linux-kernel - [RFC PATCH 55/56] x86/debug: Show return thunk in debugfs

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Message-ID: <20251013143444.3999-56-david.kaplan@amd.com>
Date: Mon, 13 Oct 2025 09:34:43 -0500
From: David Kaplan <david.kaplan@....com>
To: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Ingo Molnar
	<mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
	<x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>
CC: Alexander Graf <graf@...zon.com>, Boris Ostrovsky
	<boris.ostrovsky@...cle.com>, <linux-kernel@...r.kernel.org>
Subject: [RFC PATCH 55/56] x86/debug: Show return thunk in debugfs

Make the value of x86_return_thunk visible in debugfs to support user-space
testing.

Signed-off-by: David Kaplan <david.kaplan@....com>
---
 arch/x86/kernel/cpu/bugs.c | 44 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 26ceb42e0cfb..8365448b3aef 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -16,6 +16,7 @@
 #include <linux/sched/smt.h>
 #include <linux/pgtable.h>
 #include <linux/bpf.h>
+#include <linux/debugfs.h>
 
 #include <asm/spec-ctrl.h>
 #include <asm/cmdline.h>
@@ -4065,6 +4066,49 @@ void arch_cpu_reset_mitigations(void)
 	tsa_reset_mitigation();
 	vmscape_reset_mitigation();
 }
+
+static int rethunk_debug_show(struct seq_file *m, void *p)
+{
+	if (x86_return_thunk == __x86_return_thunk)
+		seq_puts(m, "__x86_return_thunk\n");
+	else if (x86_return_thunk == retbleed_return_thunk)
+		seq_puts(m, "retbleed_return_thunk\n");
+	else if (x86_return_thunk == call_depth_return_thunk)
+		seq_puts(m, "call_depth_return_thunk\n");
+	else if (x86_return_thunk == its_return_thunk)
+		seq_puts(m, "its_return_thunk\n");
+	else if (x86_return_thunk == srso_alias_return_thunk)
+		seq_puts(m, "srso_alias_return_thunk\n");
+	else if (x86_return_thunk == srso_return_thunk)
+		seq_puts(m, "srso_return_thunk\n");
+	else
+		seq_puts(m, "unknown\n");
+
+	return 0;
+}
+
+static int rethunk_debug_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, rethunk_debug_show, inode->i_private);
+}
+
+static const struct file_operations dfs_thunk_ops = {
+	.open		= rethunk_debug_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
+static int __init mitigations_debugfs_init(void)
+{
+	struct dentry *dir;
+
+	dir = debugfs_create_dir("mitigations", arch_debugfs_dir);
+	debugfs_create_file("x86_return_thunk", 0400, dir, NULL, &dfs_thunk_ops);
+
+	return 0;
+}
+late_initcall(mitigations_debugfs_init);
 #endif
 
 void cpu_bugs_update_speculation_msrs(void)
-- 
2.34.1