rseq/selftests: Use __rseq_handled symbol to coexist with glibc

In order to integrate rseq into user-space applications, expose a
__rseq_handled symbol so many rseq users can be linked into the same
application (e.g. librseq and glibc).

The __rseq_refcount TLS variable is static to the librseq library. It
ensures that rseq syscall registration/unregistration happens only for
the most early/late caller to rseq_{,un}register_current_thread for each
thread, thus ensuring that rseq is registered across the lifetime of all
rseq users for a given thread.

Signed-off-by: Mathieu Desnoyers <[e-mail]>
CC: Shuah Khan <[e-mail]>
CC: Carlos O'Donell <[e-mail]>
CC: Florian Weimer <[e-mail]>
CC: Joseph Myers <[e-mail]>
CC: Szabolcs Nagy <[e-mail]>
CC: Thomas Gleixner <[e-mail]>
CC: Ben Maurer <[e-mail]>
CC: Peter Zijlstra <[e-mail]>
CC: "Paul E. McKenney" <[e-mail]>
CC: Boqun Feng <[e-mail]>
CC: Will Deacon <[e-mail]>
CC: Dave Watson <[e-mail]>
CC: Paul Turner <[e-mail]>
CC: [e-mail]
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/rseq/rseq.c

···
 #include <syscall.h>
 #include <assert.h>
 #include <signal.h>
+#include <limits.h>
 
 #include "rseq.h"
 
 #define ARRAY_SIZE(arr)	(sizeof(arr) / sizeof((arr)[0]))
 
-__attribute__((tls_model("initial-exec"))) __thread
-volatile struct rseq __rseq_abi = {
+__thread volatile struct rseq __rseq_abi = {
 	.cpu_id = RSEQ_CPU_ID_UNINITIALIZED,
 };
 
-static __attribute__((tls_model("initial-exec"))) __thread
-volatile int refcount;
+/*
+ * Shared with other libraries. This library may take rseq ownership if it is
+ * still 0 when executing the library constructor. Set to 1 by library
+ * constructor when handling rseq. Set to 0 in destructor if handling rseq.
+ */
+int __rseq_handled;
+
+/* Whether this library have ownership of rseq registration. */
+static int rseq_ownership;
+
+static __thread volatile uint32_t __rseq_refcount;
 
 static void signal_off_save(sigset_t *oldset)
 {
···
 	int rc, ret = 0;
 	sigset_t oldset;
 
+	if (!rseq_ownership)
+		return 0;
 	signal_off_save(&oldset);
-	if (refcount++)
+	if (__rseq_refcount == UINT_MAX) {
+		ret = -1;
+		goto end;
+	}
+	if (__rseq_refcount++)
 		goto end;
 	rc = sys_rseq(&__rseq_abi, sizeof(struct rseq), 0, RSEQ_SIG);
 	if (!rc) {
···
 		goto end;
 	}
 	if (errno != EBUSY)
-		__rseq_abi.cpu_id = -2;
+		__rseq_abi.cpu_id = RSEQ_CPU_ID_REGISTRATION_FAILED;
 	ret = -1;
-	refcount--;
+	__rseq_refcount--;
 end:
 	signal_restore(oldset);
 	return ret;
···
 	int rc, ret = 0;
 	sigset_t oldset;
 
+	if (!rseq_ownership)
+		return 0;
 	signal_off_save(&oldset);
-	if (--refcount)
+	if (!__rseq_refcount) {
+		ret = -1;
+		goto end;
+	}
+	if (--__rseq_refcount)
 		goto end;
 	rc = sys_rseq(&__rseq_abi, sizeof(struct rseq),
 		      RSEQ_FLAG_UNREGISTER, RSEQ_SIG);
 	if (!rc)
 		goto end;
+	__rseq_refcount = 1;
 	ret = -1;
 end:
 	signal_restore(oldset);
···
 		abort();
 	}
 	return cpu;
+}
+
+void __attribute__((constructor)) rseq_init(void)
+{
+	/* Check whether rseq is handled by another library. */
+	if (__rseq_handled)
+		return;
+	__rseq_handled = 1;
+	rseq_ownership = 1;
+}
+
+void __attribute__((destructor)) rseq_fini(void)
+{
+	if (!rseq_ownership)
+		return;
+	__rseq_handled = 0;
+	rseq_ownership = 0;
 }

tools/testing/selftests/rseq/rseq.h

···
 #endif
 
 extern __thread volatile struct rseq __rseq_abi;
+extern int __rseq_handled;
 
 #define rseq_likely(x)		__builtin_expect(!!(x), 1)
 #define rseq_unlikely(x)	__builtin_expect(!!(x), 0)