usdt: is-enabled probe support

This implementation is in many ways similar to v1's, but contains one
significant simplification: rather than is-enabled probes appearing to
the user as a function that returns a 1 or 0, they appear as a function
that returns void and takes the address of a local variable: the
_IS_ENABLED macros use the GCC statement-expression extension
to declare and initialize a distinct local for each use of the
is-enabled probe:

	({ uint32_t enabled = 0; \
	__dtraceenabled_foo___bar(&enabled); \
	   enabled; })

The is-enabled trampoline writes a 1 into this local variable.  This
allows the dt_link code to discard all the architecture-dependent code
dealing with return value handling and treat is-enabled probes just like
other USDT probes: it still tracks whether is-enabled probes are used,
but doesn't use the info for anything but setting the DOF version.

The downside of this approach, of course, is that users need the
statement-expression extension to work, and that old is-enabled probes
from DTrace v1 aren't going to work without regeneration with dtrace
-G. Neither of these seem likely to be serious problems in practice, and
the commit before this one should help.

We use #pragma system_header to suppress -pedantic warnings about
use of the extension.

Internally, there are tiny changes in the dtprobed and dt_pid
uprobe-registration-and-creation layers to put is-enabled probes in a
different uprobe group to prevent their names clashing with USDT probes
with the same name (dt_pid_is_enabled/ rather than dt_pid/). Normal USDT
probes enable the correspondinng is-enabled probe, if any, when they are
enabled: those probes have custom trampoline that copy a 64-bit-wide 1
into the first function parameter (and have truncated parameter-copying
code that avoids preserving any parameter but the first, because there
will never be any more).

Please ignore the code duplication between trampoline() and
trampoline_is_enabled(): this is temporary to make it easier to make
changes for getting globbed systemwide probing working later on.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

dtprobed/dtprobed.c

@@ -1,6 +1,6 @@
 /*
  * Oracle Linux DTrace; DOF-consumption and USDT-probe-creation daemon.
- * Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2022, 2023, Oracle and/or its affiliates. All rights reserved.
  * Licensed under the Universal Permissive License v 1.0 as shown at
  * http://oss.oracle.com/licenses/upl.
  */
@@ -323,15 +323,13 @@ create_probe(pid_t pid, dof_parsed_t *provider, dof_parsed_t *probe,
 {
 	const char *mod, *fun, *prb;
 
-	if (tp->tracepoint.is_enabled)
-		return;				/* Not yet implemented.  */
-
 	mod = probe->probe.name;
 	fun = mod + strlen(mod) + 1;
 	prb = fun + strlen(fun) + 1;
 
 	free(uprobe_create_from_addr(pid, tp->tracepoint.addr,
-		provider->provider.name, mod, fun, prb));
+		tp->tracepoint.is_enabled, provider->provider.name,
+		mod, fun, prb));
 }
 
 /*

libcommon/uprobes.c

@@ -1,6 +1,6 @@
 /*
  * Oracle Linux DTrace.
- * Copyright (c) 2019, 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2019, 2023, Oracle and/or its affiliates. All rights reserved.
  * Licensed under the Universal Permissive License v 1.0 as shown at
  * http://oss.oracle.com/licenses/upl.
  */
@@ -209,13 +209,13 @@ uprobe_decode_name(const char *name)
 }
 
 char *
-uprobe_name(dev_t dev, ino_t ino, uint64_t addr, int isret)
+uprobe_name(dev_t dev, ino_t ino, uint64_t addr, int isret, int is_enabled)
 {
 	char	*name;
 
-	if (asprintf(&name, "dt_pid/%c_%llx_%llx_%lx", isret ? 'r' : 'p',
-		     (unsigned long long)dev, (unsigned long long)ino,
-		     (unsigned long)addr) < 0)
+	if (asprintf(&name, "dt_pid%s/%c_%llx_%llx_%lx", is_enabled?"_is_enabled":"",
+		     isret ? 'r' : 'p', (unsigned long long)dev,
+		     (unsigned long long)ino, (unsigned long)addr) < 0)
 		return NULL;
 
 	return name;
@@ -229,7 +229,7 @@ uprobe_name(dev_t dev, ino_t ino, uint64_t addr, int isret)
  */
 char *
 uprobe_create_named(dev_t dev, ino_t ino, uint64_t addr, const char *spec, int isret,
-		    const char *prv, const char *mod, const char *fun,
+		    int is_enabled, const char *prv, const char *mod, const char *fun,
 		    const char *prb)
 {
 	int	fd = -1;
@@ -261,7 +261,7 @@ uprobe_create_named(dev_t dev, ino_t ino, uint64_t addr, const char *spec, int i
 			return NULL;
 	}
 
-	name = uprobe_name(dev, ino, addr, isret);
+	name = uprobe_name(dev, ino, addr, isret, is_enabled);
 	if (!name)
 		goto out;
 
@@ -292,9 +292,10 @@ uprobe_create_named(dev_t dev, ino_t ino, uint64_t addr, const char *spec, int i
  * systemwide uprobe list.)
  */
 char *
-uprobe_create(dev_t dev, ino_t ino, uint64_t addr, const char *spec, int isret)
+uprobe_create(dev_t dev, ino_t ino, uint64_t addr, const char *spec, int isret,
+	int is_enabled)
 {
-	return uprobe_create_named(dev, ino, addr, spec, isret,
+	return uprobe_create_named(dev, ino, addr, spec, isret, is_enabled,
 				   NULL, NULL, NULL, NULL);
 }
 
@@ -304,7 +305,7 @@ uprobe_create(dev_t dev, ino_t ino, uint64_t addr, const char *spec, int isret)
  * are set, they are passed down as the name of the corresponding DTrace probe.
  */
 char *
-uprobe_create_from_addr(pid_t pid, uint64_t addr, const char *prv,
+uprobe_create_from_addr(pid_t pid, uint64_t addr, int is_enabled, const char *prv,
 			const char *mod, const char *fun, const char *prb)
 {
 	char *spec;
@@ -316,7 +317,7 @@ uprobe_create_from_addr(pid_t pid, uint64_t addr, const char *prv,
 		return NULL;
 
 	name = uprobe_create_named(mapp.pr_dev, mapp.pr_inum, addr, spec, 0,
-				   prv, mod, fun, prb);
+				   is_enabled, prv, mod, fun, prb);
 	free(spec);
 	return name;
 }
@@ -325,13 +326,13 @@ uprobe_create_from_addr(pid_t pid, uint64_t addr, const char *prv,
  * Destroy a uprobe for a given device, address, and spec.
  */
 int
-uprobe_delete(dev_t dev, ino_t ino, uint64_t addr, int isret)
+uprobe_delete(dev_t dev, ino_t ino, uint64_t addr, int isret, int is_enabled)
 {
 	int	fd = -1;
 	int	rc = -1;
 	char	*name;
 
-	name = uprobe_name(dev, ino, addr, isret);
+	name = uprobe_name(dev, ino, addr, isret, is_enabled);
 	if (!name)
 		goto out;
 

libcommon/uprobes.h

@@ -1,6 +1,6 @@
 /*
  * Oracle Linux DTrace; simple uprobe helper functions
- * Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2022, 2023, Oracle and/or its affiliates. All rights reserved.
  * Licensed under the Universal Permissive License v 1.0 as shown at
  * http://oss.oracle.com/licenses/upl.
  */
@@ -15,17 +15,19 @@
 
 extern char *uprobe_spec_by_addr(pid_t pid, ps_prochandle *P, uint64_t addr,
 				 prmap_t *mapp);
-extern char *uprobe_name(dev_t dev, ino_t ino, uint64_t addr, int isret);
+extern char *uprobe_name(dev_t dev, ino_t ino, uint64_t addr, int isret,
+			 int is_enabled);
 extern char *uprobe_create_named(dev_t dev, ino_t ino, uint64_t addr,
-				 const char *spec, int isret, const char *prv,
-				 const char *mod, const char *fun,
-				 const char *prb);
+				 const char *spec, int isret, int is_enabled,
+				 const char *prv, const char *mod,
+				 const char *fun, const char *prb);
 extern char *uprobe_create(dev_t dev, ino_t ino, uint64_t addr, const char *spec,
-			   int isret);
-extern char *uprobe_create_from_addr(pid_t pid, uint64_t addr, const char *prv,
-				     const char *mod, const char *fun,
-				     const char *prb);
-extern int uprobe_delete(dev_t dev, ino_t ino, uint64_t addr, int isret);
+			   int isret, int is_enabled);
+extern char *uprobe_create_from_addr(pid_t pid, uint64_t addr, int is_enabled,
+				     const char *prv, const char *mod,
+				     const char *fun, const char *prb);
+extern int uprobe_delete(dev_t dev, ino_t ino, uint64_t addr, int isret,
+			 int is_enabled);
 extern char *uprobe_encode_name(const char *);
 extern char *uprobe_decode_name(const char *);