Support systemwide probe firing, more tracers by adding a USDT probe to libstapsdt for probe firing

Makefile

@@ -19,6 +19,7 @@ install:
 	mkdir -p $(DESTDIR)$(PREFIX)/include
 	cp out/$(SONAME) $(DESTDIR)$(PREFIX)/lib/$(SONAME)
 	cp src/libstapsdt.h $(DESTDIR)$(PREFIX)/include/
+	rm -f $(DESTDIR)$(PREFIX)/lib/$(SOLINK)
 	ln -s $(DESTDIR)$(PREFIX)/lib/$(SONAME) $(DESTDIR)$(PREFIX)/lib/$(SOLINK)
 
 uninstall:

README.md

@@ -94,6 +94,24 @@ Here's an example using [eBPF/bcc](https://github.com/iovisor/bcc) trace tool
 sudo /usr/share/bcc/tools/trace -p $(pgrep demo) 'u::PROBE_NAME'
 ```
 
+Other tracing tools that do not support tracing of the libstapsdt-created
+libraries can trace the USDT stapsdt/probe. It will fire when a
+libstapsdt probe fires.  It is passed the provider and probe names
+along with arguments.  For example, the following BPF C program could
+be used to show probe firing:
+
+```
+SEC("usdt//usr/lib/libstapsdt.so:stapsdt:probe")
+int BPF_USDT(args, const char *provider, const char *probename)
+{
+  __bpf_printk("%s/%s fired\n", provider, probename);
+  return 0;
+}
+```
+
+One benefit of this form of tracing is that by tracing libstapsdt.so
+itself, we can trace libstapsdt probe firings system-wide.
+
 ## Run tests
 
 To run tests, just run the command below. Please be aware that there are only

src/libstapsdt.c

@@ -17,6 +17,7 @@
 #include "shared-lib.h"
 #include "util.h"
 #include "libstapsdt.h"
+#include "stapsdt-probe.h"
 #include "errors.h"
 
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(3, 17, 0)
@@ -227,6 +228,12 @@ void probeFire(SDTProbe_t *probe, ...) {
     arg[i] = va_arg(vl, uint64_t);
   }
 
+  /* fire static probe for libstapsdt signifying a dynamic probe has
+   * fired; tracers can attach to libstapsdt to see all dynamic probe
+   * firings system-wide.
+   */
+  stapsdtProbeFire(probe, arg);
+
   switch(probe->argCount) {
     case 0:
       ((void (*)())probe->_fire) ();
@@ -259,6 +266,11 @@ int probeIsEnabled(SDTProbe_t *probe) {
   if(probe->_fire == NULL) {
     return 0;
   }
+  /* tracers can attach to stapsdt/probe to see probe firings across
+   * libstapsdt.
+   */
+  if (stapsdtProbeIsEnabled(probe))
+    return 1;
   if(((*(char *)probe->_fire) & 0x90) == 0x90) {
     return 0;
   }

src/sdt-config.h

@@ -0,0 +1,6 @@
+/* includes/sys/sdt-config.h.  Generated from sdt-config.h.in by configure.
+
+   This file just defines _SDT_ASM_SECTION_AUTOGROUP_SUPPORT to 0 or 1 to
+   indicate whether the assembler supports "?" in .pushsection directives.  */
+
+#define _SDT_ASM_SECTION_AUTOGROUP_SUPPORT 1