[RFC PATCH 32/35] xattr: add a function to check if a namespace is supported

[Frank van der Linden <fllinden@amazon.com>]

Add a function that checks is an extended attribute namespace is
supported for an inode.

To be used by the nfs server code when being queried for extended
attributes support.

Signed-off-by: Frank van der Linden <fllinden@amazon.com>
---
 fs/xattr.c            | 27 +++++++++++++++++++++++++++
 include/linux/xattr.h |  2 ++
 2 files changed, 29 insertions(+)

diff --git a/fs/xattr.c b/fs/xattr.c
index 58013bcbc333..7d0ebab1df30 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -134,6 +134,33 @@ xattr_permission(struct inode *inode, const char *name, int mask)
 	return inode_permission(inode, mask);
 }
 
+/*
+ * Look for any handler that deals with the specified namespace.
+ */
+int
+xattr_supported_namespace(struct inode *inode, const char *prefix)
+{
+	const struct xattr_handler **handlers = inode->i_sb->s_xattr;
+	const struct xattr_handler *handler;
+	size_t preflen;
+
+	if (!(inode->i_opflags & IOP_XATTR)) {
+		if (unlikely(is_bad_inode(inode)))
+			return -EIO;
+		return -EOPNOTSUPP;
+	}
+
+	preflen = strlen(prefix);
+
+	for_each_xattr_handler(handlers, handler) {
+		if (!strncmp(xattr_prefix(handler), prefix, preflen))
+			return 0;
+	}
+
+	return -EOPNOTSUPP;
+}
+EXPORT_SYMBOL(xattr_supported_namespace);
+
 int
 __vfs_setxattr(struct dentry *dentry, struct inode *inode, const char *name,
 	       const void *value, size_t size, int flags)
diff --git a/include/linux/xattr.h b/include/linux/xattr.h
index 3a71ad716da5..32e377602068 100644
--- a/include/linux/xattr.h
+++ b/include/linux/xattr.h
@@ -61,6 +61,8 @@ ssize_t generic_listxattr(struct dentry *dentry, char *buffer, size_t buffer_siz
 ssize_t vfs_getxattr_alloc(struct dentry *dentry, const char *name,
 			   char **xattr_value, size_t size, gfp_t flags);
 
+int xattr_supported_namespace(struct inode *inode, const char *prefix);
+
 static inline const char *xattr_prefix(const struct xattr_handler *handler)
 {
 	return handler->prefix ?: handler->name;
-- 
2.17.2