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From Chris Nauroth <cnaur...@hortonworks.com>
Subject Re: Mask value not shown in GETFACL using webhdfs
Date Tue, 24 May 2016 17:18:45 GMT
Hello Kumar,

I answered at the Stack Overflow link.  I'll repeat the same information here for everyone's
benefit.

HDFS implements the POSIX ACL model [1].  The linked documentation explains that the mask
entry is persisted into the group permission bits of the classic POSIX permission model. 
This is done to support the requirements of POSIX ACLs and also support backwards-compatibility
with existing tools like chmod, which are unaware of the extended ACL entries.  Quoting that
document:

> In minimal ACLs, the group class permissions are identical to the
> owning group permissions. In extended ACLs, the group class may
> contain entries for additional users or groups. This results in a
> problem: some of these additional entries may contain permissions that
> are not contained in the owning group entry, so the owning group entry
> permissions may differ from the group class permissions.
>
> This problem is solved by the virtue of the mask entry. With minimal
> ACLs, the group class permissions map to the owning group entry
> permissions. With extended ACLs, the group class permissions map to
> the mask entry permissions, whereas the owning group entry still
> defines the owning group permissions.
>
> ...
>
> When an application changes any of the owner, group, or other class
> permissions (e.g., via the chmod command), the corresponding ACL entry
> changes as well. Likewise, when an application changes the permissions
> of an ACL entry that maps to one of the user classes, the permissions
> of the class change.

This is relevant to your question, because it means the mask is not in fact persisted as an
extended ACL entry.  Instead, it's in the permission bits.  When querying WebHDFS, you've
made a "raw" API call to retrieve information about the ACL.  When running getfacl, you've
run an application that layers additional display logic on top of that API call.  getfacl
is aware that for a file with an ACL, the group permission bits are interpreted as the mask,
and so it displays accordingly.

This is not specific to WebHDFS.  If an application were to call getAclStatus through the
NameNode's RPC protocol, then it would see the equivalent of the WebHDFS response.  Also,
if you were to use the getfacl command on a webhdfs:// URI, then the command would still display
the mask, because the application knows to apply that logic regardless of the FileSystem implementation.

[1] http://www.vanemery.com/Linux/ACL/POSIX_ACL_on_Linux.html

--Chris Nauroth

From: kumar r <kumarccpp@gmail.com<mailto:kumarccpp@gmail.com>>
Date: Monday, May 23, 2016 at 10:20 PM
To: "user@hadoop.apache.org<mailto:user@hadoop.apache.org>" <user@hadoop.apache.org<mailto:user@hadoop.apache.org>>
Subject: Mask value not shown in GETFACL using webhdfs

Hi,


In Hadoop, i have enabled authorization. I have set few acl for a directory.

When i execute getfacl command in hadoop bin, i can see mask value in that.

hadoop fs -getfacl /Kumar

# file: /Kumar
# owner: Kumar
# group: Hadoop
user::rwx
user:Babu:rwx
group::r-x
mask::rwx
other::r-x


If i run the same command using webhdfs, mask value not shown.

http://localhost:50070/webhdfs/v1/Kumar?op=GETACLSTATUS

{
  "AclStatus": {
    "entries": [
      "user:Babu:rwx",
      "group::r-x"
    ],
    "group": "Hadoop",
    "owner": "Kumar",
    "permission": "775",
    "stickyBit": false
  }
}


What the reason for not showing mask value in webhdfs for GETFACL command?


Find the stack overflow question,

http://stackoverflow.com/questions/37404899/mask-value-not-shown-in-getfacl-using-webhdfs


Thanks,

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