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

[Steven Rostedt]

[ I should have added Mark as Cc ]

Dear ARM64 folks,

Are you OK with this patch set?

If so, please ACK.

Should it be marked for stable?

Hmm, I'm starting to think not.

-- Steve


On Wed, 07 Aug 2019 13:28:27 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> 
> 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@xxxxxxxxxxxxx
> 
> Reported-by: Jiping Ma <jiping.ma2@xxxxxxxxxxxxx>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> ---
>  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
> +
>  #ifndef __ASSEMBLY__
>  #include <linux/compat.h>
>  
> diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
> index 5d16f73898db..40e4a88eea8f 100644
> --- a/kernel/trace/trace_stack.c
> +++ b/kernel/trace/trace_stack.c
> @@ -158,6 +158,20 @@ static void check_stack(unsigned long ip, unsigned long *stack)
>  			i++;
>  	}
>  
> +#ifdef ARCH_RET_ADDR_AFTER_LOCAL_VARS
> +	/*
> +	 * Some archs will store the link register before calling
> +	 * nested functions. This means the saved return address
> +	 * comes after the local storage, and we need to shift
> +	 * for that.
> +	 */
> +	if (x > 1) {
> +		memmove(&stack_trace_index[0], &stack_trace_index[1],
> +			sizeof(stack_trace_index[0]) * (x - 1));
> +		x--;
> +	}
> +#endif
> +
>  	stack_trace_nr_entries = x;
>  
>  	if (task_stack_end_corrupted(current)) {