[PATCH v6 02/13] riscv/kprobe: Allocate detour buffer from module region

[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 has to be allocated
at virtual address that can access from kernel and modules text via
AUIPC/JALR.

For the time being, the vmalloc region is far from kernel/modules text,
the distance between them is half of kernel address space [1], which
can't transfer control to 32-bit pc-relative address, hence it needs to
override the alloc_optinsn_page() to allocate detour buffer from module
region.

[1] Documentation/riscv/vm-layout.rst

Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx>
Co-developed-by: Chen Guokai <chenguokai17@xxxxxxxxxxxxxxxx>
Signed-off-by: Chen Guokai <chenguokai17@xxxxxxxxxxxxxxxx>
---
 arch/riscv/kernel/probes/kprobes.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
index f21592d20306..e1856b04db04 100644
--- a/arch/riscv/kernel/probes/kprobes.c
+++ b/arch/riscv/kernel/probes/kprobes.c
@@ -6,6 +6,7 @@
 #include <linux/extable.h>
 #include <linux/slab.h>
 #include <linux/stop_machine.h>
+#include <linux/set_memory.h>
 #include <asm/ptrace.h>
 #include <linux/uaccess.h>
 #include <asm/sections.h>
@@ -84,6 +85,29 @@ 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_rox((unsigned long)page, 1);
+
+	return page;
+}
+#endif
+
 void *alloc_insn_page(void)
 {
 	return  __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START, VMALLOC_END,
-- 
2.34.1