linux-kernel - [tip: core/rseq] rseq: Separate the signal delivery path

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Message-ID: <176173341514.2601451.16659594890035832293.tip-bot2@tip-bot2>
Date: Wed, 29 Oct 2025 10:23:35 -0000
From: "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: core/rseq] rseq: Separate the signal delivery path

The following commit has been merged into the core/rseq branch of tip:

Commit-ID:     7315751a4e9e4c00278717c9f344a5929ca9898c
Gitweb:        https://git.kernel.org/tip/7315751a4e9e4c00278717c9f344a5929ca9898c
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Mon, 27 Oct 2025 09:45:10 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Wed, 29 Oct 2025 11:07:18 +01:00

rseq: Separate the signal delivery path

Completely separate the signal delivery path from the notify handler as
they have different semantics versus the event handling.

The signal delivery only needs to ensure that the interrupted user context
was not in a critical section or the section is aborted before it switches
to the signal frame context. The signal frame context does not have the
original instruction pointer anymore, so that can't be handled on exit to
user space.

No point in updating the CPU/CID ids as they might change again before the
task returns to user space for real.

The fast path optimization, which checks for the 'entry from user via
interrupt' condition is only available for architectures which use the
generic entry code.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Link: https://patch.msgid.link/20251027084307.455429038@linutronix.de
---
 include/linux/rseq.h | 21 ++++++++++++++++-----
 kernel/rseq.c        | 30 ++++++++++++++++++++++--------
 2 files changed, 38 insertions(+), 13 deletions(-)

diff --git a/include/linux/rseq.h b/include/linux/rseq.h
index a526074..06c21f6 100644
--- a/include/linux/rseq.h
+++ b/include/linux/rseq.h
@@ -7,22 +7,33 @@
 
 #include <uapi/linux/rseq.h>
 
-void __rseq_handle_notify_resume(struct ksignal *sig, struct pt_regs *regs);
+void __rseq_handle_notify_resume(struct pt_regs *regs);
 
 static inline void rseq_handle_notify_resume(struct pt_regs *regs)
 {
 	if (current->rseq.event.has_rseq)
-		__rseq_handle_notify_resume(NULL, regs);
+		__rseq_handle_notify_resume(regs);
 }
 
+void __rseq_signal_deliver(int sig, struct pt_regs *regs);
+
+/*
+ * Invoked from signal delivery to fixup based on the register context before
+ * switching to the signal delivery context.
+ */
 static inline void rseq_signal_deliver(struct ksignal *ksig, struct pt_regs *regs)
 {
-	if (current->rseq.event.has_rseq) {
-		current->rseq.event.sched_switch = true;
-		__rseq_handle_notify_resume(ksig, regs);
+	if (IS_ENABLED(CONFIG_GENERIC_IRQ_ENTRY)) {
+		/* '&' is intentional to spare one conditional branch */
+		if (current->rseq.event.has_rseq & current->rseq.event.user_irq)
+			__rseq_signal_deliver(ksig->sig, regs);
+	} else {
+		if (current->rseq.event.has_rseq)
+			__rseq_signal_deliver(ksig->sig, regs);
 	}
 }
 
+/* Raised from context switch and exevce to force evaluation on exit to user */
 static inline void rseq_sched_switch_event(struct task_struct *t)
 {
 	if (t->rseq.event.has_rseq) {
diff --git a/kernel/rseq.c b/kernel/rseq.c
index 32d4ab7..4d7ba22 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -250,13 +250,12 @@ efault:
  * respect to other threads scheduled on the same CPU, and with respect
  * to signal handlers.
  */
-void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
+void __rseq_handle_notify_resume(struct pt_regs *regs)
 {
 	struct task_struct *t = current;
 	struct rseq_ids ids;
 	u32 node_id;
 	bool event;
-	int sig;
 
 	/*
 	 * If invoked from hypervisors before entering the guest via
@@ -275,10 +274,7 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
 	if (unlikely(t->flags & PF_EXITING))
 		return;
 
-	if (ksig)
-		rseq_stat_inc(rseq_stats.signal);
-	else
-		rseq_stat_inc(rseq_stats.slowpath);
+	rseq_stat_inc(rseq_stats.slowpath);
 
 	/*
 	 * Read and clear the event pending bit first. If the task
@@ -317,8 +313,26 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
 	return;
 
 error:
-	sig = ksig ? ksig->sig : 0;
-	force_sigsegv(sig);
+	force_sig(SIGSEGV);
+}
+
+void __rseq_signal_deliver(int sig, struct pt_regs *regs)
+{
+	rseq_stat_inc(rseq_stats.signal);
+	/*
+	 * Don't update IDs, they are handled on exit to user if
+	 * necessary. The important thing is to abort a critical section of
+	 * the interrupted context as after this point the instruction
+	 * pointer in @regs points to the signal handler.
+	 */
+	if (unlikely(!rseq_handle_cs(current, regs))) {
+		/*
+		 * Clear the errors just in case this might survive
+		 * magically, but leave the rest intact.
+		 */
+		current->rseq.event.error = 0;
+		force_sigsegv(sig);
+	}
 }
 
 /*