[RFC PATCH for 5.8 2/4] rseq: Introduce RSEQ_FLAG_REGISTER

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

Introduce RSEQ_FLAG_REGISTER with the same behavior as the flag value
"0". The main advantage of introducing this flag as a non-zero (1 << 1)
value is that it can be combined with other flags to register and check
for features with a single system call. Considering that this system
call needs to be performed for each new thread in glibc, minimize the
amount of overhead required.

This is needed for introducing a new RSEQ_FLAG_RELIABLE_CPU_ID flag in a
later change.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Florian Weimer <fw@deneb.enyo.de>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Paul Turner <pjt@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Neel Natu <neelnatu@google.com>
Cc: linux-api@vger.kernel.org
---
 include/uapi/linux/rseq.h | 10 ++++-
 kernel/rseq.c             | 77 +++++++++++++++++++++------------------
 2 files changed, 51 insertions(+), 36 deletions(-)

diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index 9a402fdb60e9..3b5fba25461a 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -18,8 +18,16 @@ enum rseq_cpu_id_state {
 	RSEQ_CPU_ID_REGISTRATION_FAILED		= -2,
 };
 
+/*
+ * RSEQ_FLAG_UNREGISTER:       Unregister rseq ABI for caller thread.
+ * RSEQ_FLAG_REGISTER:         Register rseq ABI for caller thread.
+ *
+ * Flag value 0 has the same behavior as RSEQ_FLAG_REGISTER, but cannot be
+ * combined with other flags. This behavior is kept for backward compatibility.
+ */
 enum rseq_flags {
-	RSEQ_FLAG_UNREGISTER = (1 << 0),
+	RSEQ_FLAG_UNREGISTER			= (1 << 0),
+	RSEQ_FLAG_REGISTER			= (1 << 1),
 };
 
 enum rseq_cs_flags_bit {
diff --git a/kernel/rseq.c b/kernel/rseq.c
index a4f86a9d6937..47ce221cd6f9 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -309,9 +309,16 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
 {
 	int ret;
 
-	if (flags & RSEQ_FLAG_UNREGISTER) {
-		if (flags & ~RSEQ_FLAG_UNREGISTER)
-			return -EINVAL;
+	/*
+	 * Flag value 0 has the same behavior as RSEQ_FLAG_REGISTER, but cannot
+	 * be combined with other flags. This behavior is kept for backward
+	 * compatibility.
+	 */
+	if (!flags)
+		flags = RSEQ_FLAG_REGISTER;
+
+	switch (flags) {
+	case RSEQ_FLAG_UNREGISTER:
 		/* Unregister rseq for current thread. */
 		if (current->rseq != rseq || !current->rseq)
 			return -EINVAL;
@@ -324,43 +331,43 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
 			return ret;
 		current->rseq = NULL;
 		current->rseq_sig = 0;
-		return 0;
-	}
-
-	if (unlikely(flags))
-		return -EINVAL;
+		break;
+	case RSEQ_FLAG_REGISTER:
+		if (current->rseq) {
+			/*
+			 * If rseq is already registered, check whether
+			 * the provided address differs from the prior
+			 * one.
+			 */
+			if (current->rseq != rseq || rseq_len != sizeof(*rseq))
+				return -EINVAL;
+			if (current->rseq_sig != sig)
+				return -EPERM;
+			/* Already registered. */
+			return -EBUSY;
+		}
 
-	if (current->rseq) {
 		/*
-		 * If rseq is already registered, check whether
-		 * the provided address differs from the prior
-		 * one.
+		 * If there was no rseq previously registered,
+		 * ensure the provided rseq is properly aligned and valid.
 		 */
-		if (current->rseq != rseq || rseq_len != sizeof(*rseq))
+		if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
+		    rseq_len != sizeof(*rseq))
 			return -EINVAL;
-		if (current->rseq_sig != sig)
-			return -EPERM;
-		/* Already registered. */
-		return -EBUSY;
-	}
-
-	/*
-	 * If there was no rseq previously registered,
-	 * ensure the provided rseq is properly aligned and valid.
-	 */
-	if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
-	    rseq_len != sizeof(*rseq))
+		if (!access_ok(rseq, rseq_len))
+			return -EFAULT;
+		current->rseq = rseq;
+		current->rseq_sig = sig;
+		/*
+		 * If rseq was previously inactive, and has just been
+		 * registered, ensure the cpu_id_start and cpu_id fields
+		 * are updated before returning to user-space.
+		 */
+		rseq_set_notify_resume(current);
+		break;
+	default:
 		return -EINVAL;
-	if (!access_ok(rseq, rseq_len))
-		return -EFAULT;
-	current->rseq = rseq;
-	current->rseq_sig = sig;
-	/*
-	 * If rseq was previously inactive, and has just been
-	 * registered, ensure the cpu_id_start and cpu_id fields
-	 * are updated before returning to user-space.
-	 */
-	rseq_set_notify_resume(current);
+	}
 
 	return 0;
 }
-- 
2.17.1