[RFC][PATCH] Export supported trace features in debugfs

[Qais Yousef <Qais.Yousef@arm.com>]

eBPF tools like bcc-tools have hard time figuring out when features like
raw_tracepoint are supported in the kernel on which we are running. At
the moment a fragile mechanism of matching bpf_find_raw_tracepoint()
function in /proc/kallsyms is used to find out whether raw tracepoints
can be used or not. But when this function was renamed recently to
bpf_get_raw_tracepoint() the tool started to fail to use raw
tracepoints.

To help in providing a more reliable way to detect features like
RAW_TRACEPOINT, add a new file in trace debugfs to export the supported
features.

$cat /sys/kernel/debug/tracing/supported_features
RAW_TRACEPOINT
EXAMPLE_FEATURE_1
EXAMPLE_FEATURE_2

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---

This is a half baked patch to probe the potential of this solution.

The breakage mentioned in the commit message is here:

https://github.com/iovisor/bcc/pull/2241/commits/0f5849187972a50adf0d9eaa8788c11f9fd926ea

I am not sure what else beside raw_tracepoint makes sense to expose right now.

 kernel/trace/trace.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c4238b441624..daae09238e62 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6670,6 +6670,28 @@ static const struct file_operations snapshot_raw_fops = {

 #endif /* CONFIG_TRACER_SNAPSHOT */

+#define TRACE_FEATURE(feat)__stringify(feat) "\n"
+
+#define TRACE_FEATURES\
+TRACE_FEATURE(RAW_TRACEPOINT)\
+TRACE_FEATURE(EXAMPLE_FEATUTE_1)\
+TRACE_FEATURE(EXAMPLE_FEATUTE_2)
+
+static ssize_t
+tracing_read_trace_features(struct file *filp, char __user *ubuf,
+size_t cnt, loff_t *ppos)
+{
+char *buf = TRACE_FEATURES;
+size_t len = sizeof(TRACE_FEATURES);
+
+return simple_read_from_buffer(ubuf, cnt, ppos, buf, len);
+}
+
+static const struct file_operations show_trace_features_fops = {
+.read           = tracing_read_trace_features,
+.llseek         = no_llseek,
+};
+
 static int tracing_buffers_open(struct inode *inode, struct file *filp)
 {
 struct trace_array *tr = inode->i_private;
@@ -8242,6 +8264,9 @@ static __init int tracer_init_tracefs(void)
 &ftrace_update_tot_cnt, &tracing_dyn_info_fops);
 #endif

+trace_create_file("trace_features", 0444, d_tracer,
+NULL, &show_trace_features_fops);
+
 create_trace_instances(d_tracer);

 update_tracer_options(&global_trace);
--
2.17.1

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