Re: [RFC 1/3] ftrace: adjust a function's pc to search for in check_stack() for arm64

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 13 Jul 2015 14:29:33 +0900
AKASHI Takahiro <takahiro.akashi@linaro.org> wrote:

> Ftace's stack tracer on arm64 returns wrong information about call stacks:
> 
>         Depth    Size   Location    (50 entries)
>         -----    ----   --------
>   0)     5256       0   notifier_call_chain+0x30/0x94
>   1)     5256       0   ftrace_call+0x0/0x4
>   2)     5256       0   notifier_call_chain+0x2c/0x94
>   3)     5256       0   raw_notifier_call_chain+0x34/0x44
>   4)     5256       0   timekeeping_update.constprop.9+0xb8/0x114
>   5)     5256       0   update_wall_time+0x408/0x6dc
> 
> Most of 'Size' fields are unexpectedly zero.
> 
> This is because stack tracer fails to recognize each function's stack frame
> in check_stack(). Stack tracer searches for a function's pc in the stack
> based on the list returned by save_stack_trace(), but save_stack_trace() on
> arm64 does not return the exact return address saved in a stack frame, but
> a value decrmented by 4 (which means a branch instruction's address).
> This behavior was introduced by
>     commit e306dfd06fcb ("ARM64: unwind: Fix PC calculation")
> 
> So the matching doesn't succeed in most cases.
> 
> This problem can be fixed either by
> a) reverting the commit above
> b) adding an arm64-specific hack to check_patch()
> 
> This patch does b).
> 
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> ---
>  kernel/trace/trace_stack.c |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
> index 3f34496..7086fc3 100644
> --- a/kernel/trace/trace_stack.c
> +++ b/kernel/trace/trace_stack.c
> @@ -143,7 +143,11 @@ check_stack(unsigned long ip, unsigned long *stack)
>  		p = start;
>  
>  		for (; p < top && i < max_stack_trace.nr_entries; p++) {
> +#ifdef CONFIG_ARM64
> +			if (*p == (stack_dump_trace[i] + 4)) {
> +#else
>  			if (*p == stack_dump_trace[i]) {
> +#endif

Instead of the ugly #ifdef in this code, please add a macro
FTRACE_STACK_FRAME_OFFSET

Then in include/linux/ftrace.h have:

#ifndef FTRACE_STACK_FRAME_OFFSET
# define FTRACE_STACK_FRAME_OFFSET 0
#endif

And in arch/arm64/include/asm/ftrace.h

#define FTRACE_STACK_FRAME_OFFSET 4

And then just do:

	if (*p == (stack_dump_trace[i] + FTRACE_STACK_FRAME_OFFSET)) {

-- Steve

>  				this_size = stack_dump_index[i++] =
>  					(top - p) * sizeof(unsigned long);
>  				found = 1;