Re: [PATCH v2] ftrace: Get the true parent ip for function tracer

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 22 Aug 2024 07:30:21 +0800
Jeff Xie <jeff.xie@linux.dev> wrote:

> Currently, when using both function tracer and function graph simultaneously,
> it is found that function tracer sometimes captures a fake parent ip(return_to_handler)
> instead of the true parent ip.
> 
> This issue is easy to reproduce. Below are my reproduction steps:
> 
> jeff-labs:~/bin # ./trace-net.sh
> 
> jeff-labs:~/bin # cat /sys/kernel/debug/tracing/instances/foo/trace | grep return_to_handler
>     trace-net.sh-405     [001] ...2.    31.859501: avc_has_perm+0x4/0x190 <-return_to_handler+0x0/0x40
>     trace-net.sh-405     [001] ...2.    31.859503: simple_setattr+0x4/0x70 <-return_to_handler+0x0/0x40
>     trace-net.sh-405     [001] ...2.    31.859503: truncate_pagecache+0x4/0x60 <-return_to_handler+0x0/0x40
>     trace-net.sh-405     [001] ...2.    31.859505: unmap_mapping_range+0x4/0x140 <-return_to_handler+0x0/0x40
>     trace-net.sh-405     [001] ...3.    31.859508: _raw_spin_unlock+0x4/0x30 <-return_to_handler+0x0/0x40
>     [...]
> 

Can you also add 

> The following is my simple trace script:
> 
> <snip>
> jeff-labs:~/bin # cat ./trace-net.sh
> TRACE_PATH="/sys/kernel/debug/tracing"
> 
> set_events() {
>         echo 1 > $1/events/net/enable
>         echo 1 > $1/events/tcp/enable
>         echo 1 > $1/events/sock/enable
>         echo 1 > $1/events/napi/enable
>         echo 1 > $1/events/fib/enable
>         echo 1 > $1/events/neigh/enable
> }
> 
> set_events ${TRACE_PATH}
> echo 1 > ${TRACE_PATH}/options/sym-offset
> echo 1 > ${TRACE_PATH}/options/funcgraph-tail
> echo 1 > ${TRACE_PATH}/options/funcgraph-proc
> echo 1 > ${TRACE_PATH}/options/funcgraph-abstime
> 
> echo 'tcp_orphan*' > ${TRACE_PATH}/set_ftrace_notrace
> echo function_graph > ${TRACE_PATH}/current_tracer
> 
> INSTANCE_FOO=${TRACE_PATH}/instances/foo
> if [ ! -e $INSTANCE_FOO ]; then
>         mkdir ${INSTANCE_FOO}
> fi
> set_events ${INSTANCE_FOO}
> echo 1 > ${INSTANCE_FOO}/options/sym-offset
> echo 'tcp_orphan*' > ${INSTANCE_FOO}/set_ftrace_notrace
> echo function > ${INSTANCE_FOO}/current_tracer
> 
> echo 1 > ${TRACE_PATH}/tracing_on
> echo 1 > ${INSTANCE_FOO}/tracing_on
> 
> echo > ${TRACE_PATH}/trace
> echo > ${INSTANCE_FOO}/trace
> </snip>
> 
> Signed-off-by: Jeff Xie <jeff.xie@linux.dev>
> ---
> v2:
> - Adding __always_inline to function_get_true_parent_ip suggested by Steve
> 
>  kernel/trace/trace_functions.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
> index 3b0cea37e029..d989e927c898 100644
> --- a/kernel/trace/trace_functions.c
> +++ b/kernel/trace/trace_functions.c
> @@ -176,6 +176,19 @@ static void function_trace_start(struct trace_array *tr)
>  	tracing_reset_online_cpus(&tr->array_buffer);
>  }
>  
> +static __always_inline unsigned long
> +function_get_true_parent_ip(unsigned long parent_ip, struct ftrace_regs *fregs)
> +{
> +	unsigned long true_parent_ip;
> +	int idx = 0;
> +
> +	true_parent_ip = parent_ip;
> +	if (unlikely(parent_ip == (unsigned long)&return_to_handler))
> +		true_parent_ip = ftrace_graph_ret_addr(current, &idx, parent_ip,
> +				(unsigned long *)fregs->regs.sp);
> +	return true_parent_ip;
> +}

Can this work without CONFIG_FUNCTION_GRAPH_TRACER? Maybe it cause a build error
because return_to_handler is not defined.

Could you check it?

Thank you,

> +
>  static void
>  function_trace_call(unsigned long ip, unsigned long parent_ip,
>  		    struct ftrace_ops *op, struct ftrace_regs *fregs)
> @@ -193,6 +206,8 @@ function_trace_call(unsigned long ip, unsigned long parent_ip,
>  	if (bit < 0)
>  		return;
>  
> +	parent_ip = function_get_true_parent_ip(parent_ip, fregs);
> +
>  	trace_ctx = tracing_gen_ctx();
>  
>  	cpu = smp_processor_id();
> @@ -241,6 +256,7 @@ function_stack_trace_call(unsigned long ip, unsigned long parent_ip,
>  	 * recursive protection is performed.
>  	 */
>  	local_irq_save(flags);
> +	parent_ip = function_get_true_parent_ip(parent_ip, fregs);
>  	cpu = raw_smp_processor_id();
>  	data = per_cpu_ptr(tr->array_buffer.data, cpu);
>  	disabled = atomic_inc_return(&data->disabled);
> @@ -309,6 +325,7 @@ function_no_repeats_trace_call(unsigned long ip, unsigned long parent_ip,
>  	if (bit < 0)
>  		return;
>  
> +	parent_ip = function_get_true_parent_ip(parent_ip, fregs);
>  	cpu = smp_processor_id();
>  	data = per_cpu_ptr(tr->array_buffer.data, cpu);
>  	if (atomic_read(&data->disabled))
> @@ -356,6 +373,7 @@ function_stack_no_repeats_trace_call(unsigned long ip, unsigned long parent_ip,
>  	 * recursive protection is performed.
>  	 */
>  	local_irq_save(flags);
> +	parent_ip = function_get_true_parent_ip(parent_ip, fregs);
>  	cpu = raw_smp_processor_id();
>  	data = per_cpu_ptr(tr->array_buffer.data, cpu);
>  	disabled = atomic_inc_return(&data->disabled);
> -- 
> 2.34.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>