[PATCH for 5.2 04/12] rseq/selftests: Use __rseq_handled symbol to coexist with glibc

Mathieu Desnoyers
2019-04-29T15:29:34Z

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
---
tools/testing/selftests/rseq/rseq.c | 55 +++++++++++++++++++++++++++++++------
tools/testing/selftests/rseq/rseq.h | 1 +
2 files changed, 48 insertions(+), 8 deletions(-)

diff --git a/tools/testing/selftests/rseq/rseq.c b/tools/testing/selftests/rseq/rseq.c
index 4847e97ed049..7159eb777fd3 100644
--- a/tools/testing/selftests/rseq/rseq.c
+++ b/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;
+}
diff --git a/tools/testing/selftests/rseq/rseq.h b/tools/testing/selftests/rseq/rseq.h
index c72eb70f9b52..26348e2c44f3 100644
--- a/tools/testing/selftests/rseq/rseq.h
+++ b/tools/testing/selftests/rseq/rseq.h
@@ -45,6 +45,7 @@
#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)
--
2.11.0