xattr: define vfs_getxattr_alloc and vfs_xattr_cmp

vfs_getxattr_alloc() and vfs_xattr_cmp() are two new kernel xattr helper
functions.  vfs_getxattr_alloc() first allocates memory for the requested
xattr and then retrieves it. vfs_xattr_cmp() compares a given value with
the contents of an extended attribute.

Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>

fs/xattr.c

@@ -166,6 +166,64 @@ out_noalloc:
 }
 EXPORT_SYMBOL_GPL(xattr_getsecurity);
 
+/*
+ * vfs_getxattr_alloc - allocate memory, if necessary, before calling getxattr
+ *
+ * Allocate memory, if not already allocated, or re-allocate correct size,
+ * before retrieving the extended attribute.
+ *
+ * Returns the result of alloc, if failed, or the getxattr operation.
+ */
+ssize_t
+vfs_getxattr_alloc(struct dentry *dentry, const char *name, char **xattr_value,
+		   size_t xattr_size, gfp_t flags)
+{
+	struct inode *inode = dentry->d_inode;
+	char *value = *xattr_value;
+	int error;
+
+	error = xattr_permission(inode, name, MAY_READ);
+	if (error)
+		return error;
+
+	if (!inode->i_op->getxattr)
+		return -EOPNOTSUPP;
+
+	error = inode->i_op->getxattr(dentry, name, NULL, 0);
+	if (error < 0)
+		return error;
+
+	if (!value || (error > xattr_size)) {
+		value = krealloc(*xattr_value, error + 1, flags);
+		if (!value)
+			return -ENOMEM;
+		memset(value, 0, error + 1);
+	}
+
+	error = inode->i_op->getxattr(dentry, name, value, error);
+	*xattr_value = value;
+	return error;
+}
+
+/* Compare an extended attribute value with the given value */
+int vfs_xattr_cmp(struct dentry *dentry, const char *xattr_name,
+		  const char *value, size_t size, gfp_t flags)
+{
+	char *xattr_value = NULL;
+	int rc;
+
+	rc = vfs_getxattr_alloc(dentry, xattr_name, &xattr_value, 0, flags);
+	if (rc < 0)
+		return rc;
+
+	if ((rc != size) || (memcmp(xattr_value, value, rc) != 0))
+		rc = -EINVAL;
+	else
+		rc = 0;
+	kfree(xattr_value);
+	return rc;
+}
+
 ssize_t
 vfs_getxattr(struct dentry *dentry, const char *name, void *value, size_t size)
 {

include/linux/xattr.h

@@ -84,7 +84,10 @@ ssize_t generic_getxattr(struct dentry *dentry, const char *name, void *buffer,
 ssize_t generic_listxattr(struct dentry *dentry, char *buffer, size_t buffer_size);
 int generic_setxattr(struct dentry *dentry, const char *name, const void *value, size_t size, int flags);
 int generic_removexattr(struct dentry *dentry, const char *name);
-
+ssize_t vfs_getxattr_alloc(struct dentry *dentry, const char *name,
+			   char **xattr_value, size_t size, gfp_t flags);
+int vfs_xattr_cmp(struct dentry *dentry, const char *xattr_name,
+		  const char *value, size_t size, gfp_t flags);
 #endif  /*  __KERNEL__  */
 
 #endif	/* _LINUX_XATTR_H */