Handle single-stepping over fast system calls without polluting

the fast case with a pushf/popf, by having the kernel debug trap
set the TIF_SINGLESTEP flag and causing the return path to dtrt.

arch/i386/kernel/signal.c

@@ -609,6 +609,11 @@ __attribute__((regparm(3)))
 void do_notify_resume(struct pt_regs *regs, sigset_t *oldset,
 		      __u32 thread_info_flags)
 {
+	/* Pending single-step? */
+	if (thread_info_flags & _TIF_SINGLESTEP) {
+		regs->eflags |= TF_MASK;
+		clear_thread_flag(TIF_SINGLESTEP);
+	}
 	/* deal with pending signal delivery */
 	if (thread_info_flags & _TIF_SIGPENDING)
 		do_signal(regs,oldset);

arch/i386/kernel/sysenter.c

@@ -54,19 +54,18 @@ static int __init sysenter_setup(void)
 		0xc3			/* ret */
 	};
 	static const char sysent[] = {
-		0x9c,			/* pushf */
 		0x51,			/* push %ecx */
 		0x52,			/* push %edx */
 		0x55,			/* push %ebp */
 		0x89, 0xe5,		/* movl %esp,%ebp */
 		0x0f, 0x34,		/* sysenter */
+		0x00,			/* align return point */
 	/* System call restart point is here! (SYSENTER_RETURN - 2) */
 		0xeb, 0xfa,		/* jmp to "movl %esp,%ebp" */
 	/* System call normal return point is here! (SYSENTER_RETURN in entry.S) */
 		0x5d,			/* pop %ebp */
 		0x5a,			/* pop %edx */
 		0x59,			/* pop %ecx */
-		0x9d,			/* popf - restore TF */
 		0xc3			/* ret */
 	};
 	unsigned long page = get_zeroed_page(GFP_ATOMIC);

arch/i386/kernel/traps.c

@@ -605,7 +605,7 @@ asmlinkage void do_debug(struct pt_regs * regs, long error_code)
 		 * interface.
 		 */
 		if ((regs->xcs & 3) == 0)
-			goto clear_TF;
+			goto clear_TF_reenable;
 		if ((tsk->ptrace & (PT_DTRACE|PT_PTRACED)) == PT_DTRACE)
 			goto clear_TF;
 	}
@@ -637,6 +637,8 @@ asmlinkage void do_debug(struct pt_regs * regs, long error_code)
 	handle_vm86_trap((struct kernel_vm86_regs *) regs, error_code, 1);
 	return;
 
+clear_TF_reenable:
+	set_tsk_thread_flag(tsk, TIF_SINGLESTEP);
 clear_TF:
 	regs->eflags &= ~TF_MASK;
 	return;

include/asm-i386/thread_info.h

@@ -109,6 +109,7 @@ static inline struct thread_info *current_thread_info(void)
 #define TIF_NOTIFY_RESUME	1	/* resumption notification requested */
 #define TIF_SIGPENDING		2	/* signal pending */
 #define TIF_NEED_RESCHED	3	/* rescheduling necessary */
+#define TIF_SINGLESTEP		4	/* restore singlestep on return to user mode */
 #define TIF_USEDFPU		16	/* FPU was used by this task this quantum (SMP) */
 #define TIF_POLLING_NRFLAG	17	/* true if poll_idle() is polling TIF_NEED_RESCHED */
 
@@ -116,6 +117,7 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_NOTIFY_RESUME	(1<<TIF_NOTIFY_RESUME)
 #define _TIF_SIGPENDING		(1<<TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1<<TIF_NEED_RESCHED)
+#define _TIF_SINGLESTEP		(1<<TIF_SINGLESTEP)
 #define _TIF_USEDFPU		(1<<TIF_USEDFPU)
 #define _TIF_POLLING_NRFLAG	(1<<TIF_POLLING_NRFLAG)