Re: [PATCH 1/3] arm64: implement ftrace with regs

Julien Thierry
2018-08-13T10:54:12Z

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