Hi Mathieu,

some small remarks around the vDSO code.

On Fri, Mar 13, 2026 at 09:39:03AM -0400, Mathieu Desnoyers wrote:

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The approach taken to fix this is to introduce two vDSO and extend the
x86 vDSO exception table to track the relevant ip ranges: one for non-PI
robust futexes, and one for PI robust futexes.

One of the central points behind the vDSO so far was that it is only a
performance optimization. It is never required for correctness.
What are applications supposed to do when the vDSO is disabled?

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This should be uapi/linux/futex.h. Headers from the linux/ namespace should
not be used in vDSO code. The definitions from them may end up being wrong
in the compat vDSO. Either use uapi/ or vdso/ headers. (linux/types.h is a bit
of an exception for historic reasons, it could be replaced by uapi/linux/types.h)

+#include <vdso/futex.h>
+#include "extable.h"

+
+#ifdef CONFIG_X86_64

This only works because of the ugly hacks in fake_32bit_build.h.
Testing for '#ifdef __x86_64__' is simpler and nicer to read.

+# define ASM_PTR_BIT_SET	"btsq "
+# define ASM_PTR_SET		"movq "
+#else
+# define ASM_PTR_BIT_SET	"btsl "
+# define ASM_PTR_SET		"movl "
+#endif
+
+u32 __vdso_robust_futex_unlock(u32 *uaddr, struct robust_list_head *robust_list_head)
+{
+	u32 val = 0;
+
+	/*
+	 * Within the ip range identified by the futex exception table,
+	 * the register "eax" contains the value loaded by xchg. This is
+	 * expected by futex_vdso_exception() to check whether waiters
+	 * need to be woken up. This register state is transferred to
+	 * bit 1 (NEED_ACTION) of *op_pending_addr before the ip range
+	 * ends.
+	 */
+	asm volatile (
+		_ASM_VDSO_EXTABLE_FUTEX_HANDLE(1f, 3f)
+		/* Exchange uaddr (store-release). */
+		"xchg %[uaddr], %[val]\n\t"
+		"1:\n\t"
+		/* Test if FUTEX_WAITERS (0x80000000) is set. */
+		"test %[val], %[val]\n\t"
+		"js 2f\n\t"
+		/* Clear *op_pending_addr if there are no waiters. */
+		ASM_PTR_SET "$0, %[op_pending_addr]\n\t"
+		"jmp 3f\n\t"
+		"2:\n\t"
+		/* Set bit 1 (NEED_ACTION) in *op_pending_addr. */
+		ASM_PTR_BIT_SET "$1, %[op_pending_addr]\n\t"
+		"3:\n\t"
+		: [val] "+a" (val),
+		  [uaddr] "+m" (*uaddr)
+		: [op_pending_addr] "m" (robust_list_head->list_op_pending)
+		: "memory"
+	);
+	return val;
+}
+
+u32 robust_futex_unlock(u32 *, struct robust_list_head *)
+	__attribute__((weak, alias("__vdso_robust_futex_unlock")));

__weak and __alias() as per checkpatch.pl.

The entries in the linkerscripts are missing.

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Same remarks about the linux/ namespace as before.

+/**
+ * __vdso_robust_futex_unlock - Architecture-specific vDSO implementation of robust futex unlock.
+ * @uaddr:		Lock address (points to a 32-bit unsigned integer type).
+ * @robust_list_head:	The thread-specific robust list that has been registered with set_robust_list.
+ *
+ * This vDSO unlocks the robust futex by exchanging the content of
+ * *@uaddr with 0 with a store-release semantic. If the futex has
+ * waiters, it sets bit 1 of *@robust_list_head->list_op_pending, else
+ * it clears *@robust_list_head->list_op_pending. Those operations are
+ * within a code region known by the kernel, making them safe with
+ * respect to asynchronous program termination either from thread
+ * context or from a nested signal handler.
+ *
+ * Returns:	The old value present at *@uaddr.
+ *
+ * Expected use of this vDSO:
+ *
+ * robust_list_head is the thread-specific robust list that has been
+ * registered with set_robust_list.
+ *
+ * if ((__vdso_robust_futex_unlock((u32 *) &mutex->__data.__lock, robust_list_head)
+ *     & FUTEX_WAITERS) != 0)
+ *         futex_wake((u32 *) &mutex->__data.__lock, 1, private);
+ * WRITE_ONCE(robust_list_head->list_op_pending, 0);
+ */
+extern u32 __vdso_robust_futex_unlock(u32 *uaddr, struct robust_list_head *robust_list_head);

Drop the extern.

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+#endif /* _VDSO_FUTEX_H */

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