[RFC PATCH for 5.8 3/4] rseq: Introduce RSEQ_FLAG_RELIABLE_CPU_ID

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

commit 93b585c08d16 ("Fix: sched: unreliable rseq cpu_id for new tasks")
addresses an issue with cpu_id field of newly created processes. Expose
a flag which can be used by user-space to query whether the kernel
implements this fix.

Considering that this issue can cause corruption of user-space per-cpu
data updated with rseq, it is recommended that user-space detects
availability of this fix by using the RSEQ_FLAG_RELIABLE_CPU_ID flag
either combined with registration or on its own before using rseq.

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 | 5 +++++
 kernel/rseq.c             | 4 ++++
 2 files changed, 9 insertions(+)

diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index 3b5fba25461a..a548b77c9520 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -21,13 +21,18 @@ enum rseq_cpu_id_state {
 /*
  * RSEQ_FLAG_UNREGISTER:       Unregister rseq ABI for caller thread.
  * RSEQ_FLAG_REGISTER:         Register rseq ABI for caller thread.
+ * RSEQ_FLAG_RELIABLE_CPU_ID:  rseq provides a reliable cpu_id field.
  *
  * 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.
+ *
+ * The flag RSEQ_FLAG_REGISTER can be combined with the RSEQ_FLAG_RELIABLE_CPU_ID
+ * flag.
  */
 enum rseq_flags {
 	RSEQ_FLAG_UNREGISTER			= (1 << 0),
 	RSEQ_FLAG_REGISTER			= (1 << 1),
+	RSEQ_FLAG_RELIABLE_CPU_ID		= (1 << 2),
 };
 
 enum rseq_cs_flags_bit {
diff --git a/kernel/rseq.c b/kernel/rseq.c
index 47ce221cd6f9..32cc2e0d961f 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -333,6 +333,8 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
 		current->rseq_sig = 0;
 		break;
 	case RSEQ_FLAG_REGISTER:
+		fallthrough;
+	case RSEQ_FLAG_REGISTER | RSEQ_FLAG_RELIABLE_CPU_ID:
 		if (current->rseq) {
 			/*
 			 * If rseq is already registered, check whether
@@ -365,6 +367,8 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
 		 */
 		rseq_set_notify_resume(current);
 		break;
+	case RSEQ_FLAG_RELIABLE_CPU_ID:
+		break;
 	default:
 		return -EINVAL;
 	}
-- 
2.17.1