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To:     Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Julien Thierry <julien.thierry@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        live-patching@vger.kernel.org
Subject: [PATCH v4 1/3] arm64: implement ftrace with regs
In-Reply-To: <20181026142008.D922868C94@newverein.lst.de>
References: <20181026142008.D922868C94@newverein.lst.de>
Message-Id: <20181026142148.6353A68C94@newverein.lst.de>
Date:   Fri, 26 Oct 2018 16:21:48 +0200 (CEST)
From:   duwe@lst.de (Torsten Duwe)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Use -fpatchable-function-entry (gcc8) to add 2 NOPs at the beginning
of each function. Replace the first NOP thus generated with a quick LR
saver (move it to scratch reg x9), so the 2nd replacement insn, the call
to ftrace, does not clobber the value. Ftrace will then generate the
standard stack frames.

Note that patchable-function-entry in GCC disables IPA-RA, which means
ABI register calling conventions are obeyed *and* scratch registers
such as x9 are available.

Introduce and handle an ftrace_regs_trampoline for module PLTs, right
after ftrace_trampoline, and double the size of this special section
if .text.ftrace_trampoline is present in the module.

Signed-off-by: Torsten Duwe <duwe@suse.de>

--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -110,6 +110,8 @@ config ARM64
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DMA_CONTIGUOUS
 	select HAVE_DYNAMIC_FTRACE
+	select HAVE_DYNAMIC_FTRACE_WITH_REGS \
+		if $(cc-option,-fpatchable-function-entry=2)
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_FUNCTION_TRACER
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -78,6 +78,10 @@ ifeq ($(CONFIG_ARM64_MODULE_PLTS),y)
 KBUILD_LDFLAGS_MODULE	+= -T $(srctree)/arch/arm64/kernel/module.lds
 endif
 
+ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_REGS),y)
+  CC_FLAGS_FTRACE := -fpatchable-function-entry=2
+endif
+
 # Default value
 head-y		:= arch/arm64/kernel/head.o
 
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h
@@ -16,6 +16,19 @@
 #define MCOUNT_ADDR		((unsigned long)_mcount)
 #define MCOUNT_INSN_SIZE	AARCH64_INSN_SIZE
 
+/*
+ * DYNAMIC_FTRACE_WITH_REGS is implemented by adding 2 NOPs at the beginning
+ * of each function, with the second NOP actually calling ftrace. In contrary
+ * to a classic _mcount call, the call instruction to be modified is thus
+ * the second one, and not the only one.
+ */
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+#define ARCH_SUPPORTS_FTRACE_OPS 1
+#define REC_IP_BRANCH_OFFSET AARCH64_INSN_SIZE
+#else
+#define REC_IP_BRANCH_OFFSET 0
+#endif
+
 #ifndef __ASSEMBLY__
 #include <linux/compat.h>
 
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -7,9 +7,9 @@ CPPFLAGS_vmlinux.lds	:= -DTEXT_OFFSET=$(
 AFLAGS_head.o		:= -DTEXT_OFFSET=$(TEXT_OFFSET)
 CFLAGS_armv8_deprecated.o := -I$(src)
 
-CFLAGS_REMOVE_ftrace.o = -pg
-CFLAGS_REMOVE_insn.o = -pg
-CFLAGS_REMOVE_return_address.o = -pg
+CFLAGS_REMOVE_ftrace.o = -pg $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_insn.o = -pg $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_return_address.o = -pg $(CC_FLAGS_FTRACE)
 
 # Object file lists.
 arm64-obj-y		:= debug-monitors.o entry.o irq.o fpsimd.o		\
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -11,7 +11,8 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__K
 				   -fPIC -fno-strict-aliasing -mno-red-zone \
 				   -mno-mmx -mno-sse -fshort-wchar
 
-cflags-$(CONFIG_ARM64)		:= $(subst -pg,,$(KBUILD_CFLAGS)) -fpie
+cflags-$(CONFIG_ARM64)		:= $(filter-out -pg $(CC_FLAGS_FTRACE)\
+				  ,$(KBUILD_CFLAGS)) -fpie
 cflags-$(CONFIG_ARM)		:= $(subst -pg,,$(KBUILD_CFLAGS)) \
 				   -fno-builtin -fpic -mno-single-pic-base
 
--- a/arch/arm64/kernel/entry-ftrace.S
+++ b/arch/arm64/kernel/entry-ftrace.S
@@ -13,6 +13,8 @@
 #include <asm/assembler.h>
 #include <asm/ftrace.h>
 #include <asm/insn.h>
+#include <asm/asm-offsets.h>
+#include <asm/assembler.h>
 
 /*
  * Gcc with -pg will put the following code in the beginning of each function:
@@ -123,6 +125,7 @@ skip_ftrace_call:			// }
 ENDPROC(_mcount)
 
 #else /* CONFIG_DYNAMIC_FTRACE */
+#ifndef CONFIG_DYNAMIC_FTRACE_WITH_REGS
 /*
  * _mcount() is used to build the kernel with -pg option, but all the branch
  * instructions to _mcount() are replaced to NOP initially at kernel start up,
@@ -162,6 +165,114 @@ ftrace_graph_call:			// ftrace_graph_cal
 
 	mcount_exit
 ENDPROC(ftrace_caller)
+#else /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
+
+/*
+ * Since no -pg or similar compiler flag is used, there should really be
+ * no reference to _mcount; so do not define one. Only some value for
+ * MCOUNT_ADDR is needed for comparison. Let it point here to have some
+ * sort of magic value that can be recognised when debugging.
+ */
+	.global _mcount
+_mcount:
+	ret	/* make it differ from regs caller */
+
+ENTRY(ftrace_regs_caller)
+	/* callee's preliminary stack frame: */
+	stp	fp, x9, [sp, #-16]!
+	mov	fp, sp
+
+	/* our stack frame: */
+	stp	fp, lr, [sp, #-S_FRAME_SIZE]!
+	add	x9, sp, #16	/* offset to pt_regs */
+
+	stp	x10, x11, [x9, #S_X10]
+	stp	x12, x13, [x9, #S_X12]
+	stp	x14, x15, [x9, #S_X14]
+	stp	x16, x17, [x9, #S_X16]
+	stp	x18, x19, [x9, #S_X18]
+	stp	x20, x21, [x9, #S_X20]
+	stp	x22, x23, [x9, #S_X22]
+	stp	x24, x25, [x9, #S_X24]
+	stp	x26, x27, [x9, #S_X26]
+
+	b	ftrace_common
+ENDPROC(ftrace_regs_caller)
+
+ENTRY(ftrace_caller)
+	/* callee's preliminary stack frame: */
+	stp	fp, x9, [sp, #-16]!
+	mov	fp, sp
+
+	/* our stack frame: */
+	stp	fp, lr, [sp, #-S_FRAME_SIZE]!
+	add	x9, sp, #16	/* offset to pt_regs */
+
+ftrace_common:
+	/*
+	 * At this point we have 2 new stack frames, and x9 pointing
+	 * at a pt_regs which we can populate as needed.
+	 */
+
+	/* save function arguments */
+	stp	x0, x1, [x9]
+	stp	x2, x3, [x9, #S_X2]
+	stp	x4, x5, [x9, #S_X4]
+	stp	x6, x7, [x9, #S_X6]
+	stp	x8, x9, [x9, #S_X8]
+
+	ldr	x0, [fp]
+	stp	x28, x0, [x9, #S_X28]	/* FP in pt_regs + "our" x28 */
+
+	/* The program counter just after the ftrace call site */
+	str	lr, [x9, #S_PC]
+	/* The stack pointer as it was on ftrace_caller entry... */
+	add	x28, fp, #16
+	str	x28, [x9, #S_SP]
+	/* The link Register at callee entry */
+	ldr	x28, [fp, 8]
+	str	x28, [x9, #S_LR]	/* to pt_regs.r[30] */
+
+	ldr_l	x2, function_trace_op, x0
+	ldr	x1, [fp, #8]
+	sub	x0, lr, #8	/* function entry == IP */
+	mov	x3, x9		/* pt_regs are @x9 */
+
+	mov	fp, sp
+
+	.global ftrace_call
+ftrace_call:
+
+	bl	ftrace_stub
+
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+	.global ftrace_graph_call
+ftrace_graph_call:			// ftrace_graph_caller();
+	nop				// If enabled, this will be replaced
+					// "b ftrace_graph_caller"
+#endif
+
+ftrace_common_return:
+	add	x9, sp, #16	/* advance to pt_regs for restore */
+
+	ldp	x0, x1, [x9]
+	ldp	x2, x3, [x9, #S_X2]
+	ldp	x4, x5, [x9, #S_X4]
+	ldp	x6, x7, [x9, #S_X6]
+	ldp	x8, x9, [x9, #S_X8]
+
+	ldp	x28, fp, [x9, #S_X28]
+
+	ldr	lr, [x9, #S_LR]
+	ldr	x9, [x9, #S_PC]
+	/* clean up both frames, ours and callee preliminary */
+	add	sp, sp, #S_FRAME_SIZE + 16
+
+	ret	x9
+
+ENDPROC(ftrace_caller)
+
+#endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
 #endif /* CONFIG_DYNAMIC_FTRACE */
 
 ENTRY(ftrace_stub)
@@ -197,12 +308,21 @@ ENDPROC(ftrace_stub)
  * and run return_to_handler() later on its exit.
  */
 ENTRY(ftrace_graph_caller)
+#ifndef CONFIG_DYNAMIC_FTRACE_WITH_REGS
 	mcount_get_lr_addr	  x0	//     pointer to function's saved lr
 	mcount_get_pc		  x1	//     function's pc
 	mcount_get_parent_fp	  x2	//     parent's fp
 	bl	prepare_ftrace_return	// prepare_ftrace_return(&lr, pc, fp)
 
 	mcount_exit
+#else
+	add	x9, sp, #16	/* advance to pt_regs to gather args */
+	add	x0, x9, #S_LR		   /* &lr */
+	ldr	x1, [x9, #S_PC]		   /* pc */
+	ldr	x2, [x9, #S_STACKFRAME]	   /* fp */
+	bl	prepare_ftrace_return
+	b	ftrace_common_return
+#endif
 ENDPROC(ftrace_graph_caller)
 
 /*
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -65,18 +65,61 @@ int ftrace_update_ftrace_func(ftrace_fun
 	return ftrace_modify_code(pc, 0, new, false);
 }
 
+#ifdef CONFIG_ARM64_MODULE_PLTS
+static int install_ftrace_trampoline(struct module *mod, unsigned long *addr)
+{
+	struct plt_entry trampoline, *mod_trampoline;
+	trampoline = get_plt_entry(*addr);
+
+	if (*addr == FTRACE_ADDR)
+		mod_trampoline = mod->arch.ftrace_trampoline;
+	else if (*addr == FTRACE_REGS_ADDR)
+		mod_trampoline = mod->arch.ftrace_regs_trampoline;
+	else
+		return -EINVAL;
+
+	if (!plt_entries_equal(mod_trampoline, &trampoline)) {
+
+			/* point the trampoline to our ftrace entry point */
+			module_disable_ro(mod);
+			*mod_trampoline = trampoline;
+			module_enable_ro(mod, true);
+
+			/* update trampoline before patching in the branch */
+			smp_wmb();
+		}
+	*addr = (unsigned long)(void *)mod_trampoline;
+
+	return 0;
+}
+#endif
+
+/*
+ * Ftrace with regs generates the tracer calls as close as possible to
+ * the function entry; no stack frame has been set up at that point.
+ * In order to make another call e.g to ftrace_caller, the LR must be
+ * saved from being overwritten.
+ * Between two functions, and with IPA-RA turned off, the scratch registers
+ * are available, so move the LR to x9 before calling into ftrace.
+ * "mov x9, lr" is officially aliased from "orr x9, xzr, lr".
+ */
+#define QUICK_LR_SAVE aarch64_insn_gen_logical_shifted_reg( \
+			AARCH64_INSN_REG_9, AARCH64_INSN_REG_ZR, \
+			AARCH64_INSN_REG_LR, 0, AARCH64_INSN_VARIANT_64BIT, \
+			AARCH64_INSN_LOGIC_ORR)
+
 /*
  * Turn on the call to ftrace_caller() in instrumented function
  */
 int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 {
-	unsigned long pc = rec->ip;
+	unsigned long pc = rec->ip + REC_IP_BRANCH_OFFSET;
+	int ret;
 	u32 old, new;
 	long offset = (long)pc - (long)addr;
 
 	if (offset < -SZ_128M || offset >= SZ_128M) {
 #ifdef CONFIG_ARM64_MODULE_PLTS
-		struct plt_entry trampoline;
 		struct module *mod;
 
 		/*
@@ -96,54 +139,65 @@ int ftrace_make_call(struct dyn_ftrace *
 		if (WARN_ON(!mod))
 			return -EINVAL;
 
-		/*
-		 * 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.
-		 */
-		trampoline = get_plt_entry(addr);
-		if (!plt_entries_equal(mod->arch.ftrace_trampoline,
-				       &trampoline)) {
-			if (!plt_entries_equal(mod->arch.ftrace_trampoline,
-					       &(struct plt_entry){})) {
-				pr_err("ftrace: far branches to multiple entry points unsupported inside a single module\n");
-				return -EINVAL;
-			}
-
-			/* point the trampoline to our ftrace entry point */
-			module_disable_ro(mod);
-			*mod->arch.ftrace_trampoline = trampoline;
-			module_enable_ro(mod, true);
-
-			/* update trampoline before patching in the branch */
-			smp_wmb();
+		/* Check against our well-known list of ftrace entry points */
+		if (addr == FTRACE_ADDR || addr == FTRACE_REGS_ADDR) {
+			ret = install_ftrace_trampoline(mod, &addr);
+			if (ret < 0)
+				return ret;
 		}
-		addr = (unsigned long)(void *)mod->arch.ftrace_trampoline;
+		else
+			return -EINVAL;
+
 #else /* CONFIG_ARM64_MODULE_PLTS */
 		return -EINVAL;
 #endif /* CONFIG_ARM64_MODULE_PLTS */
 	}
 
 	old = aarch64_insn_gen_nop();
+	if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS)) {
+		new = QUICK_LR_SAVE;
+		ret = ftrace_modify_code(pc - AARCH64_INSN_SIZE,
+					 old, new, true);
+		if (ret)
+			return ret;
+	}
 	new = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
 
 	return ftrace_modify_code(pc, old, new, true);
 }
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
+			unsigned long addr)
+{
+	unsigned long pc = rec->ip + REC_IP_BRANCH_OFFSET;
+	u32 old, new;
+
+	old = aarch64_insn_gen_branch_imm(pc, old_addr, true);
+	new = aarch64_insn_gen_branch_imm(pc, addr, true);
+
+	return ftrace_modify_code(pc, old, new, true);
+}
+#endif
+
 /*
  * Turn off the call to ftrace_caller() in instrumented function
  */
 int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
 		    unsigned long addr)
 {
-	unsigned long pc = rec->ip;
+	unsigned long pc = rec->ip + REC_IP_BRANCH_OFFSET;
 	bool validate = true;
+	int ret;
 	u32 old = 0, new;
 	long offset = (long)pc - (long)addr;
 
+	/* -fpatchable-function-entry= does not generate a profiling call
+	 *  initially; the NOPs are already there.
+	 */
+	if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS) && addr == MCOUNT_ADDR)
+		return 0;
+
 	if (offset < -SZ_128M || offset >= SZ_128M) {
 #ifdef CONFIG_ARM64_MODULE_PLTS
 		u32 replaced;
@@ -188,7 +242,15 @@ int ftrace_make_nop(struct module *mod,
 
 	new = aarch64_insn_gen_nop();
 
-	return ftrace_modify_code(pc, old, new, validate);
+	ret = ftrace_modify_code(pc, old, new, validate);
+	if (ret)
+		return ret;
+	if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS)) {
+		old = QUICK_LR_SAVE;
+		ret = ftrace_modify_code(pc - AARCH64_INSN_SIZE,
+					 old, new, true);
+	}
+	return ret;
 }
 
 void arch_ftrace_update_code(int command)
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -115,6 +115,7 @@
 #define MCOUNT_REC()	. = ALIGN(8);				\
 			__start_mcount_loc = .;			\
 			KEEP(*(__mcount_loc))			\
+			KEEP(*(__patchable_function_entries))	\
 			__stop_mcount_loc = .;
 #else
 #define MCOUNT_REC()
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -61,8 +61,12 @@ extern void __chk_io_ptr(const volatile
 #if defined(CC_USING_HOTPATCH) && !defined(__CHECKER__)
 #define notrace __attribute__((hotpatch(0,0)))
 #else
+#if defined(CONFIG_ARM64) && defined(CONFIG_DYNAMIC_FTRACE_WITH_REGS)
+#define notrace __attribute__((patchable_function_entry(0)))
+#else
 #define notrace __attribute__((no_instrument_function))
 #endif
+#endif
 
 /* Intel compiler defines __GNUC__. So we will overwrite implementations
  * coming from above header files here
--- a/arch/arm64/include/asm/module.h
+++ b/arch/arm64/include/asm/module.h
@@ -33,6 +33,7 @@ struct mod_arch_specific {
 
 	/* for CONFIG_DYNAMIC_FTRACE */
 	struct plt_entry 	*ftrace_trampoline;
+	struct plt_entry 	*ftrace_regs_trampoline;
 };
 #endif
 
--- a/arch/arm64/kernel/module.c
+++ b/arch/arm64/kernel/module.c
@@ -452,8 +452,11 @@ int module_finalize(const Elf_Ehdr *hdr,
 			apply_alternatives_module((void *)s->sh_addr, s->sh_size);
 #ifdef CONFIG_ARM64_MODULE_PLTS
 		if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
-		    !strcmp(".text.ftrace_trampoline", secstrs + s->sh_name))
+		    !strcmp(".text.ftrace_trampoline", secstrs + s->sh_name)) {
 			me->arch.ftrace_trampoline = (void *)s->sh_addr;
+			me->arch.ftrace_regs_trampoline =
+				(void *)(s->sh_addr + sizeof(struct plt_entry));
+		}
 #endif
 	}
 
--- a/arch/arm64/kernel/module-plts.c
+++ b/arch/arm64/kernel/module-plts.c
@@ -272,7 +272,7 @@ int module_frob_arch_sections(Elf_Ehdr *
 		tramp->sh_type = SHT_NOBITS;
 		tramp->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
 		tramp->sh_addralign = __alignof__(struct plt_entry);
-		tramp->sh_size = sizeof(struct plt_entry);
+		tramp->sh_size = 2 * sizeof(struct plt_entry);
 	}
 
 	return 0;