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

David Kaplan
2025-10-13T14:36:28Z

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

Signed-off-by: David Kaplan <david.kaplan@amd.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