[PATCH v4 14/30] internal: add may_write_xattr()

[Christian Brauner]

Split out the generic checks whether an inode allows writing xattrs. Since
security.* and system.* xattrs don't have any restrictions and we're going
to split out posix acls into a dedicated api we will use this helper to
check whether we can write posix acls.

Signed-off-by: Christian Brauner (Microsoft) <brauner at kernel.org>
---

Notes:
    /* v2 */
    patch not present
    
    /* v3 */
    patch not present
    
    /* v4 */
    Christoph Hellwig <hch at lst.de>:
    - Split out checks whether an inode can have xattrs written to into a helper.

 fs/internal.h |  1 +
 fs/xattr.c    | 40 +++++++++++++++++++++++++++-------------
 2 files changed, 28 insertions(+), 13 deletions(-)

diff --git a/fs/internal.h b/fs/internal.h
index 87e96b9024ce..a95b1500ed65 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -221,3 +221,4 @@ ssize_t do_getxattr(struct user_namespace *mnt_userns,
 int setxattr_copy(const char __user *name, struct xattr_ctx *ctx);
 int do_setxattr(struct user_namespace *mnt_userns, struct dentry *dentry,
 		struct xattr_ctx *ctx);
+int may_write_xattr(struct user_namespace *mnt_userns, struct inode *inode);
diff --git a/fs/xattr.c b/fs/xattr.c
index 61107b6bbed2..57148c207545 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -80,6 +80,28 @@ xattr_resolve_name(struct inode *inode, const char **name)
 	return ERR_PTR(-EOPNOTSUPP);
 }
 
+/**
+ * may_write_xattr - check whether inode allows writing xattr
+ * @mnt_userns:	User namespace of the mount the inode was found from
+ * @inode: the inode on which to set an xattr
+ *
+ * Check whether the inode allows writing xattrs. Specifically, we can never
+ * set or remove an extended attribute on a read-only filesystem  or on an
+ * immutable / append-only inode.
+ *
+ * We also need to ensure that the inode has a mapping in the mount to
+ * not risk writing back invalid i_{g,u}id values.
+ *
+ * Return: On success zero is returned. On error a negative errno is returned.
+ */
+int may_write_xattr(struct user_namespace *mnt_userns, struct inode *inode)
+{
+	if (IS_IMMUTABLE(inode) || IS_APPEND(inode) ||
+	    HAS_UNMAPPED_ID(mnt_userns, inode))
+		return -EPERM;
+	return 0;
+}
+
 /*
  * Check permissions for extended attribute access.  This is a bit complicated
  * because different namespaces have very different rules.
@@ -88,20 +110,12 @@ static int
 xattr_permission(struct user_namespace *mnt_userns, struct inode *inode,
 		 const char *name, int mask)
 {
-	/*
-	 * We can never set or remove an extended attribute on a read-only
-	 * filesystem  or on an immutable / append-only inode.
-	 */
 	if (mask & MAY_WRITE) {
-		if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
-			return -EPERM;
-		/*
-		 * Updating an xattr will likely cause i_uid and i_gid
-		 * to be writen back improperly if their true value is
-		 * unknown to the vfs.
-		 */
-		if (HAS_UNMAPPED_ID(mnt_userns, inode))
-			return -EPERM;
+		int ret;
+
+		ret = may_write_xattr(mnt_userns, inode);
+		if (ret)
+			return ret;
 	}
 
 	/*
-- 
2.34.1