Re: [PATCH v3 2/4] arm64: implement ftrace with regs

From: Ard Biesheuvel
Date: Mon Oct 01 2018 - 11:57:58 EST

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In reply to: Torsten Duwe: "[PATCH v3 2/4] arm64: implement ftrace with regs"
Next in thread: Torsten Duwe: "Re: [PATCH v3 2/4] arm64: implement ftrace with regs"
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On 1 October 2018 at 16:16, Torsten Duwe <duwe@xxxxxx> wrote:
> Check for compiler support of -fpatchable-function-entry and use it
> to intercept functions immediately on entry, saving the LR in x9.
> patchable-function-entry in GCC disables IPA-RA, which means ABI
> register calling conventions are obeyed *and* scratch registers are
> available.
> 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.
> Introduce and handle an ftrace_regs_trampoline for module PLTs.
>
> Signed-off-by: Torsten Duwe <duwe@xxxxxxx>
>
> --- 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 $(srctree)/arch/arm64/kernel/module.lds
> endif
>
> +ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_REGS),y)
> + 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 +16,17 @@
> #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. */

OK, so the first slot will be patched unconditionally to do the 'mov x9, x30' ?

> +#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 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 +165,92 @@ ftrace_graph_call: // ftrace_graph_cal
>
> mcount_exit
> ENDPROC(ftrace_caller)
> +#else /* CC_USING_PATCHABLE_FUNCTION_ENTRY */
> +
> +/* Since no -pg or similar compiler flag is used, there should really be
> + no reference to _mcount; so do not define one. Only a function address
> + is needed in order to refer to it. */
> +ENTRY(_mcount)
> + ret /* just in case, prevent any fall through. */
> +ENDPROC(_mcount)
> +
> +ENTRY(ftrace_regs_caller)
> + sub sp, sp, #S_FRAME_SIZE
> + stp x29, x9, [sp, #-16] /* FP/LR link */
> +

You cannot write below the stack pointer. So you are missing a
trailing ! here. Note that you can fold the sub

stp x29, x9, [sp, #-(S_FRAME_SIZE+16)]!

> + stp x10, x11, [sp, #S_X10]
> + stp x12, x13, [sp, #S_X12]
> + 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]
> +

All these will shift by 16 bytes though

I am now wondering if it wouldn't be better to create 2 stack frames:
one for the interrupted function, and one for this function.

So something like

stp x29, x9, [sp, #-16]!
mov x29, sp

stp x29, x30, [sp, #-(S_FRAME_SIZE + 16]!

... store all registers including x29 ...

and do another mov x29, sp before calling into the handler. That way
everything should be visible on the call stack when we do a backtrace.

> + b ftrace_common
> +ENDPROC(ftrace_regs_caller)
> +
> +ENTRY(ftrace_caller)
> + sub sp, sp, #S_FRAME_SIZE
> + stp x29, x9, [sp, #-16] /* FP/LR link */
> +

Same as above

> +ftrace_common:
> + stp x28, x29, [sp, #224] /* FP in pt_regs + "our" x28 */
> +
> + /* 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] /* to pt_regs.r[30] */
> + /* 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
> + str x29, [sp, #S_SP]
> +
> + ldr_l x2, function_trace_op, x0
> + mov x1, x9 /* saved LR == parent IP */
> + sub x0, lr, #8 /* function entry == IP */
> + mov x3, sp /* pt_regs are @sp */
> + sub sp, sp, #16 /* skip over FP/LR link */
> +
> + .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 sp, sp, #16 /* advance to pt_regs for restore */
> +
> + 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
> +
> + ret x9
> +
> +ENDPROC(ftrace_caller)
> +
> +#endif /* CC_USING_PATCHABLE_FUNCTION_ENTRY */
> #endif /* CONFIG_DYNAMIC_FTRACE */
>
> ENTRY(ftrace_stub)
> @@ -197,12 +286,20 @@ 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+16 /* address of (LR pointing into caller) */
> + ldr x1, [sp, #S_PC+16]
> + ldr x2, [sp, #S_X28 + 8+16] /* 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
> @@ -65,18 +65,66 @@ int ftrace_update_ftrace_func(ftrace_fun
> return ftrace_modify_code(pc, 0, new, false);
> }
>
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +/* Have the assembler generate a known "mov x9,x30" at compile time. */
> +static void notrace noinline __attribute__((used)) mov_x9_x30(void)
> +{
> + asm(" .global insn_mov_x9_x30\n"
> + "insn_mov_x9_x30: mov x9,x30\n" : : : "x9");
> +}

You cannot rely on the compiler putting the mov at the beginning. I
think some well commented #define should do for the opcode (or did you
just remove that?)

> +#endif
> +
> +#ifdef CONFIG_ARM64_MODULE_PLTS
> +int use_ftrace_trampoline(struct module *mod, unsigned long *addr)
> +{
> + struct plt_entry trampoline;
> + trampoline = get_plt_entry(*addr);
> + if (*addr == FTRACE_ADDR) {
> + if (!plt_entries_equal(mod->arch.ftrace_trampoline,
> + &trampoline)) {
> +
> + /* 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();
> + }
> + *addr = (unsigned long)(void *)mod->arch.ftrace_trampoline;
> + }
> + else if (*addr == FTRACE_REGS_ADDR) {
> + if (!plt_entries_equal(mod->arch.ftrace_regs_trampoline,
> + &trampoline)) {
> +
> + /* point the trampoline to our ftrace entry point */
> + module_disable_ro(mod);
> + *mod->arch.ftrace_regs_trampoline = trampoline;
> + module_enable_ro(mod, true);
> +
> + /* update trampoline before patching in the branch */
> + smp_wmb();
> + }
> + *addr = (unsigned long)(void *)mod->arch.ftrace_regs_trampoline;
> + }
> + else
> + return -EINVAL;
> + return 0;
> +}
> +#endif
> +
> /*
> * 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 +144,67 @@ 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 = use_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 = *(u32*)&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 */
> + }
> 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 +249,16 @@ 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;
> + if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS)) {
> + smp_wmb(); /* ftrace call must not remain without LR saver. */
> + old = *(u32*)&mov_x9_x30;
> + ret = ftrace_modify_code(pc - REC_IP_BRANCH_OFFSET,
> + 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
> +#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
> --- 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
> @@ -454,6 +454,10 @@ int module_finalize(const Elf_Ehdr *hdr,
> if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
> !strcmp(".text.ftrace_trampoline", secstrs + s->sh_name))
> me->arch.ftrace_trampoline = (void *)s->sh_addr;
> + if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS) &&
> + !strcmp(".text.ftrace_regs_trampoline",
> + secstrs + s->sh_name))
> + me->arch.ftrace_regs_trampoline = (void *)s->sh_addr;
> #endif
> }
>

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