[PATCH v4] ftrace: Get the true parent ip for function tracer

[Jeff Xie]

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
    [...]

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@xxxxxxxxx>
---
v4:
  https://lore.kernel.org/linux-trace-kernel/20241005101320.766c1100@xxxxxxxxxxxxxxxxxxxx/
- fixed the crash when accessing the "current" if the arch has not implemented noinstr
  thanks steve for the testing and the detailed explanation

v3:
  https://lore.kernel.org/linux-trace-kernel/20240910001326.87f27e6b312f1d956cf352a2@xxxxxxxxxx/
- fixed build error when CONFIG_FUNCTION_GRAPH_TRACER=n suggested by Masami

v2:
  https://lore.kernel.org/linux-trace-kernel/20240821095910.1888d7fa@xxxxxxxxxxxxxxxxxx/
- Adding __always_inline to function_get_true_parent_ip suggested by Steve

 kernel/trace/trace_functions.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
index 3b0cea37e029..46b663d22c37 100644
--- a/kernel/trace/trace_functions.c
+++ b/kernel/trace/trace_functions.c
@@ -176,6 +176,27 @@ static void function_trace_start(struct trace_array *tr)
 	tracing_reset_online_cpus(&tr->array_buffer);
 }
 
+#if defined(CONFIG_FUNCTION_GRAPH_TRACER) && defined(CONFIG_ARCH_WANTS_NO_INSTR)
+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;
+}
+#else
+static __always_inline unsigned long
+function_get_true_parent_ip(unsigned long parent_ip, struct ftrace_regs *fregs)
+{
+	return parent_ip;
+}
+#endif
+
 static void
 function_trace_call(unsigned long ip, unsigned long parent_ip,
 		    struct ftrace_ops *op, struct ftrace_regs *fregs)
@@ -193,6 +214,8 @@ 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 +264,7 @@ 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 +333,7 @@ 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 +381,7 @@ 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);
-- 
2.43.0