to run. */
 		wake_up_process(child);
@@ -528,7 +532,8 @@ asmlinkage void syscall_trace_leave(struct pt_regs *regs)
 	if (unlikely(current->audit_context))
 		audit_syscall_exit(current, regs->rax);
 
-	if (test_thread_flag(TIF_SYSCALL_TRACE)
+	if ((test_thread_flag(TIF_SYSCALL_TRACE)
+	     || test_thread_flag(TIF_SINGLESTEP))
 	    && (current->ptrace & PT_PTRACED))
 		syscall_trace(regs);
 }