fs: add copy_mount_setattr() helper

Split out copy_mount_setattr() from mount_setattr() so we can use it in
later patches.

Link: https://lore.kernel.org/r/20250128-work-mnt_idmap-update-v2-v1-2-c25feb0d2eb3@kernel.org
Reviewed-by: "Seth Forshee (DigitalOcean)" <sforshee@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -4814,7 +4814,7 @@ static int do_mount_setattr(struct path *path, struct mount_kattr *kattr)
 }
 
 static int build_mount_idmapped(const struct mount_attr *attr, size_t usize,
-				struct mount_kattr *kattr, unsigned int flags)
+				struct mount_kattr *kattr)
 {
 	struct ns_common *ns;
 	struct user_namespace *mnt_userns;
@@ -4865,22 +4865,8 @@ static int build_mount_idmapped(const struct mount_attr *attr, size_t usize,
 }
 
 static int build_mount_kattr(const struct mount_attr *attr, size_t usize,
-			     struct mount_kattr *kattr, unsigned int flags)
+			     struct mount_kattr *kattr)
 {
-	unsigned int lookup_flags = LOOKUP_AUTOMOUNT | LOOKUP_FOLLOW;
-
-	if (flags & AT_NO_AUTOMOUNT)
-		lookup_flags &= ~LOOKUP_AUTOMOUNT;
-	if (flags & AT_SYMLINK_NOFOLLOW)
-		lookup_flags &= ~LOOKUP_FOLLOW;
-	if (flags & AT_EMPTY_PATH)
-		lookup_flags |= LOOKUP_EMPTY;
-
-	*kattr = (struct mount_kattr) {
-		.lookup_flags	= lookup_flags,
-		.recurse	= !!(flags & AT_RECURSIVE),
-	};
-
 	if (attr->propagation & ~MOUNT_SETATTR_PROPAGATION_FLAGS)
 		return -EINVAL;
 	if (hweight32(attr->propagation & MOUNT_SETATTR_PROPAGATION_FLAGS) > 1)
@@ -4928,7 +4914,7 @@ static int build_mount_kattr(const struct mount_attr *attr, size_t usize,
 			return -EINVAL;
 	}
 
-	return build_mount_idmapped(attr, usize, kattr, flags);
+	return build_mount_idmapped(attr, usize, kattr);
 }
 
 static void finish_mount_kattr(struct mount_kattr *kattr)
@@ -4940,23 +4926,14 @@ static void finish_mount_kattr(struct mount_kattr *kattr)
 		mnt_idmap_put(kattr->mnt_idmap);
 }
 
-SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
-		unsigned int, flags, struct mount_attr __user *, uattr,
-		size_t, usize)
+static int copy_mount_setattr(struct mount_attr __user *uattr, size_t usize,
+			      struct mount_kattr *kattr)
 {
-	int err;
-	struct path target;
+	int ret;
 	struct mount_attr attr;
-	struct mount_kattr kattr;
 
 	BUILD_BUG_ON(sizeof(struct mount_attr) != MOUNT_ATTR_SIZE_VER0);
 
-	if (flags & ~(AT_EMPTY_PATH |
-		      AT_RECURSIVE |
-		      AT_SYMLINK_NOFOLLOW |
-		      AT_NO_AUTOMOUNT))
-		return -EINVAL;
-
 	if (unlikely(usize > PAGE_SIZE))
 		return -E2BIG;
 	if (unlikely(usize < MOUNT_ATTR_SIZE_VER0))
@@ -4965,17 +4942,47 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
 	if (!may_mount())
 		return -EPERM;
 
-	err = copy_struct_from_user(&attr, sizeof(attr), uattr, usize);
-	if (err)
-		return err;
+	ret = copy_struct_from_user(&attr, sizeof(attr), uattr, usize);
+	if (ret)
+		return ret;
 
 	/* Don't bother walking through the mounts if this is a nop. */
 	if (attr.attr_set == 0 &&
 	    attr.attr_clr == 0 &&
 	    attr.propagation == 0)
 		return 0;
 
-	err = build_mount_kattr(&attr, usize, &kattr, flags);
+	return build_mount_kattr(&attr, usize, kattr);
+}
+
+SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
+		unsigned int, flags, struct mount_attr __user *, uattr,
+		size_t, usize)
+{
+	int err;
+	struct path target;
+	struct mount_kattr kattr;
+	unsigned int lookup_flags = LOOKUP_AUTOMOUNT | LOOKUP_FOLLOW;
+
+	if (flags & ~(AT_EMPTY_PATH |
+		      AT_RECURSIVE |
+		      AT_SYMLINK_NOFOLLOW |
+		      AT_NO_AUTOMOUNT))
+		return -EINVAL;
+
+	if (flags & AT_NO_AUTOMOUNT)
+		lookup_flags &= ~LOOKUP_AUTOMOUNT;
+	if (flags & AT_SYMLINK_NOFOLLOW)
+		lookup_flags &= ~LOOKUP_FOLLOW;
+	if (flags & AT_EMPTY_PATH)
+		lookup_flags |= LOOKUP_EMPTY;
+
+	kattr = (struct mount_kattr) {
+		.lookup_flags	= lookup_flags,
+		.recurse	= !!(flags & AT_RECURSIVE),
+	};
+
+	err = copy_mount_setattr(uattr, usize, &kattr);
 	if (err)
 		return err;