linux-kernel - [PATCH 1/5] rseq: Allow registering RSEQ with slice extension

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Message-ID: <20260121143207.814193010@infradead.org>
Date: Wed, 21 Jan 2026 15:24:58 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: tglx@...utronix.de,
 mathieu.desnoyers@...icios.com
Cc: linux-kernel@...r.kernel.org,
 peterz@...radead.org,
 paulmck@...nel.org,
 boqun.feng@...il.com,
 corbet@....net,
 prakash.sangappa@...cle.com,
 vineethr@...ux.ibm.com,
 kprateek.nayak@....com,
 rostedt@...dmis.org,
 bigeasy@...utronix.de,
 arnd@...db.de,
 rdunlap@...radead.org,
 rongevarg@...il.com,
 longman@...hat.com
Subject: [PATCH 1/5] rseq: Allow registering RSEQ with slice extension

Since glibc cares about the number of syscalls required to initialize a new
thread, allow initializing rseq with slice extension on. This avoids having to
do another prctl().

Requested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 include/uapi/linux/rseq.h |    3 ++-
 kernel/rseq.c             |   12 ++++++++++--
 2 files changed, 12 insertions(+), 3 deletions(-)

--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -19,7 +19,8 @@ enum rseq_cpu_id_state {
 };
 
 enum rseq_flags {
-	RSEQ_FLAG_UNREGISTER = (1 << 0),
+	RSEQ_FLAG_UNREGISTER			= (1 << 0),
+	RSEQ_FLAG_SLICE_EXT_DEFAULT_ON		= (1 << 1),
 };
 
 enum rseq_cs_flags_bit {
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -424,7 +424,7 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
 		return 0;
 	}
 
-	if (unlikely(flags))
+	if (unlikely(flags & ~(RSEQ_FLAG_SLICE_EXT_DEFAULT_ON)))
 		return -EINVAL;
 
 	if (current->rseq.usrptr) {
@@ -459,8 +459,12 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
 	if (!access_ok(rseq, rseq_len))
 		return -EFAULT;
 
-	if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION))
+	if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) {
 		rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
+		if (rseq_slice_extension_enabled() &&
+		    (flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON))
+			rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
+	}
 
 	scoped_user_write_access(rseq, efault) {
 		/*
@@ -488,6 +492,10 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
 	current->rseq.len = rseq_len;
 	current->rseq.sig = sig;
 
+#ifdef CONFIG_RSEQ_SLICE_EXTENSION
+	current->rseq.slice.state.enabled = !!(rseqfl & RSEQ_CS_FLAG_SLICE_EXT_ENABLED);
+#endif
+
 	/*
 	 * If rseq was previously inactive, and has just been
 	 * registered, ensure the cpu_id_start and cpu_id fields