Document that FUSE_CAP_POSIX_ACL turns on -o default_permissions

doc/mount.fuse.8

@@ -55,9 +55,12 @@ filesystem. The check by the kernel is done in addition to any
 permission checks by the filesystem, and both have to succeed for an
 operation to be allowed. The kernel performs a standard UNIX permission
 check (based on mode bits and ownership of the directory entry, and
-uid/gid of the client). If the filesystem supports extended
-attributes and the kernel is sufficiently recent, it may also take
-into account access control lists (ACLs).
+uid/gid of the client).
+
+This mount option is activated implicitly if the filesystem enables
+ACL support during the initial feature negotiation when opening the
+device fd. In this case, the kernel performs both ACL and standard
+unix permission checking.
 
 Filesystems that do not implement any permission checking should
 generally add this option internally.

include/fuse_common.h

@@ -253,7 +253,7 @@ struct fuse_file_info {
 #define FUSE_CAP_PARALLEL_DIROPS        (1 << 18)
 
 /**
- * Indicates support for POSIX ACL.
+ * Indicates support for POSIX ACLs.
  *
  * If this feature is enabled, the kernel will cache and have
  * responsibility for enforcing ACLs. ACL will be stored as xattrs and
@@ -263,6 +263,10 @@ struct fuse_file_info {
  * created. Note that this requires that the file system is able to
  * parse and interpret the xattr representation of ACLs.
  *
+ * Enabling this feature implicitly turns on the
+ * ``default_permissions`` mount option (even if it was not passed to
+ * mount(2)).
+ *
  * This feature is disabled by default.
  */
 #define FUSE_CAP_POSIX_ACL              (1 << 19)