Subject: [PATCHv2] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

From: [email-masked] (Abel Vesa)
Date: Tue, 7 Feb 2017 19:18:29 +0000

On Wed, Jan 25, 2017 at 07:30:32PM +0100, Jean-Jacques Hiblot wrote:

> Abel, I tested it partially.

OK, so I managed to find exactly what is happening.

> It's seems OK if I have FRAME_POINTER enabled and it breaks otherwise.

FRAME_POINTER is needed by OLD_MCOUNT. 

OLD_MCOUNT is needed by __ftrace_modify_code in order to be able to do set_kernel_text_rw.
(The other implementation does not allow writing to kernel text therefore the warning 
happens)

If you try the same thing without the patch you will get an "Invalid argument" at 
module loading.

> 
> My test case attaches a  tracing handler to a function "myfunc":
> 
> static struct ftrace_ops kprobe_ftrace_ops __read_mostly = {
> .func = jjh_ftrace_handler,
> .flags = FTRACE_OPS_FL_SAVE_REGS | FTRACE_OPS_FL_DYNAMIC,
> };
> 
> ftrace_set_filter(&kprobe_ftrace_ops,  "myfunc", 4, 0);
> register_ftrace_function(&kprobe_ftrace_ops);
> 
> I won't send the actual code because it's too messy. If you need help
> to reproduce, I'll try to extract the relevant part.
> 
> Before the crash I observe the following warning.
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 9 at kernel/trace/ftrace.c:2526
> ftrace_modify_all_code+0xd8/0x158
> Modules linked in: jjh_test(+)
> CPU: 0 PID: 9 Comm: migration/0 Tainted: G        W       4.7.0+ #53
> Hardware name: ARM-Versatile Express
> [<8010f7bc>] (unwind_backtrace) from [<8010b6f8>] (show_stack+0x18/0x1c)
> [<8010b6f8>] (show_stack) from [<803a1548>] (dump_stack+0x80/0xa0)
> [<803a1548>] (dump_stack) from [<8011ef20>] (__warn+0xe0/0x10c)
> [<8011ef20>] (__warn) from [<8011ef74>] (warn_slowpath_null+0x28/0x30)
> [<8011ef74>] (warn_slowpath_null) from [<8019d2a4>]
> (ftrace_modify_all_code+0xd8/0x158)
> [<8019d2a4>] (ftrace_modify_all_code) from [<8019d330>]
> (__ftrace_modify_code+0xc/0x14)
> [<8019d330>] (__ftrace_modify_code) from [<80198908>]
> (multi_cpu_stop+0xe8/0x114)
> [<80198908>] (multi_cpu_stop) from [<80198588>] (cpu_stopper_thread+0x60/0x134)
> [<80198588>] (cpu_stopper_thread) from [<8013f428>]
> (smpboot_thread_fn+0x148/0x1c4)
> [<8013f428>] (smpboot_thread_fn) from [<8013c00c>] (kthread+0xe0/0xfc)
> [<8013c00c>] (kthread) from [<80107db8>] (ret_from_fork+0x14/0x3c)
> ---[ end trace f7d50cc2a4627fb1 ]---
> 
> Jean-Jacques
> 
> 
> 
> 
> 2017-01-25 0:24 GMT+01:00 Abel Vesa <[email-masked]>:
> > The DYNAMIC_FTRACE_WITH_REGS configuration makes it possible for a ftrace
> > operation to specify if registers need to saved/restored by the ftrace handler.
> > This is needed by kgraft and possibly other ftrace-based tools, and the ARM
> > architecture is currently lacking this feature. It would also be the first step
> > to support the "Kprobes-on-ftrace" optimization on ARM.
> >
> > This patch introduces a new ftrace handler that stores the registers on the
> > stack before calling the next stage. The registers are restored from the stack
> > before going back to the instrumented function.
> >
> > A side-effect of this patch is to activate the support for ftrace_modify_call()
> > as it defines ARCH_SUPPORTS_FTRACE_OPS for the ARM architecture.
> >
> > Signed-off-by: Abel Vesa <[email-masked]>
> > ---
> >  arch/arm/Kconfig               |  1 +
> >  arch/arm/include/asm/ftrace.h  |  4 +++
> >  arch/arm/kernel/entry-ftrace.S | 79 ++++++++++++++++++++++++++++++++++++++++++
> >  arch/arm/kernel/ftrace.c       | 33 ++++++++++++++++++
> >  4 files changed, 117 insertions(+)
> >
> > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > index 186c4c2..db40973 100644
> > --- a/arch/arm/Kconfig
> > +++ b/arch/arm/Kconfig
> > @@ -50,6 +50,7 @@ config ARM
> >         select HAVE_DMA_API_DEBUG
> >         select HAVE_DMA_CONTIGUOUS if MMU
> >         select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU

The solution is to replace this:
> > +       select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
with this:
+       select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE && OLD_MCOUNT

> >         select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
> >         select HAVE_EXIT_THREAD
> >         select HAVE_FTRACE_MCOUNT_RECORD if (!XIP_KERNEL)
> > diff --git a/arch/arm/include/asm/ftrace.h b/arch/arm/include/asm/ftrace.h
> > index 22b7311..f379881 100644
> > --- a/arch/arm/include/asm/ftrace.h
> > +++ b/arch/arm/include/asm/ftrace.h
> > @@ -1,6 +1,10 @@
> >  #ifndef _ASM_ARM_FTRACE
> >  #define _ASM_ARM_FTRACE
> >
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > +#define ARCH_SUPPORTS_FTRACE_OPS 1
> > +#endif
> > +
> >  #ifdef CONFIG_FUNCTION_TRACER
> >  #define MCOUNT_ADDR            ((unsigned long)(__gnu_mcount_nc))
> >  #define MCOUNT_INSN_SIZE       4 /* sizeof mcount call */
> > diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S
> > index c73c403..fcdcc3a 100644
> > --- a/arch/arm/kernel/entry-ftrace.S
> > +++ b/arch/arm/kernel/entry-ftrace.S
> > @@ -92,12 +92,74 @@
> >  2:     mcount_exit
> >  .endm
> >
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > +
> > +.macro __ftrace_regs_caller
> > +
> > +       add     ip, sp, #4      @ move in IP the value of SP as it was
> > +                               @ before the push {lr} of the mcount mechanism
> > +       stmdb   sp!, {ip,lr,pc}
> > +       stmdb   sp!, {r0-r11,lr}
> > +
> > +       @ stack content at this point:
> > +       @ 0  4          44    48   52       56   60   64
> > +       @ RO | R1 | ... | R11 | LR | SP + 4 | LR | PC | previous LR |
> > +
> > +       mov r3, sp                              @ struct pt_regs*
> > +       ldr r2, =function_trace_op
> > +       ldr r2, [r2]                            @ pointer to the current
> > +                                               @ function tracing op
> > +       ldr     r1, [sp, #64]                   @ lr of instrumented func
> > +       mcount_adjust_addr      r0, lr          @ instrumented function
> > +
> > +       .globl ftrace_regs_call
> > +ftrace_regs_call:
> > +       bl      ftrace_stub
> > +
> > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > +       .globl ftrace_graph_regs_call
> > +ftrace_graph_regs_call:
> > +       mov     r0, r0
> > +#endif
> > +       ldr     lr, [sp, #64]           @ get the previous LR value from stack
> > +       ldmia   sp, {r0-r11, ip, sp}    @ pop the saved registers INCLUDING
> > +                                       @ the stack pointer
> > +       ret     ip
> > +.endm
> > +
> > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > +.macro __ftrace_graph_regs_caller
> > +
> > +       sub     r0, fp, #4              @ lr of instrumented routine (parent)
> > +
> > +       @ called from __ftrace_regs_caller
> > +       ldr     r1, [sp, #56]           @ instrumented routine (func)
> > +       mcount_adjust_addr      r1, r1
> > +
> > +       sub     r2, r0, #4              @ frame pointer
> > +       bl      prepare_ftrace_return
> > +
> > +       ldr     lr, [sp, #64]           @ restore lr from the stack
> > +       ldmia   sp, {r0-r11, ip, sp}    @ restore r0 through sp
> > +       ret     ip
> > +
> > +.endm
> > +#endif
> > +#endif
> > +
> >  .macro __ftrace_caller suffix
> >         mcount_enter
> >
> >         mcount_get_lr   r1                      @ lr of instrumented func
> >         mcount_adjust_addr      r0, lr          @ instrumented function
> >
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > +       ldr r2, =function_trace_op
> > +       ldr r2, [r2]                            @ pointer to the current
> > +                                               @ function tracing op
> > +       mov r3, #0                              @ regs is NULL
> > +#endif
> > +
> >         .globl ftrace_call\suffix
> >  ftrace_call\suffix:
> >         bl      ftrace_stub
> > @@ -212,6 +274,15 @@ UNWIND(.fnstart)
> >         __ftrace_caller
> >  UNWIND(.fnend)
> >  ENDPROC(ftrace_caller)
> > +
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > +ENTRY(ftrace_regs_caller)
> > +UNWIND(.fnstart)
> > +       __ftrace_regs_caller
> > +UNWIND(.fnend)
> > +ENDPROC(ftrace_regs_caller)
> > +#endif
> > +
> >  #endif
> >
> >  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > @@ -220,6 +291,14 @@ UNWIND(.fnstart)
> >         __ftrace_graph_caller
> >  UNWIND(.fnend)
> >  ENDPROC(ftrace_graph_caller)
> > +
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > +ENTRY(ftrace_graph_regs_caller)
> > +UNWIND(.fnstart)
> > +       __ftrace_graph_regs_caller
> > +UNWIND(.fnend)
> > +ENDPROC(ftrace_graph_regs_caller)
> > +#endif
> >  #endif
> >
> >  .purgem mcount_enter
> > diff --git a/arch/arm/kernel/ftrace.c b/arch/arm/kernel/ftrace.c
> > index 3f17594..d8d4753 100644
> > --- a/arch/arm/kernel/ftrace.c
> > +++ b/arch/arm/kernel/ftrace.c
> > @@ -139,6 +139,15 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
> >
> >         ret = ftrace_modify_code(pc, 0, new, false);
> >
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > +       if (!ret) {
> > +               pc = (unsigned long)&ftrace_regs_call;
> > +               new = ftrace_call_replace(pc, (unsigned long)func);
> > +
> > +               ret = ftrace_modify_code(pc, 0, new, false);
> > +       }
> > +#endif
> > +
> >  #ifdef CONFIG_OLD_MCOUNT
> >         if (!ret) {
> >                 pc = (unsigned long)&ftrace_call_old;
> > @@ -157,6 +166,20 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
> >         unsigned long ip = rec->ip;
> >
> >         old = ftrace_nop_replace(rec);
> > +
> > +       new = ftrace_call_replace(ip, adjust_address(rec, addr));
> > +
> > +       return ftrace_modify_code(rec->ip, old, new, true);
> > +}
> > +

Also this function implementation needs to be in an #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS clause:

> > +int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
> > +                               unsigned long addr)
> > +{
> > +       unsigned long new, old;
> > +       unsigned long ip = rec->ip;
> > +
> > +       old = ftrace_call_replace(ip, adjust_address(rec, old_addr));
> > +
> >         new = ftrace_call_replace(ip, adjust_address(rec, addr));
> >
> >         return ftrace_modify_code(rec->ip, old, new, true);
> > @@ -229,6 +252,8 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
> >  extern unsigned long ftrace_graph_call;
> >  extern unsigned long ftrace_graph_call_old;
> >  extern void ftrace_graph_caller_old(void);
> > +extern unsigned long ftrace_graph_regs_call;
> > +extern void ftrace_graph_regs_caller(void);
> >
> >  static int __ftrace_modify_caller(unsigned long *callsite,
> >                                   void (*func) (void), bool enable)
> > @@ -251,6 +276,14 @@ static int ftrace_modify_graph_caller(bool enable)
> >                                      ftrace_graph_caller,
> >                                      enable);
> >
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > +       if (!ret)
> > +               ret = __ftrace_modify_caller(&ftrace_graph_regs_call,
> > +                                    ftrace_graph_regs_caller,
> > +                                    enable);
> > +#endif
> > +
> > +
> >  #ifdef CONFIG_OLD_MCOUNT
> >         if (!ret)
> >                 ret = __ftrace_modify_caller(&ftrace_graph_call_old,
> > --
> > 2.7.4
> >

I tested it and it works now. An update version of this patch will follow.
(It also include all fixes for all the other comments on this thread)

Thanks,
Abel