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Subject: Re: [PATCH 1/3] arm64: implement ftrace with regs
To:     Torsten Duwe <duwe@lst.de>, Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        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
References: <20180810160043.9E45568C76@newverein.lst.de>
 <20180810160223.D360D68C76@newverein.lst.de>
From:   Julien Thierry <julien.thierry@arm.com>
Message-ID: <2b5c1092-2cc0-00ec-ba3d-341dc61452b4@arm.com>
Date:   Mon, 13 Aug 2018 11:54:06 +0100
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Hi Torsten,

On 10/08/18 17:02, 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 used to be disabled already).
> Add an ftrace_caller which can handle LR in x9.
> This code has successfully passed the FTRACE_STARTUP_TEST.
> 
> Signed-off-by: Torsten Duwe <duwe@suse.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
>   	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 $(srctree)/arch/arm64/kernel/module.lds
>   endif
>   
> +ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +  CC_FLAGS_FTRACE := -fpatchable-function-entry=2
> +  KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
> +  ifeq ($(call cc-option,-fpatchable-function-entry=2),)
> +    $(warning Cannot use CONFIG_DYNAMIC_FTRACE_WITH_REGS: \
> +             -fpatchable-function-entry not supported by compiler)

Shouldn't this be an error? The option -fpatchable-function-entry has 
been added to the CC_FLAGS_FTRACE, so any call to the compiler is gonna 
break anyway. Or am I missing something?

> +  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 +16,14 @@
>   #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
> +#define FTRACE_REGS_ADDR FTRACE_ADDR
> +#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 $(CC_FLAGS_FTRACE)
>   
>   # Object file lists.
>   arm64-obj-y		:= debug-monitors.o entry.o irq.o fpsimd.o		\
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -11,7 +11,8 @@
>   				   -fPIC -fno-strict-aliasing -mno-red-zone \
>   				   -mno-mmx -mno-sse -fshort-wchar
>   
> -cflags-$(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 +165,84 @@ ftrace_graph_call:			// ftrace_graph_cal
>   
>   	mcount_exit
>   ENDPROC(ftrace_caller)
> +#else /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
> +ENTRY(_mcount)
> +	mov     x10, lr
> +	mov     lr, x9
> +	ret     x10
> +ENDPROC(_mcount)
> +
> +ENTRY(ftrace_caller)
> +	stp	x29, x9, [sp, #-16]!
> +	sub	sp, sp, #S_FRAME_SIZE
> +
> +	stp	x0, x1, [sp]
> +	stp	x2, x3, [sp, #16]
> +	stp	x4, x5, [sp, #32]
> +	stp	x6, x7, [sp, #48]
> +	stp	x8, x9, [sp, #64]
> +	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]
> +	stp	x28, x29, [sp, #224]
> +	/* 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		/* complete pt_regs are @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_regs_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	x10, x11, [sp, #80]
> +	ldp	x12, x13, [sp, #96]
> +	ldp	x14, x15, [sp, #112]
> +	ldp	x16, x17, [sp, #128]
> +	ldp	x18, x19, [sp, #144]
> +	ldp	x20, x21, [sp, #160]
> +	ldp	x22, x23, [sp, #176]
> +	ldp	x24, x25, [sp, #192]
> +	ldp	x26, x27, [sp, #208]
> +	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 /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
>   #endif /* CONFIG_DYNAMIC_FTRACE */
>   
>   ENTRY(ftrace_stub)
> @@ -197,12 +278,20 @@ 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	x0, sp, #S_LR	/* address of (LR pointing into caller) */
> +	ldr	x1, [sp, #S_PC]
> +	ldr	x2, [sp, #232]	/* caller's frame pointer */

Nit:
Can we change the offset to #S_X28 + 8 ?

> +	bl	prepare_ftrace_return
> +	b	ftrace_regs_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;
>   
> @@ -127,23 +128,51 @@ int ftrace_make_call(struct dyn_ftrace *
>   #endif /* CONFIG_ARM64_MODULE_PLTS */
>   	}
>   
> -	old = aarch64_insn_gen_nop();
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +	old = aarch64_insn_gen_nop();

Doesn't moving 'old' in the #ifdef break things when 
CONFIG_DYNAMIC_FTRACE_WITH_REGS is not defined? I think the last 
ftrace_modifyi_code() is using it uninitialized in that case.

> +	new = 0xaa1e03e9;       /* mov x9,x30 */

I see there isn't a aarch64_insn_get_mov function, maybe it would be 
worth adding it. If not you could use:

aarch64_insn_gen_add_sub_imm(AARCH64_INSN_REG_9, AARCH64_INSN_REG_30, 0,
			     AARCH64_INSN_VARIANT_64BIT,
			     AARCH64_INSN_ADSB_ADD);

> +	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)

Is this getting called? I don't see it being referenced.

> +{
> +	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 +218,16 @@ 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 = 0xaa1e03e9;       /* mov x9,x30 */
> +	new = aarch64_insn_gen_nop();

'new' is already a nop here.

Cheers,

-- 
Julien Thierry