linux-kernel - Re: [PATCHv4] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH

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Date:   Mon, 27 Feb 2017 16:52:06 +0100
From:   Nicolai Stange <nicstange@...il.com>
To:     Abel Vesa <abelvesa@...ux.com>
Cc:     robin.murphy@....com, jjhiblot@...phandler.com,
        Russell King <linux@...linux.org.uk>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, pmladek@...e.com,
        mhiramat@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

Hi Abel,

On Fri, Feb 24 2017, Abel Vesa wrote:

<snip>

> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index fda6a46..877df5b 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -55,6 +55,7 @@ config ARM
>  	select HAVE_DMA_API_DEBUG
>  	select HAVE_DMA_CONTIGUOUS if MMU
>  	select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU
> +	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE && OLD_MCOUNT


AFAICS, your code depends on the __gnu_mcount_nc calling conventions,
i.e. on gcc pushing the original lr on the stack. In particular, there's
no implementation of a ftrace_regs_caller_old or so.

So shouldn't this read as !OLD_MCOUNT instead?

Also, at least the ldmia ..., {..., sp, ...} insn needs a !THUMB2_KERNEL.


>  	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)

<snip>

> diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S
> index c73c403..3916dd6 100644
> --- a/arch/arm/kernel/entry-ftrace.S
> +++ b/arch/arm/kernel/entry-ftrace.S
> @@ -92,12 +92,78 @@
>  2:	mcount_exit
>  .endm
>  
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +
> +.macro __ftrace_regs_caller
> +
> +	sub	sp, sp, #8	@ space for CPSR and OLD_R0 (not used)
> +
> +	add 	ip, sp, #12	@ 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          48   52       56   60   64    68       72
> +	@ R0 | R1 | ... | LR | SP + 4 | LR | PC | PSR | OLD_R0 | previous LR |

Being a constant, the saved pc is not very useful, I think.

Wouldn't it be better (and more consistent with other archs) to have

  pt_regs->ARM_lr = original lr
  pt_refs->ARM_pc = current lr

instead?

A (hypothetical) klp_arch_set_pc(struct pt_regs *regs, unsigned long ip)
would do the more intuitive

  regs->ARM_pc = ip;

rather than a

  regs->ARM_lr = ip

then.

In addition, the original lr register would be made available to ftrace
handlers this way.


> +	mov r3, sp				@ struct pt_regs*
> +	ldr r2, =function_trace_op
> +	ldr r2, [r2]				@ pointer to the current
> +						@ function tracing op
> +	ldr	r1, [sp, #PT_REGS_SIZE]		@ 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
> +	@ pop saved regs
> +	@ first, get the previous LR value from stack
> +	ldr	lr, [sp, #PT_REGS_SIZE]
> +	@ now pop the rest of the saved registers INCLUDING stack pointer
> +	ldmia   sp, {r0-r11, ip, sp, pc}
> +.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, #S_LR]		@ instrumented routine (func)
> +	mcount_adjust_addr	r1, r1
> +
> +	sub	r2, r0, #4		@ frame pointer

Given that r2 is prepare_ftrace_return()'s frame_pointer argument, is

  r2 = fp - 4 - 4 = fp - 8

really correct / what is wanted here?

> +	bl	prepare_ftrace_return
> +
> +	@ pop registers saved in ftrace_regs_caller
> +	@ first, get the previous LR value from stack
> +	ldr	lr, [sp, #PT_REGS_SIZE]
> +	@ now pop the rest of the saved registers INCLUDING stack pointer
> +	ldmia   sp, {r0-r11, ip, sp, pc}
> +.endm
> +#endif
> +#endif
> +

<snip>


Thanks,

Nicolai