LoongArch: Trigger user-space watchpoints correctly

In the current code, gdb can set the watchpoint successfully through
ptrace interface, but watchpoint will not be triggered.

When debugging the following code using gdb.

lihui@bogon:~$ cat test.c
  #include <stdio.h>
  int a = 0;
  int main()
  {
	a = 1;
	printf("a = %d\n", a);
	return 0;
  }
lihui@bogon:~$ gcc -g test.c -o test
lihui@bogon:~$ gdb test
...
(gdb) watch a
...
(gdb) r
...
a = 1
[Inferior 1 (process 4650) exited normally]

No watchpoints were triggered, the root causes are:

1. Kernel uses perf_event and hw_breakpoint framework to control
   watchpoint, but the perf_event corresponding to watchpoint is
   not enabled. So it needs to be enabled according to MWPnCFG3
   or FWPnCFG3 PLV bit field in ptrace_hbp_set_ctrl(), and privilege
   is set according to the monitored addr in hw_breakpoint_control().
   Furthermore, add a judgment in ptrace_hbp_set_addr() to ensure
   kernel-space addr cannot be monitored in user mode.

2. The global enable control for all watchpoints is the WE bit of
   CSR.CRMD, and hardware sets the value to 0 when an exception is
   triggered. When the ERTN instruction is executed to return, the
   hardware restores the value of the PWE field of CSR.PRMD here.
   So, before a thread containing watchpoints be scheduled, the PWE
   field of CSR.PRMD needs to be set to 1. Add this modification in
   hw_breakpoint_control().

All changes according to the LoongArch Reference Manual:
https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#control-and-status-registers-related-to-watchpoints
https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#basic-control-and-status-registers

With this patch:

lihui@bogon:~$ gdb test
...
(gdb) watch a
Hardware watchpoint 1: a
(gdb) r
...
Hardware watchpoint 1: a

Old value = 0
New value = 1
main () at test.c:6
6		printf("a = %d\n", a);
(gdb) c
Continuing.
a = 1
[Inferior 1 (process 775) exited normally]

Cc: stable@vger.kernel.org
Signed-off-by: Hui Li <lihui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/hw_breakpoint.h

@@ -75,6 +75,8 @@ do {								\
 #define CSR_MWPC_NUM		0x3f
 
 #define CTRL_PLV_ENABLE		0x1e
+#define CTRL_PLV0_ENABLE	0x02
+#define CTRL_PLV3_ENABLE	0x10
 
 #define MWPnCFG3_LoadEn		8
 #define MWPnCFG3_StoreEn	9

arch/loongarch/kernel/hw_breakpoint.c

@@ -174,11 +174,21 @@ void flush_ptrace_hw_breakpoint(struct task_struct *tsk)
 static int hw_breakpoint_control(struct perf_event *bp,
 				 enum hw_breakpoint_ops ops)
 {
-	u32 ctrl;
+	u32 ctrl, privilege;
 	int i, max_slots, enable;
+	struct pt_regs *regs;
 	struct perf_event **slots;
 	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
 
+	if (arch_check_bp_in_kernelspace(info))
+		privilege = CTRL_PLV0_ENABLE;
+	else
+		privilege = CTRL_PLV3_ENABLE;
+
+	/*  Whether bp belongs to a task. */
+	if (bp->hw.target)
+		regs = task_pt_regs(bp->hw.target);
+
 	if (info->ctrl.type == LOONGARCH_BREAKPOINT_EXECUTE) {
 		/* Breakpoint */
 		slots = this_cpu_ptr(bp_on_reg);
@@ -204,13 +214,15 @@ static int hw_breakpoint_control(struct perf_event *bp,
 		write_wb_reg(CSR_CFG_ASID, i, 0, 0);
 		write_wb_reg(CSR_CFG_ASID, i, 1, 0);
 		if (info->ctrl.type == LOONGARCH_BREAKPOINT_EXECUTE) {
-			write_wb_reg(CSR_CFG_CTRL, i, 0, CTRL_PLV_ENABLE);
+			write_wb_reg(CSR_CFG_CTRL, i, 0, privilege);
 		} else {
 			ctrl = encode_ctrl_reg(info->ctrl);
-			write_wb_reg(CSR_CFG_CTRL, i, 1, ctrl | CTRL_PLV_ENABLE);
+			write_wb_reg(CSR_CFG_CTRL, i, 1, ctrl | privilege);
 		}
 		enable = csr_read64(LOONGARCH_CSR_CRMD);
 		csr_write64(CSR_CRMD_WE | enable, LOONGARCH_CSR_CRMD);
+		if (bp->hw.target)
+			regs->csr_prmd |= CSR_PRMD_PWE;
 		break;
 	case HW_BREAKPOINT_UNINSTALL:
 		/* Reset the FWPnCFG/MWPnCFG 1~4 register. */
@@ -222,6 +234,8 @@ static int hw_breakpoint_control(struct perf_event *bp,
 		write_wb_reg(CSR_CFG_CTRL, i, 1, 0);
 		write_wb_reg(CSR_CFG_ASID, i, 0, 0);
 		write_wb_reg(CSR_CFG_ASID, i, 1, 0);
+		if (bp->hw.target)
+			regs->csr_prmd &= ~CSR_PRMD_PWE;
 		break;
 	}
 

arch/loongarch/kernel/ptrace.c

@@ -608,9 +608,14 @@ static int ptrace_hbp_set_ctrl(unsigned int note_type,
 		return -EINVAL;
 	}
 
-	err = ptrace_hbp_fill_attr_ctrl(note_type, ctrl, &attr);
-	if (err)
-		return err;
+	if (uctrl & CTRL_PLV_ENABLE) {
+		err = ptrace_hbp_fill_attr_ctrl(note_type, ctrl, &attr);
+		if (err)
+			return err;
+		attr.disabled = 0;
+	} else {
+		attr.disabled = 1;
+	}
 
 	return modify_user_hw_breakpoint(bp, &attr);
 }
@@ -641,6 +646,10 @@ static int ptrace_hbp_set_addr(unsigned int note_type,
 	struct perf_event *bp;
 	struct perf_event_attr attr;
 
+	/* Kernel-space address cannot be monitored by user-space */
+	if ((unsigned long)addr >= XKPRANGE)
+		return -EINVAL;
+
 	bp = ptrace_hbp_get_initialised_bp(note_type, tsk, idx);
 	if (IS_ERR(bp))
 		return PTR_ERR(bp);