[PATCH RFC v2 1/2] arm64: vdso: Prepare for robust futex unlock support

André Almeida
2026-04-24T18:56:26Z

There will be a VDSO function to unlock non-contended robust futexes in
user space. The unlock sequence is racy vs. clearing the list_pending_op
pointer in the task's robust list head. To plug this race the kernel needs
to know the critical section window so it can clear the pointer when the
task is interrupted within that race window. The window is determined by
labels in the inline assembly.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
---
Changes from v1:
- Fixed linker not finding VDSO symbols
---
arch/arm64/kernel/vdso.c | 30 ++++++++++++++++++++++++++++++
arch/arm64/kernel/vdso/vdso.lds.S | 7 +++++++
2 files changed, 37 insertions(+)

diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index 592dd8668de4..f9c520a1c942 100644
--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c
@@ -11,6 +11,7 @@
#include <linux/clocksource.h>
#include <linux/elf.h>
#include <linux/err.h>
+#include <linux/futex.h>
#include <linux/errno.h>
#include <linux/gfp.h>
#include <linux/kernel.h>
@@ -57,6 +58,33 @@ static struct vdso_abi_info vdso_info[] __ro_after_init = {
#endif /* CONFIG_COMPAT_VDSO */
};

+#ifdef CONFIG_FUTEX_ROBUST_UNLOCK
+static void vdso_futex_robust_unlock_update_ips(enum vdso_abi abi, struct mm_struct *mm)
+{
+ unsigned long vdso = (unsigned long) mm->context.vdso;
+ struct futex_mm_data *fd = &mm->futex;
+ uintptr_t success, end;
+
+ if (abi == VDSO_ABI_AA64) {
+ success = (uintptr_t) VDSO_SYMBOL(vdso, futex_list64_try_unlock_cs_success);
+ end = (uintptr_t) VDSO_SYMBOL(vdso, futex_list64_try_unlock_cs_end);
+
+ futex_set_vdso_cs_range(fd, 0, vdso, success, end, false);
+ }
+
+#ifdef CONFIG_COMPAT_VDSO
+ if (abi == VDSO_ABI_AA32) {
+ success = (uintptr_t) VDSO_SYMBOL(vdso, futex_list32_try_unlock_cs_success);
+ end = (uintptr_t) VDSO_SYMBOL(vdso, futex_list32_try_unlock_cs_end);
+
+ futex_set_vdso_cs_range(fd, 1, vdso, success, end, true);
+ }
+#endif
+}
+#else
+static inline void vdso_futex_robust_unlock_update_ips(enum vdso_abi abi, struct mm_struct *mm) { }
+#endif /* CONFIG_FUTEX_ROBUST_UNLOCK */
+
static int vdso_mremap(const struct vm_special_mapping *sm,
struct vm_area_struct *new_vma)
{
@@ -134,6 +162,8 @@ static int __setup_additional_pages(enum vdso_abi abi,
if (IS_ERR(ret))
goto up_fail;

+ vdso_futex_robust_unlock_update_ips(abi, mm);
+
return 0;

up_fail:
diff --git a/arch/arm64/kernel/vdso/vdso.lds.S b/arch/arm64/kernel/vdso/vdso.lds.S
index 52314be29191..c532dae6ac8e 100644
--- a/arch/arm64/kernel/vdso/vdso.lds.S
+++ b/arch/arm64/kernel/vdso/vdso.lds.S
@@ -112,3 +112,10 @@ VERSION
* Make the sigreturn code visible to the kernel.
*/
VDSO_sigtramp = __kernel_rt_sigreturn;
+
+VDSO_futex_list64_try_unlock_cs_success = __futex_list64_try_unlock_cs_success;
+VDSO_futex_list64_try_unlock_cs_end = __futex_list64_try_unlock_cs_end;
+#ifdef CONFIG_COMPAT_VDSO
+VDSO_futex_list32_try_unlock_cs_success = __futex_list32_try_unlock_cs_success;
+VDSO_futex_list32_try_unlock_cs_end = __futex_list32_try_unlock_cs_end;
+#endif

--
2.53.0