Message-ID: <f6b441b4-4fa4-9a52-e4ba-04b63201278d@arm.com>
Date:   Fri, 17 Aug 2018 18:20:53 +0100
From:   Julien Thierry <julien.thierry@....com>
To:     Torsten Duwe <duwe@....de>, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v2 1/3] arm64: implement ftrace with regs

Hi Torsten,

On 17/08/18 11:27, Torsten Duwe wrote:
> Check for compiler support of -fpatchable-function-entry and use it
> to intercept functions immediately on entry, saving the LR in x9.
> Disable ftracing in efi/libstub, because this triggers cross-section
> linker errors now (-pg is disabled already for those files).
> Add an ftrace_caller which can handle LR in x9, as well as an
> ftrace_regs_caller that additionally writes out a set of pt_regs
> for inspection.
> 
> Signed-off-by: Torsten Duwe <duwe@...e.de>
> 
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -110,6 +110,7 @@ config ARM64
>   	select HAVE_DEBUG_KMEMLEAK
>   	select HAVE_DMA_CONTIGUOUS
>   	select HAVE_DYNAMIC_FTRACE
> +	select HAVE_DYNAMIC_FTRACE_WITH_REGS

Actually there's an issue here. CONFIG_DYNAMIC_FTRACE_WITH_REGS is 
automatically selected if the arch has HAVE_DYNAMIC_FTRACE_WITH_REGS and 
DYNAMIC_FTRACE. Meaning we can no longer enable DYNAMIC_FTRACE without 
CONFIG_DYNAMIC_FTRACE_WITH_REGS.

So we wouldn't build an arm64 kernel with dynamic function tracing 
anymore without a recent enough compiler :( .

Is there a way to have "HAVE_DYNAMIC_FTRACE_WITH_REGS" depending on 
whether the compiler supports the patchable function entry or not?

(Not familiar enough with the build system to suggest something here).

>   	select HAVE_EFFICIENT_UNALIGNED_ACCESS
>   	select HAVE_FTRACE_MCOUNT_RECORD
>   	select HAVE_FUNCTION_TRACER
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -78,6 +78,15 @@ ifeq ($(CONFIG_ARM64_MODULE_PLTS),y)
>   KBUILD_LDFLAGS_MODULE	+= -T $(srctree)/arch/arm64/kernel/module.lds
>   endif
>   
> +ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS

use "ifeq($(CONFIG_DYNAMIC_FTRACE_WITH_REGS),y)", 
CONFIG_DYNAMIC_FTRACE_WITH_REGS is defined even if it is set to 'n' and 
will still try to use the compiler option.

> +  CC_FLAGS_FTRACE := -fpatchable-function-entry=2
> +  KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
> +  ifeq ($(call cc-option,-fpatchable-function-entry=2),)
> +    $(error Cannot use CONFIG_DYNAMIC_FTRACE_WITH_REGS: \
> +             -fpatchable-function-entry not supported by compiler)
> +  endif
> +endif
> +
>   # Default value
>   head-y		:= arch/arm64/kernel/head.o
>   
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -16,6 +16,13 @@
>   #define MCOUNT_ADDR		((unsigned long)_mcount)
>   #define MCOUNT_INSN_SIZE	AARCH64_INSN_SIZE
>   
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +#define ARCH_SUPPORTS_FTRACE_OPS 1
> +#define REC_IP_BRANCH_OFFSET 4
> +#else
> +#define REC_IP_BRANCH_OFFSET 0
> +#endif
> +
>   #ifndef __ASSEMBLY__
>   #include <linux/compat.h>
>   
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -7,9 +7,9 @@ CPPFLAGS_vmlinux.lds	:= -DTEXT_OFFSET=$(
>   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 CC_USING_PATCHABLE_FUNCTION_ENTRY
>   /*
>    * _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 +165,88 @@ ftrace_graph_call:			// ftrace_graph_cal
>   
>   	mcount_exit
>   ENDPROC(ftrace_caller)
> +#else /* CC_USING_PATCHABLE_FUNCTION_ENTRY */
> +ENTRY(_mcount)
> +	mov     x10, lr
> +	mov     lr, x9
> +	ret     x10
> +ENDPROC(_mcount)
> +
> +ENTRY(ftrace_regs_caller)
> +	stp	x29, x9, [sp, #-16]!
> +	sub	sp, sp, #S_FRAME_SIZE
> +
> +	stp	x10, x11, [sp, #80]
> +	stp	x12, x13, [sp, #96]
> +	stp	x14, x15, [sp, #112]
> +	stp	x16, x17, [sp, #128]
> +	stp	x18, x19, [sp, #144]
> +	stp	x20, x21, [sp, #160]
> +	stp	x22, x23, [sp, #176]
> +	stp	x24, x25, [sp, #192]
> +	stp	x26, x27, [sp, #208]
> +
> +	b	ftrace_common
> +ENDPROC(ftrace_regs_caller)
> +
> +ENTRY(ftrace_caller)
> +	stp	x29, x9, [sp, #-16]!
> +	sub	sp, sp, #S_FRAME_SIZE
> +
> +ftrace_common:
> +	stp	x28, x29, [sp, #224]
> +
> +	/* save function arguments */
> +	stp	x0, x1, [sp]
> +	stp	x2, x3, [sp, #16]
> +	stp	x4, x5, [sp, #32]
> +	stp	x6, x7, [sp, #48]
> +	stp	x8, x9, [sp, #64]
> +
> +	/* The link Register at callee entry */
> +	str	x9, [sp, #S_LR]
> +	/* The program counter just after the ftrace call site */
> +	str	lr, [sp, #S_PC]
> +	/* The stack pointer as it was on ftrace_caller entry... */
> +	add	x29, sp, #S_FRAME_SIZE+16	/* ...is also our new FP */
> +	str	x29, [sp, #S_SP]
> +
> +	adrp    x0, function_trace_op
> +	ldr     x2, [x0, #:lo12:function_trace_op]
> +	mov	x1, x9		/* saved LR == parent IP */
> +	sub	x0, lr, #8	/* function entry == IP */
> +	mov	x3, sp		/* pt_regs are @sp */
> +
> +	.global ftrace_call
> +ftrace_call:
> +
> +	bl	ftrace_stub

I should've asked this in my initial review. But why are we always 
calling ftrace_stub rather than have a patchable "nop" like in the 
!CC_USING_PATCHABLE_FUNCTION_ENTRY version?

> +
> +#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:
> +	ldp	x0, x1, [sp]
> +	ldp	x2, x3, [sp, #16]
> +	ldp	x4, x5, [sp, #32]
> +	ldp	x6, x7, [sp, #48]
> +	ldp	x8, x9, [sp, #64]
> +
> +	ldp	x28, x29, [sp, #224]
> +
> +	ldr	x9, [sp, #S_PC]
> +	ldr	lr, [sp, #S_LR]
> +	add	sp, sp, #S_FRAME_SIZE+16
> +
> +	ret	x9
> +
> +ENDPROC(ftrace_caller)
> +
> +#endif /* CC_USING_PATCHABLE_FUNCTION_ENTRY */
>   #endif /* CONFIG_DYNAMIC_FTRACE */
>   
>   ENTRY(ftrace_stub)
> @@ -197,12 +284,20 @@ ENDPROC(ftrace_stub)
>    * and run return_to_handler() later on its exit.
>    */
>   ENTRY(ftrace_graph_caller)
> +#ifndef CC_USING_PATCHABLE_FUNCTION_ENTRY
>   	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	x0, sp, #S_LR	/* address of (LR pointing into caller) */
> +	ldr	x1, [sp, #S_PC]
> +	ldr	x2, [sp, #S_X28 + 8]	/* caller's frame pointer */
> +	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
> @@ -70,7 +70,8 @@ int ftrace_update_ftrace_func(ftrace_fun
>    */
>   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;
>   
> @@ -128,22 +129,54 @@ int ftrace_make_call(struct dyn_ftrace *
>   	}
>   
>   	old = aarch64_insn_gen_nop();
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +#define INSN_MOV_X9_X30 aarch64_insn_gen_add_sub_imm(AARCH64_INSN_REG_9, \
> +		AARCH64_INSN_REG_30, 0, \
> +		AARCH64_INSN_VARIANT_64BIT, AARCH64_INSN_ADSB_ADD)

It might just be me, but since this macro is used in two different 
functions I'm not overly fond of defining in the middle of the function 
code.

Thanks,

> +	new = INSN_MOV_X9_X30;
> +	ret = ftrace_modify_code(pc-REC_IP_BRANCH_OFFSET, old, new, true);
> +	if (ret)
> +		return ret;
> +	smp_wmb();	/* ensure LR saver is in place before ftrace call */
> +#endif
>   	new = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
>   
>   	return ftrace_modify_code(pc, old, new, true);
>   }
>   
> +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);
> +}
> +
> +
>   /*
>    * 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;
>   
> +#ifdef CC_USING_PATCHABLE_FUNCTION_ENTRY
> +	/* -fpatchable-function-entry= does not generate a profiling call
> +	 *  initially; the NOPs are already there.
> +	 */
> +	if (addr == MCOUNT_ADDR)
> +		return 0;
> +#endif
> +
>   	if (offset < -SZ_128M || offset >= SZ_128M) {
>   #ifdef CONFIG_ARM64_MODULE_PLTS
>   		u32 replaced;
> @@ -188,7 +221,15 @@ 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;
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +	smp_wmb();	/* ftrace call must not remain without LR saver. */
> +	old = INSN_MOV_X9_X30;
> +	ret = ftrace_modify_code(pc-REC_IP_BRANCH_OFFSET, old, new, true);
> +#endif
> +	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
> +#ifdef CC_USING_PATCHABLE_FUNCTION_ENTRY
> +#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
> 

-- 
Julien Thierry

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