linux-kernel - Re: [PATCH 1/2 v2] tracing/arm64: Have max stack tracer handle the case of return address after data

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Date:   Fri, 9 Aug 2019 09:55:21 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Jiping Ma <jiping.ma2@...driver.com>, mingo@...hat.com,
        catalin.marinas@....com, will.deacon@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2 v2] tracing/arm64: Have max stack tracer handle the
 case of return address after data

On Wed, Aug 07, 2019 at 01:28:27PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> Most archs (well at least x86) store the function call return address on the
> stack before storing the local variables for the function. The max stack
> tracer depends on this in its algorithm to display the stack size of each
> function it finds in the back trace.
> 
> Some archs (arm64), may store the return address (from its link register)
> just before calling a nested function. There's no reason to save the link
> register on leaf functions, as it wont be updated. This breaks the algorithm
> of the max stack tracer.
> 
> Add a new define ARCH_RET_ADDR_AFTER_LOCAL_VARS that an architecture may set
> if it stores the return address (link register) after it stores the
> function's local variables, and have the stack trace shift the values of the
> mapped stack size to the appropriate functions.
> 
> Link: 20190802094103.163576-1-jiping.ma2@...driver.com
> 
> Reported-by: Jiping Ma <jiping.ma2@...driver.com>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>  arch/arm64/include/asm/ftrace.h | 13 +++++++++++++
>  kernel/trace/trace_stack.c      | 14 ++++++++++++++
>  2 files changed, 27 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index 5ab5200b2bdc..961e98618db4 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -14,6 +14,19 @@
>  #define MCOUNT_ADDR		((unsigned long)_mcount)
>  #define MCOUNT_INSN_SIZE	AARCH64_INSN_SIZE
>  
> +/*
> + * Currently, gcc tends to save the link register after the local variables
> + * on the stack. This causes the max stack tracer to report the function
> + * frame sizes for the wrong functions. By defining
> + * ARCH_RET_ADDR_AFTER_LOCAL_VARS, it will tell the stack tracer to expect
> + * to find the return address on the stack after the local variables have
> + * been set up.
> + *
> + * Note, this may change in the future, and we will need to deal with that
> + * if it were to happen.
> + */
> +#define ARCH_RET_ADDR_AFTER_LOCAL_VARS 1

FWIW (with whatever this got renamed to):

Acked-by: Mark Rutland <mark.rutland@....com>

Thanks,
Mark.