LoongArch: kprobes: Use larch_insn_text_copy() to patch instructions

On SMP systems, kprobe handlers would occasionally fail to execute on
certain CPU cores. The issue is hard to reproduce and typically occurs
randomly under high system load.

The root cause is a software-side instruction hazard. According to the
LoongArch Reference Manual, while the cache coherency is maintained by
hardware, software must explicitly use the "IBAR" instruction to ensure
the instruction fetch unit (IFU) observes the effects of recent stores.

The current arch_arm_kprobe() and arch_disarm_kprobe() only execute the
"IBAR" barrier (via flush_insn_slot -> local_flush_icache_range) on the
local CPU. This leaves a vulnerable window where remote CPU cores may
continue executing stale instructions from their pipelines or prefetch
buffers, as they have not executed an "IBAR" since the code modification.

Switch to larch_insn_text_copy() to fix this:
1. Synchronization: It uses stop_machine_cpuslocked() to synchronize all
   online CPUs, ensuring no CPU is executing the target code area during
   modification.
2. Visibility: By passing cpu_online_mask to stop_machine_cpuslocked(),
   the callback text_copy_cb() is executed on all online cores. Each CPU
   core invokes local_flush_icache_range() to execute "IBAR", clearing
   instruction hazards system-wide and ensuring the "break" instruction
   is visible to the fetch units of all cores.
3. Robustness: It properly manages memory write permissions (ROX/RW) for
   the kernel text segment during patching, ensuring compatibility with
   CONFIG_STRICT_KERNEL_RWX.

Cc: <stable@vger.kernel.org>  # 6.18+
Fixes: 6d4cc40 ("LoongArch: Add kprobes support")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: seehearfeel

arch/loongarch/kernel/kprobes.c

@@ -60,16 +60,18 @@ NOKPROBE_SYMBOL(arch_prepare_kprobe);
 /* Install breakpoint in text */
 void arch_arm_kprobe(struct kprobe *p)
 {
-	*p->addr = KPROBE_BP_INSN;
-	flush_insn_slot(p);
+	u32 insn = KPROBE_BP_INSN;
+
+	larch_insn_text_copy(p->addr, &insn, LOONGARCH_INSN_SIZE);
 }
 NOKPROBE_SYMBOL(arch_arm_kprobe);
 
 /* Remove breakpoint from text */
 void arch_disarm_kprobe(struct kprobe *p)
 {
-	*p->addr = p->opcode;
-	flush_insn_slot(p);
+	u32 insn = p->opcode;
+
+	larch_insn_text_copy(p->addr, &insn, LOONGARCH_INSN_SIZE);
 }
 NOKPROBE_SYMBOL(arch_disarm_kprobe);