As with openat2(2), this allows userspace to easily figure out what
flags and fields are supported by mount_setattr(2). As with clone3(2),
for fields which are not flag-based, we simply set every bit in the
field so that a naive bitwise-and would show that any value of the field
is valid.

The intended way of using this interface to get feature information
looks something like the following:

  static bool mountattr_nosymfollow_supported;
  static bool mountattr_idmap_supported;

  int check_clone3_support(void)
  {
      int err;
      struct mount_attr attr = {};

      err = mount_attr(-EBADF, "", 0, &args, CHECK_FIELDS | sizeof(args));
      assert(err < 0);
      switch (errno) {
      case EFAULT: case E2BIG:
          /* Old kernel... */
          check_support_the_old_way();
          break;
      case EEXTSYS_NOOP:
          mountattr_nosymfollow_supported =
              ((attr.attr_clr | attr.attr_set) & MOUNT_ATTR_NOSYMFOLLOW);
          mountattr_idmap_supported =
              ((attr.attr_clr | attr.attr_set) & MOUNT_ATTR_IDMAP) &&
              (attr.userns_fd != 0);
          break;
      }
  }

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 fs/namespace.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 328087a4df8a..c7ae8d96b7b7 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4771,6 +4771,7 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
 		size_t, usize)
 {
 	int err;
+	bool check_fields;
 	struct path target;
 	struct mount_attr attr;
 	struct mount_kattr kattr;
@@ -4783,11 +4784,27 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
 		      AT_NO_AUTOMOUNT))
 		return -EINVAL;
 
+	check_fields = usize & CHECK_FIELDS;
+	usize &= ~CHECK_FIELDS;
+
 	if (unlikely(usize > PAGE_SIZE))
 		return -E2BIG;
 	if (unlikely(usize < MOUNT_ATTR_SIZE_VER0))
 		return -EINVAL;
 
+	if (unlikely(check_fields)) {
+		memset(&attr, 0, sizeof(attr));
+		attr = (struct mount_attr) {
+			.attr_set = MOUNT_SETATTR_VALID_FLAGS,
+			.attr_clr = MOUNT_SETATTR_VALID_FLAGS,
+			.propagation = MOUNT_SETATTR_PROPAGATION_FLAGS,
+			.userns_fd = 0xFFFFFFFFFFFFFFFF,
+		};
+
+		err = copy_struct_to_user(uattr, usize, &attr, sizeof(attr), NULL);
+		return err ?: -EEXTSYS_NOOP;
+	}
+
 	if (!may_mount())
 		return -EPERM;
 

-- 
2.46.1