uprobes/x86: Implement x86 specific arch_uprobe_*_step

The arch specific implementation behaves like user_enable_single_step()
except that it does not disable single stepping if it was already
enabled by ptrace. This allows the debugger to single step over an
uprobe. The state of block stepping is not restored. It makes only sense
together with TF and if that was enabled then the debugger is notified.

Note: this is still not correct. For example, TIF_SINGLESTEP check
is not right, the application itself can set X86_EFLAGS_TF. And otoh
we leak TIF_SINGLESTEP (set by enable) if the probed insn is "popf".
See the next patches, we need the changes in arch/x86/kernel/step.c
first.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

arch/x86/include/asm/uprobes.h

@@ -46,6 +46,8 @@ struct arch_uprobe_task {
 #ifdef CONFIG_X86_64
 	unsigned long			saved_scratch_register;
 #endif
+#define UPROBE_CLEAR_TF			(1 << 0)
+	unsigned int			restore_flags;
 };
 
 extern int  arch_uprobe_analyze_insn(struct arch_uprobe *aup, struct mm_struct *mm, unsigned long addr);

arch/x86/kernel/uprobes.c

@@ -41,6 +41,9 @@
 /* Adjust the return address of a call insn */
 #define UPROBE_FIX_CALL	0x2
 
+/* Instruction will modify TF, don't change it */
+#define UPROBE_FIX_SETF	0x4
+
 #define UPROBE_FIX_RIP_AX	0x8000
 #define UPROBE_FIX_RIP_CX	0x4000
 
@@ -239,6 +242,10 @@ static void prepare_fixups(struct arch_uprobe *auprobe, struct insn *insn)
 	insn_get_opcode(insn);	/* should be a nop */
 
 	switch (OPCODE1(insn)) {
+	case 0x9d:
+		/* popf */
+		auprobe->fixups |= UPROBE_FIX_SETF;
+		break;
 	case 0xc3:		/* ret/lret */
 	case 0xcb:
 	case 0xc2:
@@ -673,3 +680,29 @@ bool arch_uprobe_skip_sstep(struct arch_uprobe *auprobe, struct pt_regs *regs)
 	}
 	return false;
 }
+
+void arch_uprobe_enable_step(struct arch_uprobe *auprobe)
+{
+	struct uprobe_task	*utask		= current->utask;
+	struct arch_uprobe_task	*autask		= &utask->autask;
+
+	autask->restore_flags = 0;
+	if (!test_tsk_thread_flag(current, TIF_SINGLESTEP) &&
+			!(auprobe->fixups & UPROBE_FIX_SETF))
+		autask->restore_flags |= UPROBE_CLEAR_TF;
+	/*
+	 * The state of TIF_BLOCKSTEP is not saved. With the TF flag set we
+	 * would to examine the opcode and the flags to make it right. Without
+	 * TF block stepping makes no sense.
+	 */
+	user_enable_single_step(current);
+}
+
+void arch_uprobe_disable_step(struct arch_uprobe *auprobe)
+{
+	struct uprobe_task *utask		= current->utask;
+	struct arch_uprobe_task	*autask		= &utask->autask;
+
+	if (autask->restore_flags & UPROBE_CLEAR_TF)
+		user_disable_single_step(current);
+}