linux-kernel - Re: [PATCH 4/7] arm: Add ftrace with regs support

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Message-ID: <cf63d647-b5ce-a41b-31ee-6d14da60f541@arm.com>
Date:   Wed, 7 Dec 2016 11:58:24 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Abel Vesa <abelvesa@...ux.com>, linux@...linux.org.uk,
        jpoimboe@...hat.com, jeyu@...hat.com, jikos@...nel.org,
        mbenes@...e.cz, pmladek@...e.com
Cc:     linux-arm-kernel@...ts.infradead.org, geert+renesas@...der.be,
        ard.biesheuvel@...aro.org, jean-philippe.brucker@....com,
        gregkh@...uxfoundation.org, stefano.stabellini@...citrix.com,
        emil.l.velikov@...il.com, linux-kernel@...r.kernel.org,
        rostedt@...dmis.org, jens.wiklander@...aro.org,
        chris.brandt@...esas.com, mingo@...hat.com,
        viro@...iv.linux.org.uk, live-patching@...r.kernel.org,
        akpm@...ux-foundation.org, mchehab@...nel.org, davem@...emloft.net,
        linux@...ck-us.net
Subject: Re: [PATCH 4/7] arm: Add ftrace with regs support

Hi Abel,

On 06/12/16 17:06, Abel Vesa wrote:
> This adds __ftrace_regs_caller which, unlike __ftrace_caller,
> adds register saving/restoring and livepatch handling if
> the pc register gets modified by klp_ftrace_handler.
> 
> Signed-off-by: Abel Vesa <abelvesa@...ux.com>
> ---
>  arch/arm/kernel/entry-ftrace.S | 49 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 49 insertions(+)
> 
> diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S
> index c73c403..b6ada5c 100644
> --- a/arch/arm/kernel/entry-ftrace.S
> +++ b/arch/arm/kernel/entry-ftrace.S
> @@ -92,6 +92,46 @@
>  2:	mcount_exit
>  .endm
>  
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +
> +.macro __ftrace_regs_caller suffix
> +
> +	stmdb	sp!, {r0-r15}

As the kbuild robot reported, you can't do this. For ARM, it's unknown
what the value stored for r13 will be, and for a Thumb-2 kernel the
whole instruction is flat out unpredictable (i.e. it might just undef or
anything).

> +	mov	r3, sp
> +
> +	ldr	r10, [sp, #60]
> +
> +	mcount_get_lr   r1                      @ lr of instrumented func
> +	mcount_adjust_addr	r0, lr		@ instrumented function
> +
> +	.globl ftrace_regs_call\suffix
> +ftrace_regs_call\suffix:
> +	bl	ftrace_stub
> +
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> +	.globl ftrace_regs_graph_call\suffix
> +ftrace_regs_graph_call\suffix:
> +	mov	r0, r0
> +#endif
> +#ifdef CONFIG_LIVEPATCH
> +	ldr	r0, [sp, #60]
> +	cmp	r0, r10
> +	beq	ftrace_regs_caller_end
> +	ldmia	sp!, {r0-r12}
> +	add	sp, sp, #8
> +	ldmia	sp!, {r11}
> +	sub	sp, r12, #16
> +	str	r11, [sp, #12]
> +	ldmia	sp!, {r11, r12, lr, pc}
> +#endif
> +ftrace_regs_caller_end\suffix:
> +	ldmia	sp!, {r0-r14}

Again, the value of SP at this point is now unknown (regardless of
whether what the value on memory was correct or not). Not good.

Either use non-writeback addressing modes, or avoid saving/restoring SP
at all (AFAICS it isn't necessary, since if the SP was changed in
between, you then wouldn't know where to restore the saved registers
from anyway).

Robin.

> +	add	sp, sp, #4
> +	mov	pc, lr
> +.endm
> +
> +#endif
> +
>  .macro __ftrace_caller suffix
>  	mcount_enter
>  
> @@ -212,6 +252,15 @@ 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
>