futex: Add entry point for FUTEX_WAIT_MULTIPLE (opcode 31)

Add an option to wait on multiple futexes using the old interface, that
uses opcode 31 through futex() syscall. Do that by just translation the
old interface to use the new code. This allows old and stable versions
of Proton to still use fsync in new kernel releases.

Signed-off-by: André Almeida <andrealmeid@collabora.com>

Author: andrealmeid

include/uapi/linux/futex.h

@@ -22,6 +22,7 @@
 #define FUTEX_WAIT_REQUEUE_PI	11
 #define FUTEX_CMP_REQUEUE_PI	12
 #define FUTEX_LOCK_PI2		13
+#define FUTEX_WAIT_MULTIPLE	31
 
 #define FUTEX_PRIVATE_FLAG	128
 #define FUTEX_CLOCK_REALTIME	256
@@ -68,6 +69,18 @@ struct futex_waitv {
 	__u32 __reserved;
 };
 
+/**
+ * struct futex_wait_block - Block of futexes to be waited for
+ * @uaddr:	User address of the futex
+ * @val:	Futex value expected by userspace
+ * @bitset:	Bitset for the optional bitmasked wakeup
+ */
+struct futex_wait_block {
+	__u32 __user *uaddr;
+	__u32 val;
+	__u32 bitset;
+};
+
 /*
  * Support for robust futexes: the kernel cleans up held futexes at
  * thread exit time.

kernel/futex/syscalls.c

@@ -142,6 +142,7 @@ static __always_inline bool futex_cmd_has_timeout(u32 cmd)
 	case FUTEX_LOCK_PI2:
 	case FUTEX_WAIT_BITSET:
 	case FUTEX_WAIT_REQUEUE_PI:
+	case FUTEX_WAIT_MULTIPLE:
 		return true;
 	}
 	return false;
@@ -154,13 +155,79 @@ futex_init_timeout(u32 cmd, u32 op, struct timespec64 *ts, ktime_t *t)
 		return -EINVAL;
 
 	*t = timespec64_to_ktime(*ts);
-	if (cmd == FUTEX_WAIT)
+	if (cmd == FUTEX_WAIT || cmd == FUTEX_WAIT_MULTIPLE)
 		*t = ktime_add_safe(ktime_get(), *t);
 	else if (cmd != FUTEX_LOCK_PI && !(op & FUTEX_CLOCK_REALTIME))
 		*t = timens_ktime_to_host(CLOCK_MONOTONIC, *t);
 	return 0;
 }
 
+/**
+ * futex_read_wait_block - Read an array of futex_wait_block from userspace
+ * @uaddr:	Userspace address of the block
+ * @count:	Number of blocks to be read
+ *
+ * This function creates and allocate an array of futex_q (we zero it to
+ * initialize the fields) and then, for each futex_wait_block element from
+ * userspace, fill a futex_q element with proper values.
+ */
+inline struct futex_vector *futex_read_wait_block(u32 __user *uaddr, u32 count)
+{
+	unsigned int i;
+	struct futex_vector *futexv;
+	struct futex_wait_block fwb;
+	struct futex_wait_block __user *entry =
+		(struct futex_wait_block __user *)uaddr;
+
+	if (!count || count > FUTEX_WAITV_MAX)
+		return ERR_PTR(-EINVAL);
+
+	futexv = kcalloc(count, sizeof(*futexv), GFP_KERNEL);
+	if (!futexv)
+		return ERR_PTR(-ENOMEM);
+
+	for (i = 0; i < count; i++) {
+		if (copy_from_user(&fwb, &entry[i], sizeof(fwb))) {
+			kfree(futexv);
+			return ERR_PTR(-EFAULT);
+		}
+
+		futexv[i].w.flags = FUTEX_32;
+		futexv[i].w.val = fwb.val;
+		futexv[i].w.uaddr = (uintptr_t) (fwb.uaddr);
+		futexv[i].q = futex_q_init;
+	}
+
+	return futexv;
+}
+
+int futex_wait_multiple(struct futex_vector *vs, unsigned int count,
+			struct hrtimer_sleeper *to);
+
+int futex_opcode_31(ktime_t *abs_time, u32 __user *uaddr, int count)
+{
+	int ret;
+	struct futex_vector *vs;
+	struct hrtimer_sleeper *to = NULL, timeout;
+
+	to = futex_setup_timer(abs_time, &timeout, 0, 0);
+
+	vs = futex_read_wait_block(uaddr, count);
+
+	if (IS_ERR(vs))
+		return PTR_ERR(vs);
+
+	ret = futex_wait_multiple(vs, count, abs_time ? to : NULL);
+	kfree(vs);
+
+	if (to) {
+		hrtimer_cancel(&to->timer);
+		destroy_hrtimer_on_stack(&to->timer);
+	}
+
+	return ret;
+}
+
 SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
 		const struct __kernel_timespec __user *, utime,
 		u32 __user *, uaddr2, u32, val3)
@@ -180,6 +247,9 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
 		tp = &t;
 	}
 
+	if (cmd == FUTEX_WAIT_MULTIPLE)
+		return futex_opcode_31(tp, uaddr, val);
+
 	return do_futex(uaddr, op, val, tp, uaddr2, (unsigned long)utime, val3);
 }
 
@@ -373,6 +443,9 @@ SYSCALL_DEFINE6(futex_time32, u32 __user *, uaddr, int, op, u32, val,
 		tp = &t;
 	}
 
+	if (cmd == FUTEX_WAIT_MULTIPLE)
+		return futex_opcode_31(tp, uaddr, val);
+
 	return do_futex(uaddr, op, val, tp, uaddr2, (unsigned long)utime, val3);
 }
 #endif /* CONFIG_COMPAT_32BIT_TIME */