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 <mathieu.desnoyers@efficios.com>
CC: Shuah Khan <shuah@kernel.org>
CC: Carlos O'Donell <carlos@redhat.com>
CC: Florian Weimer <fweimer@redhat.com>
CC: Joseph Myers <joseph@codesourcery.com>
CC: Szabolcs Nagy <szabolcs.nagy@arm.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Ben Maurer <bmaurer@fb.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
CC: Boqun Feng <boqun.feng@gmail.com>
CC: Will Deacon <will.deacon@arm.com>
CC: Dave Watson <davejwatson@fb.com>
CC: Paul Turner <pjt@google.com>
CC: linux-api@vger.kernel.org
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/rseq/rseq.c

@@ -25,18 +25,27 @@
 #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)
 {
@@ -69,8 +78,14 @@ int rseq_register_current_thread(void)
 	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) {
@@ -78,9 +93,9 @@ int rseq_register_current_thread(void)
 		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;
@@ -91,13 +106,20 @@ int rseq_unregister_current_thread(void)
 	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);
@@ -115,3 +137,20 @@ int32_t rseq_fallback_current_cpu(void)
 	}
 	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

@@ -44,6 +44,7 @@
 #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)