Initial support for Trace-flags

CHANGELOG.md

@@ -10,6 +10,7 @@ OpenTelemetry Go Automatic Instrumentation adheres to [Semantic Versioning](http
 
 ### Added
 
+- Initial support for `trace-flags`. ([#868](https://github.com/open-telemetry/opentelemetry-go-instrumentation/pull/868))
 - Support `google.golang.org/grpc` `1.66.0-dev`. ([#872](https://github.com/open-telemetry/opentelemetry-go-instrumentation/pull/872))
 
 ## [v0.13.0-alpha] - 2024-06-04

internal/include/span_context.h

@@ -20,13 +20,40 @@
 #define SPAN_CONTEXT_STRING_SIZE 55
 #define W3C_KEY_LENGTH 11 // length of the "traceparent" key
 #define W3C_VAL_LENGTH 55
+#define TRACE_ID_SIZE 16
+#define TRACE_ID_STRING_SIZE 32
+#define SPAN_ID_SIZE 8
+#define SPAN_ID_STRING_SIZE 16
+#define TRACE_FLAGS_SIZE 1
+#define TRACE_FLAGS_STRING_SIZE 2
+
+static const u8 FLAG_SAMPLED = 1;
 
 struct span_context
 {
-    unsigned char TraceID[TRACE_ID_SIZE];
-    unsigned char SpanID[SPAN_ID_SIZE];
+    u8 TraceID[TRACE_ID_SIZE];
+    u8 SpanID[SPAN_ID_SIZE];
+    u8 TraceFlags;
+    u8 padding[7];
 };
 
+// Fill the child span context based on the parent span context,
+// generating a new span id and copying the trace id and trace flags
+static __always_inline void get_span_context_from_parent(struct span_context *parent, struct span_context *child) {
+    copy_byte_arrays(parent->TraceID, child->TraceID, TRACE_ID_SIZE);
+    generate_random_bytes(child->SpanID, SPAN_ID_SIZE);
+    child->TraceFlags = parent->TraceFlags;
+}
+
+// Fill the passed span context as root span context
+static __always_inline void get_root_span_context(struct span_context *sc) {
+    generate_random_bytes(sc->TraceID, TRACE_ID_SIZE);
+    generate_random_bytes(sc->SpanID, SPAN_ID_SIZE);
+    // currently we always sample
+    sc->TraceFlags = FLAG_SAMPLED;
+}
+
+// TODO: remove this function once all the probes move to the above functions
 static __always_inline struct span_context generate_span_context()
 {
     struct span_context context = {};
@@ -55,17 +82,23 @@ static __always_inline void span_context_to_w3c_string(struct span_context *ctx,
     out += SPAN_ID_STRING_SIZE;
     *out++ = '-';
 
-    // Write sampled
-    *out++ = '0';
-    *out = '1';
+    // Write trace flags
+    bytes_to_hex_string(&ctx->TraceFlags, TRACE_FLAGS_SIZE, out);
 }
 
 static __always_inline void w3c_string_to_span_context(char *str, struct span_context *ctx)
 {
     u32 trace_id_start_pos = 3;
-    u32 span_id_start_pod = 36;
+    u32 span_id_start_pos = 36;
+    u32 trace_flags_start_pos = 53;
     hex_string_to_bytes(str + trace_id_start_pos, TRACE_ID_STRING_SIZE, ctx->TraceID);
-    hex_string_to_bytes(str + span_id_start_pod, SPAN_ID_STRING_SIZE, ctx->SpanID);
+    hex_string_to_bytes(str + span_id_start_pos, SPAN_ID_STRING_SIZE, ctx->SpanID);
+    hex_string_to_bytes(str + trace_flags_start_pos, TRACE_FLAGS_STRING_SIZE, &ctx->TraceFlags);
+}
+
+static __always_inline bool is_sampled(struct span_context *ctx)
+{
+    return ((ctx->TraceFlags & FLAG_SAMPLED) == FLAG_SAMPLED);
 }
 
 #endif

internal/include/utils.h

@@ -17,10 +17,6 @@
 
 #include "bpf_helpers.h"
 
-#define TRACE_ID_SIZE 16
-#define TRACE_ID_STRING_SIZE 32
-#define SPAN_ID_SIZE 8
-#define SPAN_ID_STRING_SIZE 16
 
 static __always_inline bool bpf_memcmp(char *s1, char *s2, s32 size)
 {

internal/pkg/instrumentation/bpf/net/http/client/bpf/probe.bpf.c

@@ -131,7 +131,7 @@ int uprobe_Transport_roundTrip(struct pt_regs *ctx) {
         copy_byte_arrays(httpReq->psc.TraceID, httpReq->sc.TraceID, TRACE_ID_SIZE);
         generate_random_bytes(httpReq->sc.SpanID, SPAN_ID_SIZE);
     } else {
-        httpReq->sc = generate_span_context();
+        get_root_span_context(&httpReq->sc);
     }
 
     if (!get_go_string_from_user_ptr((void *)(req_ptr+method_ptr_pos), httpReq->method, sizeof(httpReq->method))) {