[PATCH 2/8] riscv/kprobe: Allocate detour buffer from module area

[Chen Guokai]

From: Liao Chang <liaochang1@xxxxxxxxxx>

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

For the time being, Modules region always lives before the kernel.
But Vmalloc region resides far from kernel, the distance is half of the
kernel address space (See Documentation/riscv/vm-layout.rst), hence it
needs to override the alloc_optinsn_page() to make sure detour buffer
is allocated from jump-safe region.

Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx>
---
 arch/riscv/kernel/probes/kprobes.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
index e6e950b7cf32..034eb7b13b3c 100644
--- a/arch/riscv/kernel/probes/kprobes.c
+++ b/arch/riscv/kernel/probes/kprobes.c
@@ -12,6 +12,7 @@
 #include <asm/cacheflush.h>
 #include <asm/bug.h>
 #include <asm/patch.h>
+#include <asm/set_memory.h>
 
 #include "decode-insn.h"
 
@@ -84,6 +85,30 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
 }
 
 #ifdef CONFIG_MMU
+#if defined(CONFIG_OPTPROBES) && defined(CONFIG_64BIT)
+void *alloc_optinsn_page(void)
+{
+	void *page;
+
+	page = __vmalloc_node_range(PAGE_SIZE, 1, MODULES_VADDR,
+				    MODULES_END, GFP_KERNEL,
+				    PAGE_KERNEL, 0, NUMA_NO_NODE,
+				    __builtin_return_address(0));
+	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;
+}
+#endif
+
 void *alloc_insn_page(void)
 {
 	return  __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START, VMALLOC_END,
-- 
2.25.1