Refactoring: Introduce extensibility scheme for tracepoint structures

- Introduce struct_size extensibility scheme for all tracepoint public
  structures, except for struct lttng_ust_tracepoint_probe which is
  embedded in a public array of structures, thus making it tricky for
  instrumentation sites to efficiently skip over the array elements
  without reading their size.
- Remove padding from those structures.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I2387a62c03f6f729c59ddc3fafa3a9b5546fb287

diff --git a/include/lttng/tracepoint-types.h b/include/lttng/tracepoint-types.h
index ceb476a6f9891d760f0e43e60bda928aed2acae0..5354c3698abc478dd41c6167f5cff07098f7de63 100644 (file)
--- a/include/lttng/tracepoint-types.h
+++ b/include/lttng/tracepoint-types.h
@@ -7,19 +7,44 @@
 #ifndef _LTTNG_TRACEPOINT_TYPES_H
 #define _LTTNG_TRACEPOINT_TYPES_H
 
+#include <stdint.h>
+
+/*
+ * Tracepoint probe definition
+ *
+ * IMPORTANT: this structure is part of the ABI between instrumented
+ * applications and UST. This structure is fixed-size because it is part
+ * of a public array of structures. Rather than extending this
+ * structure, struct lttng_ust_tracepoint should be extended instead.
+ */
+
 struct lttng_ust_tracepoint_probe {
        void (*func)(void);
        void *data;
 };
 
-#define LTTNG_UST_TRACEPOINT_PADDING   16
+/*
+ * Tracepoint definition
+ *
+ * IMPORTANT: this structure is part of the ABI between instrumented
+ * applications and UST. Fields need to be only added at the end, never
+ * reordered, never removed.
+ *
+ * The field @struct_size should be used to determine the size of the
+ * structure. It should be queried before using additional fields added
+ * at the end of the structure.
+ */
+
 struct lttng_ust_tracepoint {
+       uint32_t struct_size;
+
        const char *name;
        int state;
        struct lttng_ust_tracepoint_probe *probes;
        int *tracepoint_provider_ref;
        const char *signature;
-       char padding[LTTNG_UST_TRACEPOINT_PADDING];
+
+       /* End of base ABI. Fields below should be used after checking struct_size. */
 };
 
 #endif /* _LTTNG_TRACEPOINT_TYPES_H */

diff --git a/include/lttng/tracepoint.h b/include/lttng/tracepoint.h
index bcab5e2d7b0fa3c2ac0efb71b2c4f2ed3a68f2fe..71f3e36c856af7ddffdddf23e5e8e57cc23e82fc 100644 (file)
--- a/include/lttng/tracepoint.h
+++ b/include/lttng/tracepoint.h
@@ -210,8 +210,18 @@ extern int __tracepoint_probe_unregister(const char *name, void (*func)(void),
 /*
  * tracepoint dynamic linkage handling (callbacks). Hidden visibility:
  * shared across objects in a module/main executable.
+ *
+ * IMPORTANT: this structure is part of the ABI between instrumented
+ * applications and UST. Fields need to be only added at the end, never
+ * reordered, never removed.
+ *
+ * The field @struct_size should be used to determine the size of the
+ * structure. It should be queried before using additional fields added
+ * at the end of the structure.
  */
 struct lttng_ust_tracepoint_dlopen {
+       uint32_t struct_size;
+
        void *liblttngust_handle;
 
        int (*tracepoint_register_lib)(struct lttng_ust_tracepoint * const *tracepoints_start,
@@ -220,6 +230,8 @@ struct lttng_ust_tracepoint_dlopen {
        void (*rcu_read_lock_sym)(void);
        void (*rcu_read_unlock_sym)(void);
        void *(*rcu_dereference_sym)(void *p);
+
+       /* End of base ABI. Fields below should be used after checking struct_size. */
 };
 
 extern struct lttng_ust_tracepoint_dlopen tracepoint_dlopen;
@@ -235,7 +247,9 @@ int __tracepoint_registered
 int __tracepoint_ptrs_registered
        __attribute__((weak, visibility("hidden")));
 struct lttng_ust_tracepoint_dlopen tracepoint_dlopen
-       __attribute__((weak, visibility("hidden")));
+               __attribute__((weak, visibility("hidden"))) = {
+       .struct_size = sizeof(struct lttng_ust_tracepoint_dlopen),
+};
 /*
  * Deal with gcc O1 optimisation issues with weak hidden symbols. gcc
  * 4.8 and prior does not have the same behavior for symbol scoping on
@@ -253,17 +267,31 @@ struct lttng_ust_tracepoint_dlopen *tracepoint_dlopen_ptr
  * Tracepoint dynamic linkage handling (callbacks). Hidden visibility: shared
  * across objects in a module/main executable. The callbacks are used to
  * control and check if the destructors should be executed.
+ *
+ * IMPORTANT: this structure is part of the ABI between instrumented
+ * applications and UST. Fields need to be only added at the end, never
+ * reordered, never removed.
+ *
+ * The field @struct_size should be used to determine the size of the
+ * structure. It should be queried before using additional fields added
+ * at the end of the structure.
  */
 struct lttng_ust_tracepoint_destructors_syms {
+       uint32_t struct_size;
+
        void (*tracepoint_disable_destructors)(void);
        int (*tracepoint_get_destructors_state)(void);
+
+       /* End of base ABI. Fields below should be used after checking struct_size. */
 };
 
 extern struct lttng_ust_tracepoint_destructors_syms tracepoint_destructors_syms;
 extern struct lttng_ust_tracepoint_destructors_syms *tracepoint_destructors_syms_ptr;
 
 struct lttng_ust_tracepoint_destructors_syms tracepoint_destructors_syms
-       __attribute__((weak, visibility("hidden")));
+       __attribute__((weak, visibility("hidden"))) = {
+       .struct_size = sizeof(struct lttng_ust_tracepoint_destructors_syms),
+};
 struct lttng_ust_tracepoint_destructors_syms *tracepoint_destructors_syms_ptr
        __attribute__((weak, visibility("hidden")));
 
@@ -414,14 +442,13 @@ extern struct lttng_ust_tracepoint * const __stop___tracepoints_ptrs[]
                __attribute__((section("__tracepoints_strings"))) =             \
                        #_provider ":" #_name;                                  \
        struct lttng_ust_tracepoint __tracepoint_##_provider##___##_name        \
-               __attribute__((section("__tracepoints"))) =                     \
-               {                                                               \
+               __attribute__((section("__tracepoints"))) = {                   \
+                       sizeof(struct lttng_ust_tracepoint),                    \
                        __tp_strtab_##_provider##___##_name,                    \
                        0,                                                      \
                        NULL,                                                   \
                        _TRACEPOINT_UNDEFINED_REF(_provider),                   \
                        _TP_EXTRACT_STRING(_args),                              \
-                       { },                                                    \
                };                                                              \
        static struct lttng_ust_tracepoint *                                    \
                __tracepoint_ptr_##_provider##___##_name                        \