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

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 1 October 2018 at 16:16, Torsten Duwe <duwe@lst.de> 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@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 $(srctree)/arch/arm64/kernel/module.lds
>  endif
>
> +ifeq ($(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
>         }
>