Subject: [PATCH V2] arm64:ftrace: add save_stack_trace_regs()

From: [email-masked] (James Morse)
Date: Fri, 02 Sep 2016 11:50:15 +0100

Hi Pratyush,

On 01/09/16 14:35, Pratyush Anand wrote:
> Implement save_stack_trace_regs, so that stacktrace of a kprobe events can be
> obtained.

Hmm, kconfig enables 'CONFIG_KPROBE_EVENT' if we select KPROBES and FTRACE, this
calls save_stack_trace_regs(), which we haven't implemented, giving:
[  373.406982] save_stack_trace_regs() not implemented yet.
[  373.413020] ------------[ cut here ]------------
[  373.417601] WARNING: CPU: 1 PID: 0 at ../kernel/stacktrace.c:74 save_stack_tr
ace_regs+0x3c/0x48
[  373.426212] Modules linked in:
[  373.429238]
[  373.430716] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.8.0-rc4-dirty #5128
[  373.437606] Hardware name: ARM Juno development board (r1) (DT)
[  373.443466] task: ffff800975dd1900 task.stack: ffff800975ddc000
[  373.449327] PC is at save_stack_trace_regs+0x3c/0x48
[  373.454242] LR is at save_stack_trace_regs+0x3c/0x48
[  373.459156] pc : [<ffff000008126c64>] lr : [<ffff000008126c64>] pstate: 600003c5
[  373.466476] sp : ffff80097ef52c00

[  373.554836] Call trace:
[  373.641153] [<ffff000008126c64>] save_stack_trace_regs+0x3c/0x48
[  373.647104] [<ffff00000817a0b0>] __ftrace_trace_stack+0x168/0x208
[  373.653138] [<ffff00000817ad40>] trace_buffer_unlock_commit_regs+0x5c/0x7c
[  373.659946] [<ffff00000818d57c>] kprobe_trace_func+0x308/0x3d8
[  373.665721] [<ffff00000818defc>] kprobe_dispatcher+0x58/0x60
[  373.671327] [<ffff0000088d8cfc>] kprobe_breakpoint_handler+0xbc/0x18c
[  373.677705] [<ffff0000080851b8>] brk_handler+0x50/0x90
[  373.682792] [<ffff000008081300>] do_debug_exception+0x50/0xbc

I guess hitting a WARN_ONCE() while using a 'kernel hacking' feature isn't
serious enough to make this a fix.

> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index d9751a4769e7..bd6d9d8087f0 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -152,6 +152,24 @@ static int save_trace(struct stackframe *frame, void *d)
>  	return trace->nr_entries >= trace->max_entries;
>  }
>  
> +void save_stack_trace_regs(struct pt_regs *regs, struct stack_trace *trace)
> +{
> +	struct stack_trace_data data;
> +	struct stackframe frame;
> +
> +	data.trace = trace;
> +	data.skip = trace->skip;
> +	data.no_sched_functions = 0;
> +
> +	frame.fp = regs->regs[29];
> +	frame.sp = regs->sp;
> +	frame.pc = regs->pc;

What happens when a kprobe-event runs over a frame the function graph tracer has
meddled with? You need:

#ifdef CONFIG_FUNCTION_GRAPH_TRACER
	frame.graph = current->curr_ret_stack;
#endif

> +
> +	walk_stackframe(current, &frame, save_trace, &data);
> +	if (trace->nr_entries < trace->max_entries)
> +		trace->entries[trace->nr_entries++] = ULONG_MAX;
> +}
> +
>  void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
>  {
>  	struct stack_trace_data data;
> 

Reviewed-by: James Morse <[email-masked]>

Thanks,

James