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

[Jiping Ma]

On 2019å08æ09æ 01:24, Steven Rostedt wrote:
> On Thu, 8 Aug 2019 18:11:53 +0100
> Will Deacon <will@xxxxxxxxxx> wrote:
>
> > > We could make it more descriptive of what it will do and not the reason
> > > for why it is done...
> > >
> > >
> > >    ARCH_FTRACE_SHIFT_STACK_TRACER
> > Acked-by: Will Deacon <will@xxxxxxxxxx>
> Thanks Will!
>
> Here's the official patch.
>
> 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

ARCH_FTRACE_SHIFT_STACK_TRACER is used in the code.

Jiping

> 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>
> Acked-by: Will Deacon <will@xxxxxxxxxx>
> 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..d48667b04c41 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_FTRACE_SHIFT_STACK_TRACER, 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_FTRACE_SHIFT_STACK_TRACER 1
> +
>   #ifndef __ASSEMBLY__
>   #include <linux/compat.h>
>   
> diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
> index 5d16f73898db..642a850af81a 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_FTRACE_SHIFT_STACK_TRACER
> +	/*
> +	 * 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)) {