[PATCH v2 2/2] arm64: ftrace: add support for far branches to dynamic ftrace

diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 1dcb69d3d0e5..f2b4e816b6de 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile

@@ -62,3 +62,6 @@ extra-y					+= $(head-y) vmlinux.lds
 ifeq ($(CONFIG_DEBUG_EFI),y)
 AFLAGS_head.o += -DVMLINUX_PATH="\"$(realpath $(objtree)/vmlinux)\""
 endif
+
+# will be included by each individual module but not by the core kernel itself
+extra-$(CONFIG_DYNAMIC_FTRACE) += ftrace-mod.o

diff --git a/arch/arm64/kernel/ftrace-mod.S b/arch/arm64/kernel/ftrace-mod.S
new file mode 100644
index 000000000000..00c4025be4ff
--- /dev/null
+++ b/arch/arm64/kernel/ftrace-mod.S

@@ -0,0 +1,18 @@
+/*
+ * Copyright (C) 2017 Linaro Ltd <ard.biesheuvel@linaro.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+
+	.section	".text.ftrace_trampoline", "ax"
+	.align		3
+0:	.quad		0
+__ftrace_trampoline:
+	ldr		x16, 0b
+	br		x16

+static u32 __ftrace_gen_branch(unsigned long pc, unsigned long addr)
+{
+	long offset = (long)pc - (long)addr;
+	unsigned long *tramp;
+	struct module *mod;
+
+	if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
+	    (offset < -SZ_128M || offset >= SZ_128M)) {
+
+		/*
+		 * On kernels that support module PLTs, the offset between the
+		 * call and its target may legally exceed the range of an
+		 * ordinary branch instruction. In this case, we need to branch
+		 * via a trampoline in the module.
+		 */
+		mod = __module_address(pc);
+		if (WARN_ON(!mod))
+			return AARCH64_BREAK_FAULT;
+
+		/*
+		 * There is only one ftrace trampoline per module. For now,
+		 * this is not a problem since on arm64, all dynamic ftrace
+		 * invocations are routed via ftrace_caller(). This will need
+		 * to be revisited if support for multiple ftrace entry points
+		 * is added in the future, but for now, the pr_err() below
+		 * deals with a theoretical issue only.
+		 */
+		tramp = (unsigned long *)mod->arch.ftrace_trampoline->sh_addr;
+		if (tramp[0] != addr) {
+			if (tramp[0] != 0) {
+				pr_err("ftrace: far branches to multiple entry points unsupported inside a single module\n");
+				return AARCH64_BREAK_FAULT;
+			}
+
+			/* point the trampoline to our ftrace entry point */
+			module_disable_ro(mod);
+			tramp[0] = addr;
+			module_enable_ro(mod, true);

+		}
+		addr = (unsigned long)&tramp[1];
+	}
+	return aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
+}
+