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

[ard.biesheuvel@linaro.org (Ard Biesheuvel)]

Currently, dynamic ftrace support in the arm64 kernel assumes that all
core kernel code is within range of ordinary branch instructions in
module code, which is usually the case, but is no longer guaranteed now
that we have support for module PLTs and address space randomization.

Since all patching of branch instructions involves function calls to
ftrace_caller(), we can emit the modules with a trampoline that has
unlimited range, and patch the branch instruction to call the trampoline
if ftrace_caller() itself is out of range.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 arch/arm64/Kconfig              |  2 +-
 arch/arm64/Makefile             |  3 ++
 arch/arm64/include/asm/module.h |  3 ++
 arch/arm64/kernel/ftrace.c      | 37 ++++++++++++++++++--
 arch/arm64/kernel/module-plts.c | 10 ++++++
 arch/arm64/lib/Makefile         |  2 ++
 arch/arm64/lib/ftrace-mod.S     | 25 +++++++++++++
 7 files changed, 78 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index e7f043efff41..31af7ea72072 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -982,7 +982,7 @@ config RANDOMIZE_BASE
 
 config RANDOMIZE_MODULE_REGION_FULL
 	bool "Randomize the module region independently from the core kernel"
-	depends on RANDOMIZE_BASE && !DYNAMIC_FTRACE
+	depends on RANDOMIZE_BASE
 	default y
 	help
 	  Randomizes the location of the module region without considering the
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index b9a4a934ca05..01498eab5ada 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -68,6 +68,9 @@ endif
 
 ifeq ($(CONFIG_ARM64_MODULE_PLTS),y)
 KBUILD_LDFLAGS_MODULE	+= -T $(srctree)/arch/arm64/kernel/module.lds
+ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
+KBUILD_LDFLAGS_MODULE	+= $(objtree)/arch/arm64/lib/ftrace-mod.o
+endif
 endif
 
 # Default value
diff --git a/arch/arm64/include/asm/module.h b/arch/arm64/include/asm/module.h
index e12af6754634..2ff2b7782957 100644
--- a/arch/arm64/include/asm/module.h
+++ b/arch/arm64/include/asm/module.h
@@ -25,6 +25,9 @@ struct mod_arch_specific {
 	struct elf64_shdr	*plt;
 	int			plt_num_entries;
 	int			plt_max_entries;
+
+	/* for CONFIG_DYNAMIC_FTRACE */
+	struct elf64_shdr	*ftrace_trampoline;
 };
 #endif
 
diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index a8db6857cad6..8f5a7dc36c1b 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -9,11 +9,14 @@
  * published by the Free Software Foundation.
  */
 
+#include <linux/elf.h>
 #include <linux/ftrace.h>
+#include <linux/module.h>
 #include <linux/swab.h>
 #include <linux/uaccess.h>
 
 #include <asm/cacheflush.h>
+#include <asm/debug-monitors.h>
 #include <asm/ftrace.h>
 #include <asm/insn.h>
 
@@ -63,6 +66,35 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
 	return ftrace_modify_code(pc, 0, new, false);
 }
 
+#ifdef CONFIG_ARM64_MODULE_PLTS
+EXPORT_SYMBOL_GPL(ftrace_caller);
+#endif
+
+static u32 __ftrace_gen_branch(unsigned long pc, unsigned long addr)
+{
+	long offset = (long)pc - (long)addr;
+	struct module *mod;
+
+	if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
+	    addr == (unsigned long)&ftrace_caller &&
+	    unlikely(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;
+
+		addr = (unsigned long)mod->arch.ftrace_trampoline->sh_addr;
+	}
+
+	return aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
+}
+
 /*
  * Turn on the call to ftrace_caller() in instrumented function
  */
@@ -72,7 +104,7 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 	u32 old, new;
 
 	old = aarch64_insn_gen_nop();
-	new = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
+	new = __ftrace_gen_branch(pc, addr);
 
 	return ftrace_modify_code(pc, old, new, true);
 }
@@ -87,8 +119,7 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
 	u32 old = 0, new;
 
 	if (!IS_ENABLED(CONFIG_ARM64_MODULE_PLTS))
-		old = aarch64_insn_gen_branch_imm(pc, addr,
-						  AARCH64_INSN_BRANCH_LINK);
+		old = __ftrace_gen_branch(pc, addr);
 	new = aarch64_insn_gen_nop();
 
 	return ftrace_modify_code(pc, old, new,
diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
index 1ce90d8450ae..859c7170e69a 100644
--- a/arch/arm64/kernel/module-plts.c
+++ b/arch/arm64/kernel/module-plts.c
@@ -162,6 +162,10 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 			mod->arch.plt = sechdrs + i;
 		else if (sechdrs[i].sh_type == SHT_SYMTAB)
 			syms = (Elf64_Sym *)sechdrs[i].sh_addr;
+		else if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
+			 strcmp(".text.ftrace_trampoline",
+				secstrings + sechdrs[i].sh_name) == 0)
+			mod->arch.ftrace_trampoline = sechdrs + i;
 	}
 
 	if (!mod->arch.plt) {
@@ -173,6 +177,12 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 		return -ENOEXEC;
 	}
 
+	if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) && !mod->arch.ftrace_trampoline) {
+		pr_err("%s: module ftrace trampoline section missing\n",
+		       mod->name);
+		return -ENOEXEC;
+	}
+
 	for (i = 0; i < ehdr->e_shnum; i++) {
 		Elf64_Rela *rels = (void *)ehdr + sechdrs[i].sh_offset;
 		int numrels = sechdrs[i].sh_size / sizeof(Elf64_Rela);
diff --git a/arch/arm64/lib/Makefile b/arch/arm64/lib/Makefile
index c86b7909ef31..b01dcfa9c002 100644
--- a/arch/arm64/lib/Makefile
+++ b/arch/arm64/lib/Makefile
@@ -4,6 +4,8 @@ lib-y		:= bitops.o clear_user.o delay.o copy_from_user.o	\
 		   memcmp.o strcmp.o strncmp.o strlen.o strnlen.o	\
 		   strchr.o strrchr.o
 
+lib-$(CONFIG_DYNAMIC_FTRACE) += ftrace-mod.o
+
 # Tell the compiler to treat all general purpose registers (with the
 # exception of the IP registers, which are already handled by the caller
 # in case of a PLT) as callee-saved, which allows for efficient runtime
diff --git a/arch/arm64/lib/ftrace-mod.S b/arch/arm64/lib/ftrace-mod.S
new file mode 100644
index 000000000000..ce15b9948851
--- /dev/null
+++ b/arch/arm64/lib/ftrace-mod.S
@@ -0,0 +1,25 @@
+/*
+ * 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"
+ENTRY(__ftrace_trampoline)
+	stp		x29, x30, [sp, #-16]!
+	mov		x29, sp
+
+	movz		x30, #:abs_g3:ftrace_caller
+	movk		x30, #:abs_g2_nc:ftrace_caller
+	movk		x30, #:abs_g1_nc:ftrace_caller
+	movk		x30, #:abs_g0_nc:ftrace_caller
+	blr		x30
+
+	ldp		x29, x30, [sp], #16
+	ret
+ENDPROC(__ftrace_trampoline)
-- 
2.9.3