x86 ptrace: unify TIF_SINGLESTEP

This unifies the treatment of TIF_SINGLESTEP on i386 and x86_64.
The bit is now excluded from _TIF_WORK_MASK on i386 as it has been
on x86_64.  This means the do_notify_resume() path using it is never
used, so TIF_SINGLESTEP is not cleared on returning to user mode.

Both now leave TIF_SINGLESTEP set when returning to user, so that
it's already set on an int $0x80 system call entry.  This removes
the need for testing TF on the system_call path.  Doing it this way
fixes the regression for PTRACE_SINGLESTEP into a sigreturn syscall,
introduced by commit 1e2e99f.

The clear_TF_reenable case that sets TIF_SINGLESTEP can only happen
on a non-exception kernel entry, i.e. sysenter/syscall instruction.
That will always get to the syscall exit tracing path.

Signed-off-by: Roland McGrath <roland@redhat.com>

arch/x86/kernel/entry_32.S

@@ -383,10 +383,6 @@ syscall_exit:
 					# setting need_resched or sigpending
 					# between sampling and the iret
 	TRACE_IRQS_OFF
-	testl $X86_EFLAGS_TF,PT_EFLAGS(%esp)	# If tracing set singlestep flag on exit
-	jz no_singlestep
-	orl $_TIF_SINGLESTEP,TI_flags(%ebp)
-no_singlestep:
 	movl TI_flags(%ebp), %ecx
 	testw $_TIF_ALLWORK_MASK, %cx	# current->work
 	jne syscall_exit_work

arch/x86/kernel/signal_32.c

@@ -657,12 +657,6 @@ static void do_signal(struct pt_regs *regs)
 void
 do_notify_resume(struct pt_regs *regs, void *unused, __u32 thread_info_flags)
 {
-	/* Pending single-step? */
-	if (thread_info_flags & _TIF_SINGLESTEP) {
-		regs->flags |= X86_EFLAGS_TF;
-		clear_thread_flag(TIF_SINGLESTEP);
-	}
-
 	/* deal with pending signal delivery */
 	if (thread_info_flags & _TIF_SIGPENDING)
 		do_signal(regs);

arch/x86/kernel/signal_64.c

@@ -487,12 +487,6 @@ static void do_signal(struct pt_regs *regs)
 void do_notify_resume(struct pt_regs *regs, void *unused,
 		      __u32 thread_info_flags)
 {
-	/* Pending single-step? */
-	if (thread_info_flags & _TIF_SINGLESTEP) {
-		regs->flags |= X86_EFLAGS_TF;
-		clear_thread_flag(TIF_SINGLESTEP);
-	}
-
 #ifdef CONFIG_X86_MCE
 	/* notify userspace of pending MCEs */
 	if (thread_info_flags & _TIF_MCE_NOTIFY)

include/asm-x86/thread_info.h

@@ -124,15 +124,15 @@ struct thread_info {
 /* work to do on interrupt/exception return */
 #define _TIF_WORK_MASK							\
 	(0x0000FFFF &							\
-	 ~(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SINGLESTEP|	\
+	 ~(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|			\
 	 _TIF_SECCOMP|_TIF_SYSCALL_EMU))
 
 /* work to do on any return to user space */
 #define _TIF_ALLWORK_MASK (0x0000FFFF & ~_TIF_SECCOMP)
 
 /* Only used for 64 bit */
 #define _TIF_DO_NOTIFY_MASK						\
-	(_TIF_SIGPENDING|_TIF_SINGLESTEP|_TIF_MCE_NOTIFY|_TIF_HRTICK_RESCHED)
+	(_TIF_SIGPENDING|_TIF_MCE_NOTIFY|_TIF_HRTICK_RESCHED)
 
 /* flags to check in __switch_to() */
 #define _TIF_WORK_CTXSW							\