linux-kernel - [PATCH V4 6/6] Add API to query supported rseq cs flags

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Message-ID: <20250513214554.4160454-7-prakash.sangappa@oracle.com>
Date: Tue, 13 May 2025 21:45:54 +0000
From: Prakash Sangappa <prakash.sangappa@...cle.com>
To: linux-kernel@...r.kernel.org
Cc: peterz@...radead.org, rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        tglx@...utronix.de, bigeasy@...utronix.de, kprateek.nayak@....com
Subject: [PATCH V4 6/6] Add API to query supported rseq cs flags

For the API, add a new flag to sys_rseq 'flags' argument called
RSEQ_FLAG_QUERY_CS_FLAGS.

When this flag is passed it returns a bit mask of all the supported
rseq cs flags in the user provided rseq struct's 'flags' member.

Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Signed-off-by: Prakash Sangappa <prakash.sangappa@...cle.com>
---
 include/uapi/linux/rseq.h |  1 +
 kernel/rseq.c             | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index 015534f064af..44baea9dd10a 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -20,6 +20,7 @@ enum rseq_cpu_id_state {
 
 enum rseq_flags {
 	RSEQ_FLAG_UNREGISTER = (1 << 0),
+	RSEQ_FLAG_QUERY_CS_FLAGS = (1 << 1),
 };
 
 enum rseq_cs_flags_bit {
diff --git a/kernel/rseq.c b/kernel/rseq.c
index c4bc52f8ba9c..997f7ca722ca 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -576,6 +576,23 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
 		return 0;
 	}
 
+	/*
+	 * return supported rseq_cs flags
+	 * It is an or of all the rseq_cs_flags;
+	 */
+	if (flags & RSEQ_FLAG_QUERY_CS_FLAGS) {
+		u32 rseq_csflags = RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT |
+				   RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL  |
+				   RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE |
+				   RSEQ_CS_FLAG_DELAY_RESCHED |
+				   RSEQ_CS_FLAG_RESCHEDULED;
+		if (!rseq)
+			return -EINVAL;
+		if (copy_to_user(&rseq->flags, &rseq_csflags, sizeof(u32)))
+			return -EFAULT;
+		return 0;
+	}
+
 	if (unlikely(flags))
 		return -EINVAL;
 
-- 
2.43.5