[PATCH] riscv/kprobes: allocate detour buffer from module area

[Liao Chang]

To address the limitation of PC-relative branch instruction
on riscv architecture, detour buffer slot is allocated from
a area, the distance of which from kernel should be less than 4GB.

For the time being, Modules region always live before the kernel.
But Vmalloc region reside far away from kernel, the distance is
half of the kernel address space.

Signed-off-by: Liao Chang <liaochang1 at huawei.com>
---
 arch/riscv/kernel/probes/kprobes.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
index e6e950b7cf32..bc027a663b17 100644
--- a/arch/riscv/kernel/probes/kprobes.c
+++ b/arch/riscv/kernel/probes/kprobes.c
@@ -6,12 +6,14 @@
 #include <linux/extable.h>
 #include <linux/slab.h>
 #include <linux/stop_machine.h>
+#include <linux/moduleloader.h>
 #include <asm/ptrace.h>
 #include <linux/uaccess.h>
 #include <asm/sections.h>
 #include <asm/cacheflush.h>
 #include <asm/bug.h>
 #include <asm/patch.h>
+#include <asm/set_memory.h>
 
 #include "decode-insn.h"
 
@@ -86,10 +88,28 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
 #ifdef CONFIG_MMU
 void *alloc_insn_page(void)
 {
+#if defined(CONFIG_MODULES) && defined(CONFIG_64BIT)
+	void *page;
+
+	page = module_alloc(PAGE_SIZE);
+	if (!page)
+		return NULL;
+
+	set_vm_flush_reset_perms(page);
+	/*
+	 * First make the page read-only, and only then make it executable to
+	 * prevent it from being W+X in between.
+	 */
+	set_memory_ro((unsigned long)page, 1);
+	set_memory_x((unsigned long)page, 1);
+
+	return page;
+#else
 	return  __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START, VMALLOC_END,
 				     GFP_KERNEL, PAGE_KERNEL_READ_EXEC,
 				     VM_FLUSH_RESET_PERMS, NUMA_NO_NODE,
 				     __builtin_return_address(0));
+#endif
 }
 #endif
 
-- 
2.17.1