[PATCH V2] 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 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 live before the kernel.
But Vmalloc region reside 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 allocate detour
buffer from jump-safe region.

Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx>
---
v2:
1. Avoid to depend on CONFIG_MODUELS.
2. Override alloc_optinsn_page to allocate slot from jump-safe region.

---
 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..fd62b0b086d2 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
+#ifdef CONFIG_OPTPROBES
+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,
-- 
2.17.1