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

[André Almeida]

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@xxxxxxxxxx>
---
RFC: Those symbols can't be found by the linker after patch 2/2, it fails with:

ld: arch/arm64/kernel/vdso.o: in function `vdso_futex_robust_unlock_update_ips':
arch/arm64/kernel/vdso.c:72:(.text+0x200): undefined reference to `__futex_list64_try_unlock_cs_success'
ld: arch/arm64/kernel/vdso.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `__futex_list64_try_unlock_cs_success' which may bind externally can not be used when making a shared object; recompile with -fPIC
arch/arm64/kernel/vdso.c:72:(.text+0x200): dangerous relocation: unsupported relocation
---
 arch/arm64/include/asm/vdso.h |  4 ++++
 arch/arm64/kernel/vdso.c      | 29 +++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/arch/arm64/include/asm/vdso.h b/arch/arm64/include/asm/vdso.h
index 232b46969088..182fde1df3dd 100644
--- a/arch/arm64/include/asm/vdso.h
+++ b/arch/arm64/include/asm/vdso.h
@@ -18,6 +18,10 @@
 
 extern char vdso_start[], vdso_end[];
 extern char vdso32_start[], vdso32_end[];
+extern char __futex_list64_try_unlock_cs_success[], __futex_list64_try_unlock_cs_end[];
+#ifdef CONFIG_COMPAT
+extern char __futex_list32_try_unlock_cs_success[], __futex_list32_try_unlock_cs_end[];
+#endif
 
 #endif /* !__ASSEMBLER__ */
 
diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index 592dd8668de4..42a82e73a774 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 +58,32 @@ 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;
+
+	/*
+	 * RFC: won't compile due to undefined reference to `__futex_list64_try_unlock_cs_...`
+
+	if (abi == VDSO_ABI_AA64) {
+		futex_set_vdso_cs_range(fd, 0, vdso, (uintptr_t) __futex_list64_try_unlock_cs_success,
+					(uintptr_t) __futex_list64_try_unlock_cs_end, false);
+	}
+
+#ifdef CONFIG_COMPAT
+	if (abi == VDSO_ABI_AA32) {
+		futex_set_vdso_cs_range(fd, 1, vdso, (uintptr_t) __futex_list32_try_unlock_cs_success,
+					(uintptr_t) __futex_list32_try_unlock_cs_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 +161,8 @@ 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:

-- 
2.53.0